B~]o December I93; Gllide I'll; P ffldllgtio" ill thg Low e . r R|o Grande Valley \ Q Q QQ QQQ QQQ QQQQ QQQQQ Q Q Q Q QQ Q Q Q Q QQ Q Q Q Q Q Q8 \ O O O \ \ O\ Q Q Q Q Q Q Q\ Q Q Q Q Q Q Q Q\ Q Q Q Q Q Q Q Q Q\ Q Q Q Q Q Q Q Q QQ Q Q Q Q Q Q Q Q QQ Q Q Q Q Q Q Q Q Q QQ Q Q Q Q Q Q Q Q Q Q QQ Q Q Q Q Q Q Q Q Q Q QQ Q Q Q Q Q Q Q Q Q Q Q QQ Q Q Q Q Q Q Q Q Q Q Q Q QQ Q Q Q Q Q Q Q Q Q Q Q Q QQ Q Q Q Q Q Q Q Q Q Q Q Q Q QQ Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q\ Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q\ Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q\ Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q\ Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q QQ Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q QQ Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q QQ Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q QQ Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q QQ Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q QQ Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q QQ Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q QQ Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q QQ Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q QQ Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q QQ Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q QQ Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q QQ Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q QQ Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q QQ O Q Q \ \ Q Q Q Q \ \ O O O O O SQ Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q QQ Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q QQ Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q QQ Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q QQ Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q QQ Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q QQ Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q QQ Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q QQ Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q QQ Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q QQ Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q QQ Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q QQ Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q QQ Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q QQ Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q QQ Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q QQ Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q QQ Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q QQ Q Q Q Q Q Q Q Q Q I Q Q Q Q Q Q QQ Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q Q QQ Q Q \ i Q ‘(D Q Q § Q ~', y! .3» Q Q Q Q Q ~e Q Q O V‘, Q Q \\\\ \ O O O \\ I Q Q \ \\ O O O Q \\ \ \ Q O 00 Q Q \ \ eh O O Q Q Q! Q O Q \ 00 Q e O e O Q0 \ O O O O0 O O O \ Q0 DIOOO Q O \ \ on \ Q Q Q 00 Q \ e \ ee Q e e \ e so 4 Q 0 e O0 \ O \ O \O assoc e\e\\ Q e O 0 00 eeee 0e \ O \ O O0 \ O O O Q0 I O \ Q O0 O \ O O \ O0 I O O \ O0 O \ \ \ \O 6 e e e es 1 \ \ Q 00 Q Q \ \ 05 _ Q \ \ \ 00 \\\\\\ es \ \ Q O \\ I O I \ \\ Q e e \ Q \\ \ O O Q it Q O \ \ 00 \ \ \ O O0 \ \ O \ 00 0 Q e \ be \ \ \ e 00 Q Q \ O \ es Q \ Q 0 ee \ O O O \\ 0 \ e Q es O \ \ \ O0 \ Q \ 0 00 O O O \ \\ e e e e ea e e \ \ no \ e e \ ee Q \ \ e \ so e \ e e so \ e \ Q ee \ e e e so e \ \ \ es e e Q e so \ \ e \ es eeeeee 00 Q Q e \ we e \ Q Q 00 Q \ e \ ee e \ e e on \ e e e ee Q e e e es e Q \ e e 00 TEXAs M n ma; “mam WITH)" ' ' ' ""8 as e we; l CONTENTS QINNOOO h-hid 28 28 28 28 VALLEY CITRUS AND ITS POTENTIAL Comparison to Other Areas General Description of Climate SELECTING A SITE Soil Factors Water Quality Water Availability Topography Factors IRRIGATION, SALINITY, AND DRAINAGE Irrigation Systems for Citrus Groves Salinity Problems Drainage Problems KINDS OF CITRUS AND THEIR VALUE Grapefruit Varieties Orange Varieties Tangerines and Tangelos Limes, Lemons and Miscellaneous Citrus Rootstocks-—Their Tolerance and Commercial Value NURSERY TREES Diseases and Insects Tree Size and Form GROVE ESTABLISHMENT Tree Spacing Planting and Initial Care Fertilizing and Watering Trees Pruning and Training Trees CARE OF BEARING TREES Fertilization Mechanical Cultivation Diseases from Fungus and Virus Physiological Disorders Nematodes Citrus Mites and Insects Mites Armored Scales Unarmored Scales Miscellaneous Insects ‘ Water Requirements of Citrus Climatic Factors Plant Factors Irrigation Schedule Pruning Chemical Weed Control AGRICULTURAL METEOROLOGY FOR CITRUS PRODUCTION Minimum Temperature Forecast Program Agricultural Interpretation of Weather Forecasts Thermometer Calibration and Exposure Interval Between Freezes Temperature-Duration Relationships in Severe Freezes COLD PROTECTION Physiological Aspects of Breeze Injury Plant Responses During‘ Freezes Freeze Injury Critical Tissue Temperatures Dormancy and Cold Hardiness Varieties and Rootstocks Mechanical Cold Protection ‘I’ I Engineering Considerations Windmachines hfigafion Cultural Practices Windbreaks CARE AND REHABILITATION OF FREEZE-DAMAGED CITRUS Young Banked Trees Bearing Trees Fertilization and Irrigation Equipment for Pruning Wound Protectants Susceptibility to Subsequent Freezes MARKETING TEXAS CITRUS Marketing Organization Harvesting Operations Packing House Operations Packing Costs Containers Processing Texas Citrus Distribution Wholesale Industry Retail Industry COST AND RETURNS OF PRODUCING TEXAS CITRUS Stages of Grove Maturity Tree Spacing and Variety I- Grove Ownership and Care Annual Operating Expense Irrigation Cists Fertilizer Costs Spraying and Dusting Weed Control Tree Banking Pruning Taxes Replacement Trees Capital Investment and Expense Return to Operator Labor and Management s ‘ Cost of Rehabilitating Freeze-Damaged Gro " Yields ' Prices Returns from Citrus .; Capital Requirements for Grove Developme i; Grove Value Estimation ' Cold Protection Value Estimation References Acknowledgments VALLEY CITRUS AND ITS POTENTIAL Norman Maxwell, Ralph Petersen, Robert Orton and Donald Haoldockl‘ p The earliest record of citrus planted in the Valley a planting of seedling orange trees, made by Don h’ cedona Vela in the early 1880’s, on the Laguna ~ l» a Ranch, north of Edinburg. John Shary was one the early pioneers in developing the citrus industry = the Valley and is generally known as the father of Texas citrus industry. One of the earliest successful commercial citrus antings on sour orange rootstock was made by i rles Volz in 1908. Previous commercial plantings id been made by others on trifoliate orange root- Jock but the trees did not survive because trifoliate ange is not adaptable to high alkaline soils and later containing high chlorides. The Valley citrus industry came into recognition A 1920 when about 124,000 trees were reported for it area. The rate of tree plantings increased to a of near 14,000,000 trees in 1949. The 1949 and £951 freezes reduced the number to 3,500,000. At f- e time of the 1962 freeze, which destroyed about fl percent of the trees, there were close to 7,000,000 lees in the Valley. A Production figures for 1919 through 1962 are hown in Table 1. p The many changes in the Valley citrus industry ave been gradual, but many were focused on disas- (rs like the 1934 hurricane, 1949, 1951, and 1962 eezes. After every disaster the industry has come ack better in some respect than it was before. Early plantings were mainly white seedy grape- ruit and seedy oranges. Varieties gradually changed o white seedless and pink grapefruit and a mixture f seedy and seedless oranges. After the discovery of udsports of red grapefruit in 1929 and 1931, new lantings of grapefruit were changed to red seedless apefruit. Most orange plantings are now of the edless varieties, but some seedy pineapple groves re planted, principally for processing. Most of the major variety changes were tied losely to freezes and market preference. Until 1951 5 ere were still many olid, groves with mixed varieties g seedy white and seedless white grapefruit, pink Rapectively, associate horticulturist, Texas Agricultural Experi- ment Substation l5, Weslaco; farm management specialist, Texas , Agricultural Extension Service, Weslaco; state climatologist, vWeather Bureau Airport Station, Austin, Texas; and advisory j agricultural meteorologist, Weather Bureau Agricultural Service Office, Walaco, Texas. seedy and seedless grapefruit, and seedless oranges. After the 1951 freeze, when 80 percent of the existing industry was destroyed, most new plantings of grape- fruit were red grapefruit and a few white marsh for specialty processing. New orange plantings were seedless early oranges, a few seedy pineapple oranges in mid-season, and the Valencia——a late season variety. Other changes over the years include: 1. Closer tree spacing 2. Greater use of mechanical grove care equip- ment Table 1. Texas citrus fruit production in boxes (1 3/5 bu. boxes). Oranges Grapefruit Total 1919-20 9,000 3,000 12,000 1920-21 5,000 5,000 10,000 1921-22 5,000 8,000 13,000 1922-23 10,000 35,000 45,000 1923-24 6,000 65,000 71 ,000 1924-25 17,000 301,000 318,000 1925-26 12,000 200,000 212,000 1926-27 41,000 361,000 402,000 1927-28 85,000 524,000 609,000 1928-29 125,000 753,000 878,000 1929-30 261,000 1,550,000 1,811,000 1930-31 250,000 1,200,000 1,450,000 1931-32 520,000 2,600,000 3,120,000 1932-33 325,000 1,440,000 1,765,000 1933-34 430,000 1,200,000 1,630,000 1934-35 650,000 2,740,000 3,390,000 1935-36 777,000 2,780,000 3,557,000 1936-37 2,000,000 9,630,000 1 1 ,630,000 1937-38 1,440,000 1 1,840,000 13,280,000 1938-39 2,815,000 15,670,000 18,485,000 1939-40 2,360,000 14,400,000 16,760,000 1940-41 2,650,000 13,650,000 16,300,000 1941-42 2,850,000 14,500,000 17,350,000 1942-43 2,550,000 17,510,000 20,060,000 1944-45 4,400,000 22,300,000 26,700,000 1945-46 4,480,000 24,000,000 28,480,000 1946-47 5,000,000 23,300,000 28,300,000 1947-48 5,200,000 23,200,000 28,400,000 1948-49 3,400,000 1 1,300,000 14,700,000 1949-50 1,760,000 6,400,000 8,160,000 1950-51 2,700,000 7,500,000 10,200,000 1951-52 300,000 200,000 500,000 1952-53 1,000,000 400,000 1,400,000 1953-54 900,000 1,200,000 2,100,000 1954-55 1,500,000 2,500,000 4,000,000 1955-56 1,600,000 2,200,000 3,800,000 1956-57 1,773,955 3,824,514 5,598,469 1957-58 2,000,000 3,500,000 5,500,000 1958-59 2,300,000 4,200,000 6,500,000 1959-60 2,800,000 5,500,000 8,300,000 1960-61 3,500,000 6,500,000 *10,000,000 1961-62 2,200,000 2,600,000 4,800,000 Source: USDA - AMS. Compiled and distributed by the Valley Chamber of Commerce, Weslaco, Texas. *USDA estimate. 3. Centralization of the citrus industry on soils most adaptable for citrus production. COMPARISON TO OTHER AREAS The potential for profitable citrus production in the Rio Grande Valley compares favorably to other U. S. Citrus producing areas, because of relatively low production costs and high quality fruit. The major disadvantage of citrus production in Texas is the hazard of a killing freeze. Unless an effective economical cold protection system can be developed, the risk of a killing freeze can offset any competitive advantage the Valley may have in production costs and fruit quality. The yearly costs of operation in Texas are much lower than corresponding costs in other citrus pro- ducing areas. These costs include fertilizer, irriga- tion water, insecticide, machinery operation, labor, and taxes. Costs of establishing and developing an orchard in Texas are also much less than similar costs in competing areas. Recurrent freezes in Texas have prevented citrus trees from maturing enough t0 produce heavily for sustained periods of time. The short time over which the establishment and development costs must be dis- tributed has, in the past, meant that investment costs were high in Texas when compared to other areas where trees have produced over 40 years. Yields from trees of the same age generally are lower in Texas than in California or Florida. How- ever, these low yields are offset by the low production costs in Texas. Costs of marketing Texas fruit probably are higher than similar costs in other areas because of the high variation in annual production. An addi- tional factor could be the large number and small costs from Texas to South Central and Mid-Western?“ states are somewhat lower than such costs from other: areas. The market area within which Texas has a» transportation advantage should be large enough to absorb a substantial part of the fruit produced in. Texas. i. A Market acceptance or consumer preference for Texas fruit is good compared to fruit from other areas. While the high quality of Texas fruit makes, it especially suited to fresh sales, the trend in cit? Q: consumption is to processed products. Facilities for, processing citrus products are available in the Valley: and should allow the Texas producer to sell his fruit, for use in the most profitable form. ‘ GENERAL DESCRIPTION OF CLIMATE The Lower Rio Grande Valley (Cameron, Wil lacy, Hidalgo, and Starr Counties) has a subtropic § semiarid climate. g Moist air from the Gulf of Mexic has a moderating effect on Lower Valley tempera‘ tures. The average daily minimum temperature o, the coldest month, January, ranges from 52 degred A at Brownsville to 46 degrees at Rio Grande City, -; . shown in Table 2. Freezes (32 degrees F or lower) do not occu“ every year in the Lower Valley. A fall freeze occur about 7 years out of every 10, on an average, at Ri Grande City; 3 years out of every 5 at Mission; year out of every 2 at Raymondville; and 2 out 0 every 5 at Harlingen. Spring freezes occur about 9 years out of every 4~, on an average, at Harlingen an Raymondville; 4 years out of 5 at Mission; and ’, years out of 1O at Rio Crande City. ' In general, average annual rainfall decreases 9_ the distance from the Gulf of Mexico increas Table 3 shows that it varies from about 26 inches ' size of agencies marketing the fruit. Transportation Willacy and Cameron Counties to around 19 to Table 2. Minimum temperature (° F.) 1931-1962. Station 1.2:; Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Annu l Brownsville Mean 52.2 55.1 59.3 65.6 71.1 75.0 76.1 75.9 73.2 67.2 58.5 53.8 65.3 Extreme 19.0 22.0 32.0 41.0 53.0 64.0 68.0 66.0 55.0 44.0 34.0 29.0 19.0 _ Raymondville Mean 49.4 52.3 57.1 63.7 69.0 72.5 73.4 73.0 70.8 63.9 55.1 50.8 62.6 l Extreme 14.0 19.0 28.0 37.0 48.0 59.0 65.0 61.0 51.0 40.0 28.0 26.0 14.0 Weslaco Mean 50.7 53.9 58.3 65.7 69.9 73.3 74.1 73.8 71.2 64.8 56.7 52.2 63.7 Extreme 16.0 19.0 31.0 38.0 47.0 61.0 67.0 62.0 48.0 40.0 30.0 24.0 16.0 i Mission Mean 48.4 51.7 56.5 63.6 69.3 73.1 74.1 73.8 71.0 63.9 54.9 49.8 62.5 V: Extreme 18.0 19.0 31.0 39.0 48.0 59.0 67.0 63.0 51.0 40.0 29.0 25.0 18.0 ' Rio Grande City Mean 46.2 49.7 54.7 62.0 68.8 73.0 74.3 73.8 70.5 62.8 52.6 47.6 61.3 s‘ Extreme 10.0 15.0 28.0 32.0 44.0 56.0 59.0 60.0 52.0 39.0 27.0 23.0 10.0 _ Table 3. Mean precipitation (inches) 1931-1960. Station Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Annual Brownsville 1.35 1.48 1.04 1.55 2.36 2.96 1.68 2.77 4.99 3.53 1.32 1.72 26.75 Raymondville 1.83 1.15 1.30 1.45 3.48 2.46 1.94 3.00 4.65 2.57 1.37 1.33 26.53 Weslaco 1.66 1.03 1.07 1.53 2.70 2.46 1.71 2.67 4.13 2.10 1.02 1.26 23.34 Mission 1.35 1.00 0.88 1.61 2.22 1.94 1.58 1.66 3.24 2.05 0.74 1.02 19.29 Rio Granule City 0.94 0.79 0.85 1.26 2.10 2.01 1.37 1.69 3.13 1.84 0.61 0.68 17.27 inches in Hidalgo, and t0 near 17 inches in Starr County. Most of the precipitation falls in the form of thundershowers, thus amounts are unevenly distrib- uted both geographically and seasonally. Large vari- ations may exist over relatively small areas. Long term weather records show that the most rain for any one month falls in September. The general inadequacy and variability of rain- fall necessitates the use of supplemental irrigation water for successful production of most agricultural crops in the Lower Rio Grande Valley. The distribution of relative humidity is similar to rainfall. Highest values are observed along the coast; the lowest in the extreme western portion of Starr County. Mean annual relative humidity ranges from about 75 to 80 percent in Willacy and Cameron Counties to 65 to 7O percent in Starr County. Al- ‘Q1 though monthly variations are small, lowest mean monthly relative humidities occur in March or April, and again in July and August. Highest monthly mean relative humidities occur in January and F eb- ruary, and again in May. Daily values are usually highest just before sunrise, and lowest during mid- afternoon. Mean relative humidity for selected hours at Brownsville and Laredo are shown in Table 4. Table 4. Mean relative humidity for selected hours at Brownsville and Laredo, Texas. Mean relative humidity (7,) Stfltiv" Y?’ Mid- 6=oo N moo recoord night a.m. 0°" p.m. Brownsville 1940-61 87 90 61 71 Laredo 1944-61 71 82 51 45 Source; Local Climatological Data, U. S. Weather Bureau, 1961. SELECTING A SITE Morris Bailey, Norman Maxwell, David Carter and Donald Haddoclfi‘ Soil, water, and topography are major factors for consideration in selecting a site for citrus pro- duction. i SOIL FACTORS Soils in the Lower Rio Crande Valley vary in texture from coarse sandy loams to fine textured clays. The best soils for citrus growing are deep and well drained. They should have a subsoil free from tight clay layers, and the free water table should not come higher than five feet from the surface. The coarser textured soils in the Valley are the best citrus soils. These are classified as Brennan, Willacy, Delmita, and Hidalgo series. Clay lenses often occur at 4- to 6 feet in some of these soils and require tile drainage in order to grow citrus. Citrus production is also possible on Laredo, Raymondville, and other soil series of finer texture where drainage is good. Before land is planted to citrus, a profile study of the area should be made to determine whether it has the characteristics to make a good orchard soil. WATER QUALITY Irrigation water quality depends upon the amount and kinds of dissolved salts the water contains. Chem- ical analyses will show what salts are present and the amount of each. From such analyses, the suitability of water for irrigation is determined, and irrigation waters are classified according to their total salt con- centration in parts per million (ppm). Citrus is salt sensitive. The recommended use of various classes of irrigation water for citrus are presented below. SALT CONCEN- TRATION, P.P.M. CLASSIFICATION OF WATER C'I—I.OW-SAI.INITY WATER can be used 0 to 200 for irrigation of citrus on most soils with little likelihood that a salinity problem will develop. Normal irri- gation practices provide the small amount of leaching required to assure no salt accumulation. ‘Respectively, area horticulturist, Texas Agricultural Extension Service, Weslaco; associate horticulturist, Texas Agricultural Ex- periment Substation 15, Weslaco; research soil scientist, USDA- SWORD, Weslaco; advisory agricultural meteorologist, Weather Bureau Agricultural Service Office, Weslaco. 6 i, - C2—MEDIUM-SAI.INITY WATER cant be used for citrus irrigation if a moderate amount of leaching occurs. Water in excess of that required to wet the rooting zone soil should be applied to , provide for moderate leaching. The s“ soil must have adequate natural or artificial drainage. C3—MODERATELY HIGH - SALlNlTY WATER can be used occasionally for citrus irrigation if high amounts of leaching occurs. Several inches of water over that needed to wet the rooting zone soil should be applied at each irriga- tion. Adequate natural or artifical leaching is required. If possible, the next irrigation should be with better quality water, with some excess ap- plied for leaching. C3B—HIGH-SALINITY WATER should not be used for citrus irrigation except to save trees that may die because of drouth. Use of this water may cause defoliation as well as other damage to trees. Following use of this low quality water, an irrigation with good quality water should be applied as soon as such water is available. Excess good quality water should be applied to provide for leaching. C4—VERY HIGH - SALINITY WATER should not be used for citrus. 200 to 500 500 to ‘I000 ‘I000 to 1500 l Above l 500 ln addition to total salt concentration, othei minerals must be considered when determining n. suitability of irrigation water. These include boro chloride, sodium, and residual carbonate plus bica bonate concentrations. Only water with less than i ppm of boron are recommended for citrus irrig tion. The best waters contain less than 0.3 ti, boron. Irrigation waters containing between 0 and 1.0 ppm boron may cause some boron toxicity i citrus. When necessary to use waters of the latt group, occasional irrigations with low boron wate (less than 0.3 ppm) should be applied in excess ~ that some leaching will occur. Waters with a sodium-adsorption-ratio level w: low 8 are safe for citrus irrigation. _ Those with S l levels of 8 to 15 are marginal, and continued use l. waters with SAR levels above 2O will undoubtedl lead to serious sodium problems. Excessive residu,‘ carbonate plus bicarbonate in irrigation waters a will cause serious sodium problems ‘with contin usage. Residual carbonate plus bicarbonate conc . trations below 1.25 milliequivalents per liter are g = erally safe. Residual carbonate plus bicarbonate c0“ ntrations between 1.25 and 2.50 milliequivalents per , r are marginal, and concentrations above 2.50 mil- pquivalents per liter are excessive. Soil drainage, water supply sufficiency, and water iplication techniques also must be considered in re- 'on to water quality. Where leaching is required, ‘e source of water must supply enough water to ac- implish the leaching, and drainage must be adequate such leaching. Generally, sprinkler irrigation is if] advisable for citrus except with the best quality aters for citrus can accumulate salts through leaves id through roots. Salt damage from a given water Spears much earlier when citrus is sprinkler irri- ted than when it is irrigated by other techniques. Irrigation waters of the Lower Rio Grande Val- ‘ are of the medium to moderately high salinity sses. Many well waters also contain excessive iron concentrations and high SAR levels. Some ._ll waters contain excessive residual carbonate plus arbonate concentrations. Most of the medium to oderately high salinity waters also contain high i oride concentrations that cause specific chloride lxicity to citrus. Rainfall in the Lower Rio Grande Valley is ade- ate to provide a sizeable fraction of the water for ‘r us. Seasonal rains usually account for several ir- lations each year. Often rains provide the occa- nal leaching with good quality water needed when jing moderate to high salinity and boron waters for i _. igation. Such leaching occurs only where soils ‘e well drained, however. Growers should use the highest quality of water ifailable. They should be particularly cautious of ill waters, since most of these waters are too high A, both total salt and boron for citrus irrigation. WA TER A VAILABILI TY The water sources for the Lower Rio Grande lley include natural precipitation, the Rio Grande 'ver, and ground water. Generally, these sources pply adequate water for most land developed for i igation in the Valley. Water shortages are common id sometimes severe, however. The annual precipitation varies widely from year 1 year and may be several inches below average ‘a R n? sometimes. When precipitation is below average, severe water shortages may result unless adequate irrigation water is available. Shallow and deep wells supply irrigation water for 8,000 to 10,000 acres in the Valley. Additional acreages are irrigated from wells during drouth pe- riods, but often, the water from these occasionally- used wells is of poor quality. The principal source of irrigation water is the Rio Grande River. The Falcon Reservoir agreement allows for irrigation of about 750,000 acres in the United States below Falcon Darn, but the quantity of water needed to irrigate this acreage is not always available. Thus, water must be withheld from some eligible land in order that acreage planted to citrus can be adequately irrigated. TOPOGRAPH Y FA C TORS Cameron, Hidalgo, and Willacy are the principal citrus producing counties in Texas. They are in a generally flat and featureless plain with poor natural drainage. The elevation increases from sea level along the coast to 37 feet at Harlingen, 75 at Weslaco, 96 at Edinburg, and to 225 feet at McCook. Even though the terrain features are poorly de- fined, they do affect the minimum temperature pat- terns under certain meteorological conditions. The elevation and slope of a site will influence the amount and rate of cold air drainage from adjacent fields into groves on calm, clear nights. Cold air, being heavier than warm air, sinks and moves downslope to a lower elevation when the nightime wind speed is generally less than three or four miles an hour. The cold air collects in bottoms or depressions, com- monly called “cold pockets.” In these cold pockets, citrus trees are subjected to temperatures a few de- grees colder than those on top of small ridges during calm clear nights when radiation is maximum. On the other hand, during periods of high north- erly winds, citrus trees on top of small ridges and exposed northerly slopes may be subjected to tem- peratures a few degrees colder than those in bottoms or depressions and on southerly slopes. IRRIGATION, SALINITY, AND DRAINAGE V. I. Myers, P. E. Ross and D. L. Carteri‘ IRRIGA TION S YS TEMS FOR CI TR US GRO VES Surface irrigation is the predominant method used in Lower Rio Grande Valley citrus groves. If the system is properly designed and installed before the grove is planted, good irrigation efficiencies may be obtained with minimum labor. It is generally difficult to use surface irrigation efficiently in non- level groves, and it is almost impossible to do any major land forming after the grove is established. Some of the early citrus orchards were planted on slopes ranging from 1.5 to 3.0 percent. A properly designed system. will include land forming to proper grade, proper length and width of areas irrigated, adequate irrigation stream size, and an adequate delivery system. Assistance in design and installation of irrigation systems may be obtained by the landowners from the Soil Conservation Service through the Soil Conservation Districts. Include as much area in one level plane as prac- tical when leveling land for either citrus or row crops. Where topography permits, entire fields are leveled to one elevation. If the natural slopes are such that excessive top soil must be removed from the cut areas of the fields, bench leveling must be used. Whether Fig. l. Flooding is the most commonly used method of irrigating orchards, but it is the least efficient method. *Respectively, project manager, research agricultural engineer, USDA-SWCRD; agricultural engineer, USDA-SWCRD; research soil scientist, USDA-SWCRD. 8 the benched areas are straightf-‘ior follow the contour l of the land depends upon the degree and direction of i slope. Less soil is removed when the benches follow the contour, but some grove owners prefer straight; borders because of field appearance and harvesjing, ease. ln some cases, the irrigation delivery system; is more costly on contour benched land than it is for“: straight benches. i‘ Zero grade is best when it can be obtained with-f out excessive removal of top soil from the cut areas; Permanent border ridges should be constructed to‘; enclose level areas. The ridges should have an effec-; tive settled height of not less than 1 foot, and where tillage equipment must pass over the ridges, the side, slopes should be flat enough for easy crossing. Side. slopes of 6:1 can be negotiated easily by grove village equipment. The border ridges limit the width of the; area irrigated with one irrigation stream and retain-é all water, whether irrigation or rainfall, which is plied to or falls in the area. The borders also prevent rainfall runoff from the tree area where they are used between each tree row. i The best Valley citrus soils are the sandy loams.) They have good soil permeability. For this reason,’- irrigation runs generally should not be longer thani 450 feet; in most of the sandier soils, better distribug tion of water can be obtained on runs of about 320i feet. The width for border irrigating should be the: tree row width, which will range from 18 to 3O feet in most of the newly-planted groves. Some grove‘, owners prefer to use a border for each two rows o, trees, but where this is practiced, large irrigatio streams are necessary to move the water over the lan fast enough to cover the area in 3O to 4-5 minutes, Border widths in excess of 5O feet are not recom. mended. 5 On level borders, irrigation stream sizes of to .08 cubic feet per second (c.f.s.) per foot of horde width are adequate for uniform water distributio ; l Where clean cultivation is practiced and smooth bor, der surfaces are maintained, the smaller stream w' travel along the irrigation run at about 1O feet minute. For soils on which citrus is planted, cover age of the soil with water in 3O to 45 minutes giv good distribution of water. If cover crops or mulch‘ are present in groves, the irrigation stream must er to overcome the friction to flow of water by the cover. The irrigation delivery system is an extremely ifrtant part of the overall system. Good physical f: ol of the water helps achieve proper water dis- iution and to minimize labor requirements. Underground concrete pipe with turnout valves ieach tree row is an excellent delivery system. The _al cost of such a system is high but maintenance , are comparatively low. ‘Furthermore, no land is § to the pipeline right-of-way, and a maximum area be’ irrigated with little labor. Water may be flied uniformly to each tree row simply by regu- fig the amount of time each valve remains open. The pipeline should be laid to a uniform grade gcovered by at least 2.5 feet of soil. Surge pipes d be installed at the distal end of each pipe, at turns of 9O degrees or less, and at the higli point ire 1O degree changes in grade are downward in direction of flow. Where long lines are laid, it is to have vents or surge pipes at least every 1,000 elliPipeline size depends, of course, upon the volume ' iwater to be handled. It is well to use sizes large gh to operate the system under minimum pressure consistent with the volume of water to be trans- '0 Pressure heads in excess of 25 feet should be "ided in ordinary concrete pipe. Turnout valves should be located in each bor- w area to be irrigated. The valve should be of fuate size to handle the irrigation stream called the designed system. In sandy or easily eroded l}: the turnout valve should be installed 4- to 6 ls below the natural surface of the land. When is done, the initial velocity of the water from the f is reduced materially before it begins to spread the border and lessens erosion ne-ar the turnout. Pipeline delivery systems are highly desirable for groves planted in contour borders. The en- I system eliminates most difficulties in transport- irrigation water down steep slopes, and it reduces ridge maintenance problems, since ridge cut- F is not required to introduce water to lower bor- "Open ditch irrigation delivery systems are com- y used on fields-ii where the gradient does not ir- erosion. Frequently, these are temporary ditches for each irrigation application, and they are koyed during subsequent tillage operations. If l earth ditches are permanent or semi-permanent, a serious weed problem develops and tillage is incon- venient.. Where open ditches are used, the water is generally delivered to the tree rows through cuts through the ditch bank. It is difficult to apply the same amount of water to each border using this meth- od, because the opening in the ditch bank often in- creases in size as water passes through it. The labor involved in opening and closing the cuts is consider- able, and the work involved is unpleasant. The initial expense of open ditch delivery systems is not great compared with other systems, but operation and main- tenance costs are high, Sprinkler irrigation systems are used effectively in groves established on sloping lands. It sometimes is tedious to fit the rate of the sprinkler output with the water intake rate of the soil, but doing so is neces- sary if best results are to be obtained. Outputs over one inch an hour generally exceed the intake rate of the soil and cause runoff, ponding, or both. The irrigator will often move the pipe when some ponding begins, without realizing that the upper slopes of the land may not have received enough water. If he does this too many times, the under-irrigated areas become drouthy, sometimes salty, and production is reduced. Two types of sprinkler irrigation systems avail- able in the Valle-y are pressure perforated pipe and rotary sprinklers. Rotary sprinkle-rs generally cover more area per setting than do perforated pipe sys- tems. But in larger trees, it is difficult to avoid blanked-out areas where tree foliage intercepts spray from rotating heads. Perforated pipe generally covers no more than one tree row width per setting and thus labor costs for moving are high. The perforate-d pipe may be operated on as little as 15 pounds per square inch (p.s.i.), however, whereas the rotary sprinklers require 4-5 p.s.i. or more. Fig. 2. Closely mowed sod in conjunction with sprinkler irrigation is a popular "system of orchard management. 9 Fig. 3. Sprinkler irrigation is most efficient, especially on rolling land. . In choosing a sprinkler system, the owner should consider the cost and availability of labor versus the cost of power for operation and initial costs of the system. Regardless of the type of irrigation system a grove owner may install, his results will only be as good as his system management. He must know when to irrigate and how much water to apply for best results. Above all, he must supervise the operation to make sure his instructions are carried out. SALINI T Y PROBLEMS Citrus is most sensitive to salinity. Both total salt concentration or osmotic effects and specific ion effects are pronounced in citrus. Osmotic effects limit water absorption and com- bine with specific ion effects to cause leaf burning, necrosis, and in extreme cases, complete defoliation, twig die-back, and death of trees. Specific ions that severely damage citrus when present in high concen- tration are chloride (CF), sodium (Na+ ) , and bicar- bonate HCO3_). Qther ions have been reported to cause damage in some cases. Boron is also extremely toxic to citrus. It is often difficult to distinguish between the effects of total salts and those of specific ions. Usually, when total salt concentration is high enough to damage citrus, specific ion accumulations have also reached toxic levels. a The total salinity level of soil is determined by measuring the electrical conductivity of the saturation extract (ECe) in millimhos per centimeter and parts per million. Generally, citrus can be produced with little likelihood of salinity damage on soils with salini- ty levels below 1280 ppm throughout the rooting 10 zone. Citrus can be grown on soils with salinity levels f i between 1280 and 2560 ppm with special manage- r1 ment practices such as intermittent leaching and more j Yields are usually lowered in? Generally, frequent irrigations. spite of special management practices. tree survival is poor and production low on soils with i’ salinity levels above 2560 ppm}: ’ Soils of many areas in the Lower Rio Grander Valley are too saline for citrus production. Many of these soils have inadequate natural drainage and gre. difficult to drain artificially. Generally, since éthéf coarser-textured, well-drained soils are least likely to; be saline, they are the best for citrus production. i“ DRAINAGE PROBLEMS g Poor drainage, with the accompanying salinity: problems, causes more damage to citrus than is ordi-g. narily realized. Damage from high water tables and; salinity usually are not recognized until trees begin to lose leaves or show other signs of extensive dam- "_ age. Fruit yields frequently are reduced even though damage is not readily apparent. Even the better citrus soils of the Valley are sub-f? ject to high water tables and surface ponding in some‘ areas. Drainage problems can exist in groves of ir- regular topography where ponding occurs or on - leveled fields as a result of a rise in the water table.’ In many cases, the soils may allow free movement of; water downward for several feet until an impervious layer is reached. High water tables affect citrus production inf many ways. Excessive soluble salts, which are toxic‘ to citrus or which make less soil moisture available Fig. 4. Surface drainage is important. Water on this le i‘ pan orchard came from a rain that occurred 3 days before i picture was made. plant use, accumulates in the soil. Soil structure ilnot be maintained if the soil stays excessively wet. aeration occurs in a poorly-drained soil, even a distance 0f 1 0r more feet above a water table. s roots cannot survive in soil that remains satu- ' w for prolonged periods. If water is not removed m the root zone within about 5 days, the root sys- can be severely damaged or destroyed. Drainage and irrigation problems are so closely ted that they must be treated as inseparable com- Sons and handled accordingly. Surface drainage j provided when the irrigation system is properly f lled. Subsurface drainage must be designed by i ompetent engineer and installed by a reliable con- tor. A drainage system must be designed to handle ' ss water. Suitable depth, spacing, and location itiles are essential. Many old drainage systems in Valley are not functioning properly. In some i ‘s. cases, tile lines have been improperly installed or have been spaced too far apart. In other cases, tile has been placed below an impermeable layer. In most instances, water and salts are not able to enter the tile fast enough because of the lack of filter or en- velope material around the tile. Where no drainage outlets are available to allow disposal of excess water, careful water management is the only alternative. The following recommendations can minimize drainage problems. 1. Use proper land forming to achieve good water distribution. 2. Apply only the quantity of water needed to fill the soil profile each irrigation. An occa- sional excessive application may be necessary to leach accumulated salts from the soil. 3. Provide sub-surface drainage where necessary. 11 KINDS OF CITRUS AND THEIR VALUE E. O. Olson, Roger Young, Morris Bailey, Norman Maxwell, W. C. Cooper and Bruce Lime* GRAPEFR UI T VARIE TIES The Valley’s reputation as a citrus area is based primarily upon the high interior quality of its grape- fruit. Valley grapefruit is sweeter than that raised in California, Arizona, and most parts of Florida. In the 1920’s, white and pink grapefruit varieties were planted extensively. About 1930, budsports of red grapefruit were found on Thompson (Pink Marsh) grapefruit trees in several locations in the Valley. Fruit from trees propagated from these bud- sports had a red blush on the rind and the pulp was a deeper red than Thompson or Foster grapefruit. New grapefruit plantings have been mainly the red variety, but some white grapefruit is planted for specialized processing. White grapefruit reaches legal maturity in October and November and its sugar and acid is similar in taste to Red grapefruit. Red grapefruit may reach legal maturity in Octo- ber and is shipped until the following June. Acid decreases as the season progresses until it is low in late spring. The interior red color gradually fades from a rose red in October to a pink in the spring. Choice of trees probably should be based on freedom from virus diseases and not on the source of the bud- sport, since a test conducted by the Experiment Sta- tion at Weslaco showed no differences in fruit or trees propagated from these red budsports. ORANGE VARIE TIES Oranges from the Lower Rio Grande Valley usually have thinner peel, less acid, and the peel is more yellow than fruit grown in areas with cooler nights. In terms of maturity, they generally are re- ferred to as early, midseason, and late. Early oranges that are most popular early-t0- midseason varieties in Texas are Marrs and Hamlin. Marrs, relatively unknown outside of Texas, passes legal maturity tests before other early oranges, primarily because of its low acidity. The Marrs orig- inated as a navel-orange budsport discovered by O. F. a-Regpectively, pathologist, USDA-CRD, Weslaco; physiologist, USDA-CRD, Weslaco; area horticulturist, Texas Agricultural Extension Service, Weslaco; associate horticulturist, Texas Agri- cultural Experiment Substation 15, Weslaco; senior plant patholo- gist, USDA-ARS, CRD, Orlando, Florida; research chemist, Fruit & Vegetable Products Laboratory, Weslaco, Southern Utili- zation Research and Development Division, Agricultural Research Service. USDA- 12 Marrs of Donna, Texas. Earlyl in the season, Marr oranges must be picked after the dew has dried p the peel, or rind oil spot will develop during storag in the packing shed. Later in the season when 1h peel matures, rind oil spot is no longer a probfe If The Marrs, which has high-solid and low-acid frui may be legally mature in September, but on the bas' of flavor, peel maturity, and color-break of rind an juice, it actually matures in November. The Mar ,_ sets fruit on young trees and consistently bears heav »; crops. It is considered seedless, but its seedin varies, probably depending on the pollinator, sinc seedless and seedy fruit occur on the same tree. Mar fruit attains larger sizes than Hamlin; it can shipped through January with little deterioration c5‘ fruit. A Hamlin matures in late October or early Novel ber. It requires less care in harvesting and packin than early-season Marrs; it has slightly more acid an less sugar than the Marrs, and its uice has less colo than the Marrs. Hamlin trees produce heavy yiel of relatively seedless fruit. Hamlin oranges tend be small sized and they dry out late in the season. Hamlin and Marrs uice will not make top-quali a concentrate consistently, because of their low acid ~; poor-color, but must be blended with Valencia jui 1 Early oranges generally are easier to raise th J late oranges and the fruit is harvested before great danger of freeze damage. However, early orang; frequently sell for low prices when markets are d‘ pressed by heavy shipments from other areas. A Some navel oranges have been raised in Tex for gift fruit and shipments for the Christmas tra The navel orange has a reputation of low yields b_ receives higher returns per box compared to oth varieties. Navel oranges generally reach legal maturity ' October and are usually shipped by Christmas. I Texas trees are mostly propagated from selectior g of the Washington navel. l Midseason oranges include several varieti Pineapple orange in Texas refers to any seedy mi season orange, including Parson Brown, Ruby, Me terranean sweet, the true Pineapple and other va ' ties, which reach legal maturity in November. A joppa and Iaffa varieties are both Shamouti gs and have been so intermingled that it is i’ le to separate them in Texas. Both mature ilember, and are relatively seedless. They tend a alternate bearers and carry lighter crops than or Hamlin. In some seasons, they are suscepti- black-core disease, which causes fruit break- i, in transit and is especially objectionable for ‘ a "‘ W plantings of varieties have decreased in re- rs. l‘te oranges are represented by the Valencia, lily late-season orange. It is probably the world’s iariety and is grown in every commercial orange in the world. alencia fruit is seedless and in Texas Valencia 1| attains legal maturity in February. 1t is i». w until May. Prices have usually been favor- Development of a Texas concentrate industry i require a high proportion of the high-acid, Nlid, good-color Valencia juice to blend with lor, low-acid juice from the popular early such as the Marrs and Hamlin. Since Va- "has a late harvest, the fruit is exposed to freeze a for longer periods than the early oranges. _'a trees yield less than early or midseason but this is generally offset by the higher price _ for the fruit. TANGERINES AND TANGELOS jangerines and tangelos (tangerine x grapefruit _‘)'have been grown mainly in small plantings i_ giving or gift-fruit sales. Trees of many u and tangelo varieties have more cold hardi- _ the Valencia orange. The fruits are easy and have deep orange color and rich flavor. lantings of tangerines and tangelos are recom- f= [to supplement orange and grapefruit plant- ientine (Algerian) tangerine matures in late _ and November. The trees bear heavy crops -flavored, small-to-medium sized fruit that i trather quickly after reaching maturity. The n; are self sterile and require cross pollination. ltine trees have exhibited a high degree of cold n; in Texas freezfesin 1949, 1951, and 1962. L cy tangerine matures in December or Janu- fruit is small to medium sized, is slightly id, does not dry out as quickly, and has a red- “re attractive rind than Clementine. Dancy trees were much less cold hardy than Clementine trees in 1951 and 1962 freezes. Orlando tangelo (Duncan grapefruit x Dancy tangerine) matures in December, has a slick orange rind, pleasing flavor, few to many seeds depending on pollen source, is low in acid, and is the size and shape of a large tangerine. The peel is tight. Since the blossom is frequently self sterile, plantings of Orlando should be interplanted with other seedy varieties which act as pollinators. Orlando trees have been planted extensively in Florida and have given good returns to growers. Orlandos were more cold hardy than most grapefruit or orange trees in Florida’s freeze in 1962. Minneola tangelo have been grown in small plant- ings for gift-fruit shipments. The fruit is shaped like a medium-large orange with stem end slightly raised. The rind is deep red-orange and the pulp is also a deep orange color with excellent flavor. The fruit matures from late-January through February. Temple “orange” is probably a tangor, a hybrid between tangerine and sweet orange. Small plantings occur throughout citrus districts. The fruit ripens in February and is attractive and richly flavored. It has a reputation for being cold-sensitive, primarily be- _ cause it makes flushes of new growth during the win- I81‘. LIMES, LEMONS, AND MISCELLANEOUS CITRUS Mexican lime produces several crops per year of small acid fruits. There are spiney strains and spine- less strains. Because they are extremely cold-sensitive, Mexican limes generally are raised only in small, non- commercial plantings. When grown as seedlings, they survive freezes even though frozen to the ground. Eustis limequat (kumquat x lime) has fruit simi- lar to Mexican lime but can withstand more cold than the lime. The Eustis limequat, however, cannot be propagated on sour orange rootstock; it can be grown on Cleopatra mandarin or calamondin (Austera tan- gerine x kumquat) rootstocks. Lisbon and Eureka lemons are_gr0wn occasionally. However, both are cold sensitive and are risky com- mercial ventures. Meyer lemon has been the commercial lemon of South Texas. It is somewhat less cold hardy than the grapefruit or sweet orange but is hardier than the Lisbon or Eureka lemon. Meyer lemon trees on their own roots, propagated from layers or cuttings, survive 13 freezes even though frozen to the ground. Fruit ripens in late summer and is thinner skinned, juicier, bigger, but a poorer shipper than California-type lemons. While some Valley Meyer lemon trees carry tristeza virus, most Valley trees are descendants of three trees which for unexplained reasons were tristeza-free. Ponderosa lemon, for lemons the size of footballs, is planted as a specialty fruit. Kumquats and satsumas are cold hardy varieties for small, noncommercial plantings. Kumquats are usually propagated on Cleopatra mandarin or cala- mondin rootstock because trifoliate rootstock, which is used in other areas, is not adaptable to the Valley soils and water. A continuing program of variety improvement is being conducted by the U. S. Department of Agricul- ture in cooperation with the Texas Agricultural Ex- periment Station. Virus-free budwood of almost every commercial variety has been found. New selections from breeding programs in California and Florida are under test, as are seedlings of varieties of commercial importance in other parts of the world. From these programs, varieties with improved tolerance to Texas hazards are expected. The present emphasis is on mandarin hybrids with increased cold hardiness. ROOTS T OCKS — THEIR TOLERANCES AND COMMERCIAL VALUE The success of a rootstock in the Valley is deter- mined by many factors such as tolerance to salt, boron, alkalinity, disease and cold, fruit production and quality, and general adaptability to Valley soil types. However, the three main limiting factors usually are salt, disease, and cold tolerance. Many rootstocks have been tested with Valencia orange and red grapefruit tops while only a few stocks have been tested with Jaffa orange, Marrs orange, and Meyer lemon tops. Tolerance to various diseases varies among root- stocks. Some are tolerant to tristeza and to most of the virus diseases known to infect the commercial strains of red grapefruit. These include Cleopatra and Ponkan mandarins; Kara and Kinnow tangors; San Jacinto, Webber, and Sampson tangelos; rough 14 lemon; Rusk and Savage citranges; Sacaton citrui melo, Citrumelo 4475; and several sweet oranges. Commercial citrus varieties which are virus-free may be compatible with many more rootstocks. f Most sweet orange rootstocks, although toleran to tristeza and several other viruses, are quite sensitiv to foot rot. 5 There also is a wide range in salt, boron, an alkaline tolerances among rootstocks. Rootstocks wi w. good salt tolerance are not necessarily tolerantft, boron or alkalinity. Stocks showing good to model“ ate tolerance to salt, boron, and alkalinity inclu Cleopatra mandarin, Mexican lime, Rangpur ma darin lime, rough lemon, Savage citrange, and sou orange. No known rootstocks have good tolerance t all three soil factors: salt, boron, and alkalinity. In mature groves, only Cleopatra mandarin hibited good tolerance to cold during the 1962 freez Many stocks have shown moderate cold toleranc These include several mandarins, tangors, tangelo, sweet and sour oranges, and grapefruit. Lim lemons, most trifoliate orange hybrids and trifolia, ‘ orange have poor cold tolerance. ‘ Two rootstocks, from all the stocks tested, ha been found to be most adaptable for Valley condition; These are sour orange and Cleopatra mandarin. Cl of patra mandarin, although more cold and salt toler 4- than sour orange, is less alkaline tolerant. Trees of sour orange generally have larger fruit of slightli better quality than on Cleo, come into bearing earlie and are easier to propagate. For these reasons, soil has been used more extensively than Cleo, although y is more susceptible to tristeza than the latter. i‘ Sweet orange and grapefruit as stocks are r recommended because of their poorer cold and a i line tolerance. Sweet orange is also susceptible to f0 rot. " Trifoliate orange and most of its hybrids ha: poor salt and alkaline tolerance. Most are susceptib to several of the viruses carried by commercial cit P’ varieties. ‘ The limes and lemons, while somewhat mo tolerant to salt and boron, are very sensitive to c0 and produce poorer fruit quality. 5 DISEASES AND INSECTS Nursery trees should be relatively free from insect The citrus nematode, Tylejnchulus semipenetrans, 'dely distributed in old groves in the lower Rio de Valley and may cause considerable root injury loss in tree vigor. It is highly desirable to plant tode-free trees on new land, or land that has 1 fumigated for parasitic citrus nematodes. To e that nursery trees are free of nematodes, nur- A stock should be grown on land that has not been jiously planted to citrus or that has been treated rly with a nematocide. oflNursery trees having areas of exposed wood on trunks, as a result of frost damage or mechanical ry, should be avoided. The original wound at the union line should be partially healed over. Pro- “ gumming or bleeding in the trunk should be ed with suspicion as a possible indication of a “sed condition. '_ Virus diseases _are not readily apparent in nursery I One virus disease, psorosis, has been a major f;- of decline in productive trees 1O years and older i A alley groves. It can be prevented by planting i: free of psorosis virus. Other virus diseases such xocortis and xyloporosis, present in certain old i’ citrus clones, can be avoided by planting trees have been certified as virus-free by, the Citrus 'sery Inspector of the Texas Department of Agri- >. re. ipectively, pathologist, Texas Agricultural Experiment Sub- '0n 15, Weslaco; and area hortieulturist, Texas Agricultural Vnsion Service, Weslaco. w‘; NURSERY TREES Bailey Sleeth and Morris Bailey* TREE SIZE AND FORM A sound, straight-trunked nursery tree 5%; to 1% inches in diameter just above the bud-union is pre- ferred for planting. A l/Z-inch tree is too small and generally will be retarded in growth and initial bear- ing. Trees larger than l inch lose much of their root system when dug and balled and may be‘slow in starting. Age is an important consideration in deter- mining tree quality. The desired tree size should be attained within 9 to 12 months after budding. Nursery trees are headed back in the nursery at an approximate height of 18 to 20 inches. Heading back is necessary in order to stimulate lateral growth which produces the framework branches. Fig. 5. Nursery stock in the Valley is sold us balled and burluped trees. 15 GROVE ESTABLISHMENT Morris Bailey and Norman Maxwell* TREE SPA CING Tree spacing is still an extremely important fac- tor in the establishment of a citrus grove although concepts about spacing have changed much since the Valley citrus industry began. Before the 194-9 freeze, most Valley groves were planted to a 25' x 30' spac- ing, allowing 58 trees per acre. After that freeze, the trend moved toward closer tree spacing, with the most common distances being 15' x 25' (116 trees per acre). Close spacing definitely gives higher yields per acre during the first few years of production, and experiments in California have shown that closely- spaced groves will outyield wider-spaced groves over a long period. In one California experiment, after 2O years, a closely-spaced grove (I2' x 22') outyielded a wider-spaced grove (245 x 22') by 62 percent. In this test yields per tree dropped as the trees became crowded, but yields per acre remained higher than in wider-spaced groves. With the costs of land, labor, equipment, and water increasing constantly, it is im- perative that unit costs be reduced. Increases in yield per acre can do that. Picking middles should be provided at regular intervals, however, in closely spaced groves. PLANTING AND INITIAL CARE In Texas, citrus trees are sold as balled trees. They will have the head cut back to correspond to the reduced root system. These trees may be planted immediately after digging or they can be held several days or longer by storing them in a sheltered place and keeping the balls wet. When the trees are set in the field, care should be exercised that the root systems do not become too dry. It is a good idea to dig the holes ahead of time and as the trees are dropped off at each place, set them into the hole to prevent drying. In planting the tree, the top of the ball should be about level or slightly above the soil around it, to allow settling after the initial irrigation. After filling most of the hole with soil and tamping it to prevent air pockets, the string around the top of the ball ‘Resrpectively, area horticulturist, Texas Agricultural Extension Service, Weslaco; and associate horticulturist, Texas Agricultural Experiment Station, Weslaco. 16 should be cut and the burlap follied back and covered with soil so that it will rot quickly. 1 Basins or strip borders should be built around the trees so they may be irrigated as soon as possible after; J setting. ~' s,’ Citrus trees may be planted successfully over a‘ long period of time in the Valley—October through May. The early planting of October through Decem- ber enables the trees to establish a root system and begin top growth that will make a large top during- the coming season. Early planting is highly successful‘ during mild winters, but in winters with one or more hard freezes, the succulent young trees often will b, damaged by the cold. An excellent time to plant is in late Decembe and January, because the balled trees are dormant 4 ‘ to the digging. Also the weather is cool enough tha top growth does not start until danger of freezes zf I past. All trees set in the fall and winter mont é should be banked with trash-free soil for protection against freezes. j Spring planting of trees, February through May‘ also is successful in the Valley. Trees planted in Ma often require more water the first season than planted during the fall, winter, and early spring, cause late planted trees generally cannot establish :1; strong root system before hot weather begins. Eve so, these plantings will do well and will start produ I ing fruit about the same time as the earlier plante trees. ‘ FER TILIZING AND WA TERING TREES For mature trees, it makes little differen whether the year’s allotment of fertilizer is one 0,‘ several applications. Fertilizer applications on youn trees definitely should be on a split basis, however since they can be damaged by large amounts and c I better utilize fertilizer when it is applied several tim Nitrogen is the only major element generally reco mended for Valley citrus trees, although in some cas a minor element deficiency may occur necessitatin corrective measures. Nitrogen applied late in the -€ may decrease cold hardiness. General fertilizer recommendations are: 1A3 poun of nitrogen per tree each year for 1-year old tre M; pound of actual nitrogen per tree each year for 2-year old trees, and 1X2 pound of actual nitrogen per tree each year for 3-year old trees. If the trees are basin-irrigated (tank watered), the fertilizer can be mixed into the water and applied at the time of irri- gation. If the trees are strip-irrigated, it may be more convenient t0 apply the fertilizer by hand. Ex- treme care should be exercised t0 avoid trunk burn and t0 assure even distribution of the material over the entire root zone area. Young citrus trees should not be allowed to wilt as a result of moisture stress. During the hot, dry summer months, it is necessary to water basin-irri- gated trees quite often (usually every 14» days). Strip- irrigated trees do not require quite so frequent water- ing since a larger area is watered during each irriga- tion. Strip-irrigated trees make faster growth than basin-irrigated trees, and, therefore, come into bear- ing earlier. PR UNIN G AND TRAINING TREES Young citrus trees do not require much pruning or training to form a good bearing tree. When the trees arrive from the nursery, the scaffold limbs have already been formed. Generally, the major work in pruning is to break sprouts off the trunk and remove limbs that are dead or rubbing one another. Occa- sionally, a water sprout or shoot will grow faster than Fig. 6. During hot Valley summers, basin-irrigated lcitrus trees usually require waterings every ‘l4 days. the rest of the tree. These either can be removed, if they are in a poor location, or cut back to correspond with the rest of the growth on the tree. Cutting back usually hardens and slows down growth so that it will make good fruiting wood. Sometimes, in the second or third year, limbs will grow down and touch the ground. These should be cut back to avoid damage during tillage. 17 CARE OF BEARING TREES Morris Bailey, Norman Maxwell, Bailey Sleeth, Herbert Dean, Cleveland Gerard and Morris Bloodworthl‘ FER TILIZA T ION Most Valley soils are quite fertile. The only major element generally required by Valley citrus trees is nitrogen, although in some groves, a minor element deficiency may exist in iron, zinc and man- ganese. Iron is by far the most commonly noted unavail- able minor element in Valley soils. When a minor element deficiency does occur, soil or foliar applica- tion of chelated materials should be made. In the early spring, trees growing on sour orange rootstock may show symptoms of iron deficiency in the first growth flush. This condition will usually clear up without treatment as the soil temperature in- creases, however. The rate of nitrogen to apply depends on tree size, age, and/ or yield. Growers who base fertilizer application on yield should add one-fifth pound actual nitrogen per 70-pound field box of fruit produced. If fertilizer application is based on tree age, the recom- mended rates are as follows: Tree Tolal pounds actual N Age applied yearlyi 4 3/4 5 l 6 l Vs 7 l l/s 3 l V2 9 l 1% ‘l0 8- over 2 ‘For sodded groves increase the fertilizer rates by 50%. It makes little difference whether nitrogen is ap- plied in one pre-bloom application 0r throughout the growing season. If two applications are made, the first should be made before the bloom period and the second either in May or August. If three applications are preferred, they should be made before bloom, dur- ing May, and finally in August. Nitrogen should be broadcast evenly over the entire root zone area and as in the case of young fltespectively, area horticulturist, Texas Agricultural Extension Service, Weslaco; associate horticulturist, Texas Agricultural Ex- periment Substation l5, Weslaoo; pathologist, Texas Agricultural Experiment Substation 15, Weslaco; associate entomologist, Texas Agricultural Experiment Substation 15, Weslaco; assolciate soil physicist, Texas Agricultural Experiment Substation 15, Weslaco; head of Department of Soil and Crop Sciences, Texas A&M University, College Station. 18 trees, care should be exercisedfb avoid trunk burn. To prevent loss of material through volatilization and to dissolve the fertilizer into the root zone, the fertili- If zation should be followed immediately by an irriga- . tion. In semi-clean groves, it is also wise to disk tlréff accumulated weeds and grasses before irrigating. A A MECHANICAL C UL TI VA TI ON Cultivation required in a citrus grove depends upon the type of soil management being used. Groves in a semi-clean management system, used i in most Valley groves, require disking and weed con- i trol from February through November. ' The first disking is done in early February so 5 that the grove can be prepared for irrigation and the‘ ensuing spring growth. From then on, a light disking will keep weeds under control if timing coincides with irrigation. Disk cutting depth should be no deeper than two or three inches to avoid damage to tree roots. Several types of offset equipment are available: for control of weeds and vines under the trees, but = there is a small area next to the trunk where weeds must be controlled by hand to avoid tree injury. I Where irrigation with sod culture is the soill management system, the weeds and grass can be kept- under control with a cotton stalk shredder and offset equipment. I No matter what the system of soil management used, weeds and grasses should not be allowed to growl into trees where they can cause damage to fruit and; young twigs. Besides, a heavy growth of weeds com-v petes for moisture during the hot summer months. A Cultivation of citrus after September may affec {l the cold hardiness of the trees. Observations afterf the 1951 and 1962 freezes indicated that disking in; late fall and winter in many groves caused the trees; to be less dormant, resulting in more cold injury than sustained by trees in groves that did not have late cultivation. l DISEASES FROM FUNGUS AND VIRUS I Cotton root rot or Phymatotrichum root rot is : A fungus that affects many species and varieties 0 plants. Under favorable conditions the fungus attac .- 'trus roots and frequently kills young trees. Young iitrus tree loss is highest in intercroppings with cot- i (n or alfalfa, two highly susceptible crops. Sour orange is resistant to Phymatotrichum root 0t and should be used as the rootstock for plantings ~ an areas not entirely free of the fungus. y FLYSPECK (regreening) is another minor fun- ’ s disease that affects citrus fruit. The small black ecks or spots, less than pinhead size, are closely oven hyphae, comparable to sclerotia. If numerous, J spots tend to give an unsightly appearance to ma- re fruit and lower its grade, making it mainly a ‘roblem to shippers of fancy grade fruit. For process- 1| it is of little or no importance. In Texas, flyspeck associated with dark green areas of mature grape- i uit. The contrast between the dark green areas and p. e attractive light yellow of fruit is pronounced. Vfvidence indicates that the flyspeck fungus in areas ,3 heavy infestation inhibits the green rind from hanging to a light yellow as the fruit matures. Lack of information on the biology of the fungus inders the development of control measures; how- er, summer fungicidal sprays have increased the V“ ount of clean fruit. GREASY SPOT, sometimes called greasy mela- pose, occurs only on the leaves. The spots, at first ves. The spots darken, become slightly thickened i - d greasy in appearance. The greasy spots vary in hape and size, from small dots to areas 0.3 inches in ‘ iameter to solid diseased areas affecting most of the surface. As the characteristic dark greasy spots jevelop, the leaves tend to become yellowish. Then ilants defoliate prematurely in late summer and fall. Greasy spot has been also associated with mite i jury as well as a fungus. Summer sprays with eutral copper (0.5 pounds metallic copper in 100 gal- Vons of water) or an oil emulsion spray have been ffective. More preferable is a summer spray of zineb in maneb at the rate of 1.5 to 2.0 pounds per 100 fallons water for then the copper injury (star mela- (prise) to the fruit can be avoided. _ MELANOSE occurs on all varieties of citrus; owever, grapefruit is somewhat more susceptible than ranges. The fungus attacks young fruit, leaves and igs, but is of economic importance in Texas only ause it lowers eye-appeal of fresh fruit. It is a wet ason disease-—a period of several days of high hu- idity is necessary for the fungus to sporulate and fect the young tender tissues. Mature or hardened i» are resistant to infection. ellowish brown, develop on one side of the older ' Melanose spots on the fruit are at first light brown, circular, and sunken; later they become dark brown to almost black with a wax-like appearance. The surface of an affected area has a rough and sand- papery feel. Tear-streak patterns are sometimes caused by spore-laden water flowing over the fruit surface during light showers or heavy dews. Solid, heavily- infected areas of roughened scar tissue may cover a large part of the fruit. “Mudcake melanose” develops when the areas of scar tissue crack into more or less irregular patterns. In severe cases, leaves may become twisted, lose their green color, and drop prematurely. Melanose may be controlled effectively with neu- tral copper (0.75 pounds actual copper per 100 gal- lons of spray) when applied after petal fall and before the fruit averages 0.5 inches in diameter. The period of effective application is short——10 to 14- days follow- ing petal fall. A single high-pressure spray treatment usually gives excellent control. If humidity is low, during the first 2 to 3 weeks following petal fall, no spraying is necessary. PHYTOPHTHORA COLLAR ROT (foot rot, brown rot gummosis) and seedling blight are caused by one or more species of Phytophthora. They attack the citrus tree both above and below ground and cause several types of injury to roots, trunk, branches, leaves, and fruit. Under favorable conditions, the fungi causing collar rot invade and kill the bark of the tree both above and below the bud union. The dis- eased tree is killed if the trunk is eventually girdled. Twig injury, leaf and blossom blight, and fruit decay may occur during or immediately after periods of rainy weather. Citrus varieties differ in susceptibility to Phy- tophthora infection. Lemons, limes, and oranges are highly susceptible; somewhat less susceptible are grapefruit, rough lemon, and mandarins. Samson tan- gelo and sour oranges are resistant. Phytophthora infection of the trunk base causes a profuse gumming on the surface of the bark lesion. The gum from infections below the soil line (collar or foot rot) is absorbed by the soil. Infections ex- tending above the ground produce typical masses of exuded gum. The gum hardens in long vertical ridges on the surface of the bark or runs down into the soil. The fungus-invaded bark is killed, remains firm, darkens in color, and in time becomes shrunken and cracked, shredding in strips as it dries. The bark that remains alive above the fungus lesions often develops callus rolls that check further spread, especially in an upward direction. Sometimes the disease appears to 19 be arrested, only to resume activity at a later date. Ultimately, the lesion may encircle the trunk and kill the tree, but the vertical spread is usually restricted to 1 to 2 feet above ground. Phythophthora frequently affects young nursery stock in the Lower Rio Grande Valley during rainy periods. Lesions on the stems and blighting of leaves may kill large numbers’ of seedlings. Sour orange seedlings, which are quite resistant, succumb to infec- tion in wet crowded nurseries. Rootstocks highly resistant to Phytophthora infec- tion should be used. Nursery seedlings should be budded fairly high, 5 to 6 inches, especially if the scion varieties are susceptible to Phytophthora. In planting trees in a grove, the bud union should be at least 5 inches above the ground line, or the tree should be planted to the same depth as grown in the nursery. Many infected trees may be saved by cutting out the trunk lesions to 0.2 inches beyond the discolored mar- gins. The wound should be scraped clean and painted immediately with a good fungicidal tree wound paint. It will help keep down basal trunk infections and tree losses to adopt grove practices to prevent water from standing around the base of the tree for any length of time and the removal of tree banks as soon as dan- ger of frost is past in the spring. Good air and soil drainage will help prevent Phytophthora infection in citrus nurseries. Neutral copper (0.75 pounds metallic copper to 100 gallons of water) or zineb or maneb (2 pounds of 65-70 percent material to 100 gallons of water) applied as a spray at weekly intervals should control the disease in the nursery. RIO GRANDE GUMMOSIS is a gum-exuding disease of grapefruit and shaddocks. It may be con- fused with other gum forming diseases as psorosis, exocortis, and Phytophthora root rot. Unprotected wounds are the main points of entry for the causal agent of Rio Grande Gummosis. Typical symptoms of the disease have been reproduced in Texas by in- oculating grapefruit trees with Diplodia natalensis. The evidence indicates that D. ndtalensis is related to the occurrence of the disease in Texas, but that there may be other contributing causes. In the early stages of the disease, gum exudes from a crack or blister in an area of darkened bark beneath which the cambium is discolored. Later, in affected trees, gum filled pockets may develop beneath the bark causing blister-like bumps at some distance from the point of infection. When the blisters break, Areas of buff discolored copious gumming occurs. 20 wood with salmon-orange margins exist below the area j of infected bark. The stained wood occurs in a band a often an inch below the surface. The band of stained , wood ranges from 0.2 to 0.5 inches in thickness, may ;j be several inches wide, and spreads upward or down- I ward two or more feet from the point of infection. In the 1930’s and 194-0’s, Rig Grande Cummosis . prevailed in many Valley grapefruit groves; 50 to 60 percent of the trees were diseased. It is not a prob- lem in uninjured trees growing on well drained soils, j Control measures consists of protection of wounds . against infection and good grove care practices. ' SOOTY MOLD is caused by fungi feeding on the i. honey-dew excreted by certain insects. In recent v years, sooty mold has been highly conspicuous in» Valley groves as the result of heavy infestations of L brown soft scale. I Sooty mold appears as a black velvety mem- r branous coating over the leaves and fruit. The * amount of sooty mold on the trees is roughly propor- I tional to the number of parasitic honey-dew-excreting insects present. The black film is superficial and no. \ parasitic relationship exists between the causal fungi ' and citrus tree. I The damage caused by sooty mold is indirect. l‘ Little or no effect on the tree may be noticed when j the amount of sooty mold is small, but when it occursr in abundance, it may seriously retard growth, cause; light blooming with reduced yield and increase sus- ' ceptibility to drought. The black sooty covering in- 2 terferes with photosynthesis and the formation of I starches and sugars. Fruits covered with sooty mold i‘ ripen late and color unevenly. Often they are small j in size and require washing at the packinghouse. Even " in processing plants, sooty mold-covered fruits add to“. the mold or contamination hazard of juice products. l Sooty mold can be limited by a program to con-j trol the honey-dew-excreting insects such as white flies, aphids, mealybugs, and certain scales—especiallyr the brown soft scale. A TWIG DIEBACK is common in Valley groves. j Several different fungi, as well as many other factors, can cause dying back of young branches. The affect-l ed twigs may be killed back from one to several inches, from the tips. Gum exudation occurs frequently at the margin of live and necrotic tissues. Damage byi twig dieback usually is not severe. Cutting out in-f fected twigs about 1 to 2 inches below the advancing margin of infection will help keep down injury. a Damage caused by virus diseases varies among scion-rootstock combinations from a slight slowing. 13, down in growth t0 loss in yield, stunting, decline, and eventual death. Three viruses—exocortis, xyloporo- sis, and tristeza—cause rootstock diseases while pso- rosis virus causes bark shelling 0n the trunk and l branches of trees 8 to 10 years and older. These _ four viruses are bud-transmissable, and certain aphids I can transmit the tristeza virus. Citrus virus diseases in Texas can be controlled by planting virus-free trees. Tolerant rootstock-scion Q combinations should be used if virus-free trees are not I obtainable. Nursery trees certified to be psorosis-free by the Nursery Inspector of the Texas Department of ; Agriculture are available at Valley citrus nurseries. EXOCORTIS virus causes bark-shelling and fl stunting of trees on trifoliate orange, trifoliate hy- Lbrids, and Rangpur lime rootstocks. 1 stages of the disease, gum exudes from pustules at the v base of the trunk which may extend from below the In the early soil line up to the bud union. New bark forms be- s‘ neath the pustules, while the outer bark sluffs off and § causes bark-shelling. The rate of tree decline varies __' with tolerance to the exocortis virus; some affected S trees may live for many years while others die within ‘A 2 or 3 years. Sour orange and Cleopatra mandarin rootstocks i are tolerant to exocortis. However, exocortis-infected y scions on tolerant rootstocks, even though rootstock scaling does not occur, grow slower than exocortis- . free trees on the same rootstock. PSOROISIS has been the most serious disease af- t fecting mature citrus trees in Texas. This disease has been spread chiefly by budding nursery stock with 1 buds from infected trees. Rootgrafting is responsible for some spread of the disease after a grove is planted I with both virus-free and virus-infected trees. Sweet orange, grapefruit, and tangerines are the more se- t, verely affected varieties. Psorosis virus strains have common leaf symp- toms, even though trunk and branch symptoms are wlifferent. The typical leaf patterns are (1) faint flecks or translucent areas between the veinlets and paralleling them and (2) chlorotic areas which re- semble an oak leaf. The leaf patterns can best be seen on young leaves during the spring flush by trans- mitted light. They are generally symmetric, the pat- tern being similar on each side of the midrib. The I flecking or chlorotic markings may be pronounced or ‘ obscure. They are not persistent and may disappear in a few days. Leaf symptoms show whether nursery stock or young grove trees are infected with the psorosis virus. This method of detection has been used effectively in selecting psorosis-free bud-wood parent trees. Bark scaling of trunks and larger branches is typical of psorosis symptoms in citrus trees 8 to 12 years old or older. The symptoms begin on the bark as scales of bark with or without gum formation. The scales of outer bark are dry, irregular flakes about 0.2 inches thick, with live, tan to buff-colored bark under- neath. As the disease advances, the deeper layers of bark, and even wood becomes affected. Within a few years gum and resinlike deposits occur in the wood, and the affected area becomes brown or reddish brown, the discoloration developing in an irregular fashion. As the disease progresses, the rate of decline rapidly increases. Unless the tree is removed, it may linger on for many years as a non-productive tree. TRISTEZA has caused tremendous losses in the citrus producing areas of South America, South Africa, Australia, California, Florida, and elsewhere. As yet, tristeza has caused no appreciable loss in Texas. Infected trees found in Texas have been traced to introduction or, as in the case of infected Meyer lemons, to propagation by the use of infected budwood. There is no evidence of aphid transmission of the virus in Texas. Susceptible-rootstock combinations are sweet orange, tangerine, grapefruit, temple, and tangelo on sour orange. Tolerant combinations include sweet orange and tangerine on rough lemon and sweet orange on Rangpur lime or Cleopatra mandarin. The symptoms of tristeza in a grove are not dis- tinctive in that they are similar to those resulting from root injury, such as retardation of growth, thinning of foliage, and twig dieback. The tristeza virus can be detected readily by leaf flecking or vein-clearing symptoms on Mexican lime seedlings if grafted with buds or tissue from citrus trees carrying the virus. Where tristeza is a major problem, tolerant root- stocks must be used. In Texas where the disease has been found with no evidence of insect transmission, no control measure is necessary, except planting virus- free trees and preventing the introduction of either virulent strains of tristeza or thelaphid vector or both. When a tristeza tolerant rootstock is found that ap- proaches sour orange in its adaptability to Texas con- ditions, it would be well for growers to consider its use as a hedge against a future outbreak of tristeza. XYLOPQROSIS virus affects many mandarin, mandarin hybrids, tangelo, and sweet lime scions and 21 rootstocks. Orlando tangelo is especially susceptible. Sweet orange, sour orange, lemon, and grapefruit are tolerant to the virus. The first external symptoms of xyloporosis ap- pear in a susceptible rootstock, such as Orlando tan- gelo, 2 to 4~ years after bud-infection as shallow elongate depressions 0.2 to 0.8 inches wide in the bark. The wood becomes channeled and pitted. The inner bark will have ridges, bumps, and peg-like struc- tures that fit into the wood depressions. The pits often are lined with a dark brown resinous substance. Cracks develop in the bark, tissues become necrotic, scaling may develop, growth is retarded, yield de- clines, and the tree may die in a few years. As for exocortis, effective control consists of planting new groves with xyloporosis-free trees. PHYSIOLOGICAL DISORDERS MESOPHYLL COLLAPSE is characterized by a sudden wilting and disorganization of the interior tis- sues of citrus leaves. One or more affected areas may develop on a leaf. The affected area is generally trans- lucent, may turn yellow, dry out, and become light gray or brown. If severe enough, defoliation occurs. Mesophyll collapse is induced by water stress caused by climatic and soil conditions that make it impossible for the tree to obtain sufficient water for all parts of the foliage. In the Valley, after several days of high, dry winds, extensive leaf damage often occurs followed by defoliation. Defoliation is great- est on the side of the tree exposed to the drying winds. Observations in Texas indicate that this condition is accentuated by heavy populations of the Texas citrus mite. Foliage loss from mesophyll collapse can best be minimized by good grove cultural practices. Mainte- nance of adequate soil moisture and windbreaks are helpful. Any sort of cultivation that cuts or destroys citrus roots if followed by high dry winds will increase the damage. BIND-OIL SPOT, Oleocellosis, has caused considerable loss to growers and shippers of Marrs orange when picked early in the season while the rind is still green. The spotting is caused by oil released from the oil glands. The spots vary in size, from less than 0.5 inches in diameter to large irregular areas involving a larger part of the surface of the orange. The spots are green in contrast to the yellow color of the normal rind after treatment with ethylene gas. High humidity is the underlying factor in suscepti- bility of Marrs orange to spotting. 22 Rind-oil spot can be prevented or greatly reduc fi by (l) picking fruit in afternoons of clear, sunn days; (2) deferring picking 2 or 3 days after a raif or an irrigation; (3) using fiberboard-lined fiel boxes or padded trailers; and (4) having pickers y cotton gloves. As a general rulp, fruit susceptible rind-oil spot injury should be picked only when th fruit surface is dry and handled carefully so as no” to puncture or rupture the oil glands. A CHLOROSIS is a general term applied to a c“ dition of citrus leaves in which, instead of the norm green, there is a yellow, light yellow, to almost whi color. There are several causes of chlorosis wi i differences and similarities in symptoms. Iron o lime-induced chlorosis and mottle leaf or zinc defi ciency are the two most common types of chloros' in Valley groves. i: IRON CHLOROSIS, a yellowing of the leaves ~- affected plants, usually occurs on trees growing in c ‘ careous soils. Citrus growing on sour orange rod stock is relatively free of iron chlorosis, since it f more tolerant to lime-induced chlorosis than Cleopat mandarin. Trees on Cleopatra mandarin rootstoc, growing in calcareous soils frequently develop chlor: ' sis that may persist indefinitely. 4" A chelated iron compound, sequestrene 138, “ effective in alkaline soils. In the nursery, it is a plied at the rate of 2 to 3 ounces per 100 sq. ft. Su gested dosages, on a trial basis, for grove trees wo ; be 1 ounce for 1 to 2 year-old trees to 0.5 to O. pounds for mature trees. ' MOTTLE-LE-AF (zinc deficiency) is not distrU: uted uniformly throughout a grove. The leaves shod, chlorotic areas, situated between the lateral veins 0 each side of the midrib. The part immediately neg to the large veins and midrib remains green, wll” '1 the chlorophyll is absent in the parts between. A results in irregular spotting or mottling. Mottle-leaf is controlled effectively through , use of neutral zinc (1 pound actual zinc per 100 g lons of water) in the post-bloom spray. Usua A mottle-leaf affected trees recover during the summ without treatment. Unless the mottle-leaf condition‘ pronounced and tends to persist throughout the gro ing season, the application of zinc in the spray probably only justifiable when it is combined wi another spraying operation. i NEMA TODES Nematodes are microscopic worms, 0.01 to ll. inches long; many are parasitic on the roots of plan The citrus nematode, Tylenchulus semipenetrans, * P,’ e . jury to leaves and green twigs. . of high relative humidity (75 to 95%) are favorable f t0 increasing populations. l_ to relative humidity are not so favorable. Apply been found in more than 50‘ percent of the older citrus groves examined in the Valley. Other parasitic nema- todes, dagger, X iphinema americanum, and stylet, T ylenchorhynchus spp. have been found and may cause damage to the roots. The burrowing nematode, Radopholus similis, has not been found on citrus in South Texas. However, it is a serious pest in Florida causing spreading decline. Good cultural practices, weed control, high fer- tility level and adequate moisture tend to offset dam- age from parasitic citrus nematodes. If an old grove is to be replanted within 2 or 3 years following re- moval, soil fumigation prior to replanting most likely will be beneficial. Some effective soil fumigants are ethylene dibromide, dichloropropane and dichloropro- pene mixture, and dibromochloropane when used as a preplant treatment. Use these fumigants at rates recommended by the manufacturer. For best results, treat the entire planting area. However, fumigation of the individual tree planting sites uses less chemical and should be effective in increasing growth of the young trees. In tree site treatment, an area approximately 9 by 9 feet should be treated. CITRUS MITES AND INSECTS For specific control recommendations, consult L-559, Texas Guide for Controlling Pests and Dis- eases on Citrus, available at county extension agents’ i offices. Mites THE CITRUS RUST MlTE, Phyllocoptruta 0lei- vora (Ashmead), is about 1/200 of an inch long, wedge-shaped and light yellow. Under optimum weather conditions, '7 to 1O days are required for de- velopment of a generation from egg to egg. Rust mites usually are more prevalent on the east side of the tree I “and lower surface of the leaf. Rust mite damage has ’ been associated with russeting of fruit which results in reduction in grade and size. They also cause in- Continuous periods Continuous periods of 2O control measures at post-bloom followed by applica- tions when needed. THE TEXAS CITRUS MITE, Eotetranychus ban- , ski (McQ), has long considered a pest of eco- _ nomic importance, although research information is I lacking on the degree of damage. These pests prefer \< the upper surface of the leaf and are sometimes re- ferred to as “spider mites.” Eggs are disk-like and usually are laid on the sides of the mid-rib and branching veins. Adult mites are 1/ 70‘ of an inch long and vary in color from a lemon yellow to a dark green with dark blotches on each side down the back. After heavy feeding by this mite, the leaf will have a grayed appearance. During most months greater numbers will be found on the leaves on the south side of the tree. During normal years, their populations are small in February and March and a great increase follows in the April-July period although development has been observed during every month of the year. Hot and dry conditions favor development while rains will decrease populations. FALSE SPIDER MITES, Brevipalpus australis, (Tucker) and B. phoenicis (Geijskes), are a potential problem if chemical control measures are not applied during the year. An association has been established with B. australis and the disease, leprosis. Leprosis was controlled in Florida by controlling this mite. These mites are small, flattened, reddish, and slow- moving. Their legs are whitish with two pair at the head end of the body and two pair slightly behind the middle. Population counts on leaves indicate an increase in June followed by greater numbers during the following months in untreated groves. Fruit and twigs are also attacked by this mite. Armored Scales Chaff scale, Parlatoria pergandii Comst., and California red scale, Aonidiella aurantii (Maskell) are the principal armored scales which have required chemical control in Texas. Long or Glover scale, Lepidosaphes gloverii (Pack.), purple scale, Lepido- saphes beckii (Newm.), and Florida red scale, Chry- somphalus aonidum (L.) have required chemical con- trol in only a few locations. These scales move for 2 to 5 days after hatching, attacking all parts of the Table 5. Characteristics for identification of the common armored scales on Texas citrus. Length or _ diameter of Scale covering 5cu|e scale covering, Shape COIN color inch Chaff 1/15 Circular Brownish Purple to f0 elongate gray Calif. red 1/13 Circular Appears Yellow red Flor. recl 1/13 Circular Reddish Yellow brown Glover’s 1/10-1/9 Long and Purplish White narrow brown to purple Purple 1/12-1/9 Oyster Purplish shell brown White 23 tree. When the scale settles, it remains in the same place through the balance of its life. Scales extract the plant juices causing defoliation, dying of small twigs, fruit drop, and failure of fruit to color. Com- plete coverage of the tree is necessary if chemical control is to be successful. These armored scales may be classified by the characteristics given in Table 5. (Unarmored Scales _ These scales do not have a separate armor cover- ing their bodies, but retain their legs and can move to other locations to feed. Chemical control generally has not been necessary, except for brown soft scale, Coccus hesperidum L. Beneficial insects have been important in maintaining economic control. The se- cretion of honey dew by these scales provides a growth media for black sooty mold fungus. In many in- stances, the fungus is noticed before the scales are found. BROWN SOFT SCALE has been the most impor- tant unarmored scale. Adults are brown to pale yel- low, mottled, and oval in shape and 1/3 to 1/6 inch long. This scale attacks leaves and twigs and occa- sionally may be found on fruit. Its reproduction po- tential is very great. During average years, increases in populations may be found during May, and follow- ing that time, depending upon weather conditions and the degree of parasitization and predation by benefi- cial insects. Tl-IE BARNACLE SCALE, Ceroplastes cirripecli- formis Comst., is found on rare occasions. The height of the adult is almost equal to its width. The six plates on the sides and one on the top distinguishes this dirty-white (mottled with brown) wax scale. This scale has been well controlled by beneficial insects. A PULVINARIA SCALE has been found on rare occasions, such as following freezes. The scale is greenish, about\3/32t inch long, but with the cottony egg sac (with 4- ridges) fully extended measures about 5/16 inch long and is 3/32 inch wide. Insects. related to scales secrete honey dew and black sooty mold fungus is indicative of their pres- ence. In a few instances, chemical control has been necessary. CUITONY-CUSI-IION SCALE, Icerya purchasi Maskell, has been the most important of these insects. They congregate along the midrib of the leaves and on twigs. The young are reddish to brown with yellow, waxy threads extending from the body. Adult females are recognized by the reddish plate in front of the white, fluted egg sac and measures overall about 1/2 24 A ground (such as the fire ant) may kill or disturi inch in length. The vedalia lady beetle is the bes controlling agent and usually is found with the scal in this area. THE CLOUDY-WINGED WHITEFLY, Diale rodes citrifolii (Morg.) and the citrus whitefly, D citri (Ashm.), occasionally invade citrus in this are The eggs, laid on the undersufface of the leaf, Q elongate and are attached to the leaf by a short sta - After the crawler settles, the nymph becomes immobil and attains a length of about 1/25 inch. The aduly are mealy-white and hold their wings roof-like o the body. Control is usually maintained by entomog nous fungi and beneficial insects. é THE CITRUS MEALYBUG, Pseudococcus Cir (Risso) , has been found in a few groves. Their are distinctly segmented with lateral filaments cover with a white wax and may reach 1/4 inch in len =f They collect around stems and where fruit touch on; another in shaded areas. Large infestations will ».' sult in fruit drop. ‘ Miscellaneous Insects . Numerous species of ants may be found in Vallel groves. They may tend insects for honey dew an are a nuisance to workers in the grove. Ants th‘ ‘ nest in the tree or those that enter the tree from beneficial insects. Ant control is a good grove pra tice but chemicals for control should not be appli to the tree, except when individual nests must l, treated. 4 THE SPIREA APHID, Aphis spiraecola Patc and the cotton or melon aphid (A. gossypii Glover i; are the most prevalent aphids found on citrus in th' area. The black citrus aphid, Toxoptera aura ' (Fonsc.) may be found on occasions and the cow aphid, A. medicaginis Koch has been found on a f young citrus trees. Aphids attack the young succf lent foliage on the undersurface and are unable - develop on mature leaves. Their feeding causes t7 leaves to curl and become distorted. Honey dew i secreted by aphids and acts as a media for black soo A mold fungus. In past years, only a few cases ha been found where chemical control was economic l, feasible. A FLATID PLANTHOPPER, Metcalfa pruino (Say), hatches during late March from eggs laid f» previous summer. Nymphs, with sucking mouth pa congregate around the fruit stems or undersurface leaves. The adult stage is reached by mid or late sf‘ and this stage may be found as late as Septem They have numerous host plants and prefer grapef to oranges. During some years, they are heavily par gtized and economic damage by this insect is ques- nable. i THE MEXICAN FRUIT FLY, Anastrepha ludens 3 ew), is sometimes a problem with late fruit. Jdults begin migrating from Mexico during late i ember or January after which quarantine regula- ins go into effect for certain interstate shipment of it. These flies cannot survive under Valley sum- weather conditions. Larvae cause breakdown of fruit either on the tree or after harvest. v THE ORANGE-DOG, Papilio cresphontes Cram- is seldom found to be numerous on more than two three trees in a grove. This caterpillar is grayish- own with lighter patches and attains a length of 2 inches. Two long horn-like reddish processes _'ch emit a substance with a disagreeable smell are if st out from behind the orange-dogs head when it I disturbed. Although the caterpillar feeds on the liage, the adult is a harmless giant swallowtail but- y. Hand-picking is the usual method for control. q PUSS CATERPILLAR LARVAE, Megalopyge .3 rcularis (J. E. Sm.) , are occasionally on citrus mid-May to late-July and mid-September to mid- vember. These caterpillars are tan or gray, have iomous setae among the soft hairs and may grow to inch in length. They damage citrus trees by feed- on the leaves. KATYDIDS usually do not attack more than a . _ trees in the grove. The most common of these s» p-backed grasshoppers lays its eggs (clam-shaped) ‘ _e a fringe around the edge of the leaf. Adults and npllS feed on the leaves. Eggs are usually heavily asitized. A DESERT DAMPWOOD TERMITE, Paraneo- g es simplicicornis (Banks), may be a problem Vere citrus is planted on recently-cleared brush land. Qt age results from severing the large lateral roots d/ or the tap root of young citrus. The termite will feed upward in the trunk causing the trees to ther and die. Affected trees have been noted more ring the winter months. i 9A STINK BUG, Loxa Florida Van Duzee, may pi on citrus fruit in September. As the area around feeding puncture begins to decay, fruit changes to yellow color and drops. This bug measures about 16 inch wide by 3/hinch long and is green with a dish tinge around tlie edge of the body. The side the body back of the head comes to a sharp point. SNOUT BEETLES sometimes feed on citrus 'age during the spring and summer months. The larvae feed on the roots of plants. The adult of one specie is about 3/8 inch long by 1A; inch wide, greenish-gray and has somewhat of a glow from greenish spots on the back. Most species are thought to have only one generation a year. WOOD BORERS often attack weakened citrus trees, particularly after freezes. Dead wood is attrac- tive to borers so they usually will not be found in healthy citrus trees. Tunnels in large branches are made by borers which remain in the tree for as long as 2 years while tunnels in the bark or just beneath are made by borers with a shorter life cycle. CICADAS sometimes lay eggs in twigs during July and August. The bark of the twigs may gum and the twigs subsequently die. WATER REQUIREMENTS OF CI TR US Efficient irrigation practices are dependent upon a sensible application of the physics of soil moisture and an understanding of the use of water by plants. The use of water by plants is an energy controlled process which is modified by climatic, plant and soil factors. Climatic Factors Solar energy from the sun is the main source of energy which causes water to be vaporized from soil or leaf surfaces. The amount of solar energy which arrives at the evaporating or transpiring surface de- pends upon factors such as locality, time of year, time of day, and color of evaporating or transpiring sur- faces. The water requirement for citrus production is high in the Rio Grande Valley. Rainfall often sup- plies an important part of the water requirements of citrus. The average annual rainfall at Weslaco for example is about 23 inches a year. However, the annual precipitation varies considerably from year to year. The highest recorded annual precipitation was 40.4 inches in 1941. The lowest recorded was 7.8 inches in 1956. Irrigation schedules should be planned to take advantage of the normally high rain- fall in May, June, and September. Climatic factors also influence the irrigation practices in the fall and winter months. Citrus trees generally should not be irrigated after November and prior to February because of the danger of inducing growth during a time when there are possibilities of severe freezes. plants can cause severe damage to trees. A freeze after flushing of citrus 25 Fig. 7. Some orchards are kept clean cultivated most of the year. This is one method of conserving water for use by the trees. Plant Factors Some of the plant factors which influence water requirements are plant spacing and type of manage- ment used in the grove. Close spacing of groves can cause an increase in water requirements of citrus trees, but this increase probably would be relatively small. A grower should consider his water supply when making decisions as to whether the grove will be clean- tilled or planted to cover crops or grass. The use of cover crops or grass in the grove will increase the moisture requirement. For example, research at Wes- laco indicates that Coastal bermudagrass required 3O to 35 percent more water than clean-tilled or straw- mulched plots. The use of water by plants is relatively high when the available moisture supply of the soil is high. Irri- gation and rainfall replenishes the available water supply and therefore generally increases the water use by plants. The availability of water at increasing soil depth generally decreases because root development of plants including citrus decreases with soil depth. Size and vigor determine the root development as well as the amount of water available ‘for plant use. Over- irrigation or heavy rainfall may reduce water use by causing root rot and an anaerobic condition which are unfavorable for plant growth and development. Chemical and physical properties indirectly in- fluence water requirement by affecting root develop- ment and plant growth. Fine-textured soils are not recommended generally for citrus production because they hold more water per foot of soil and may impede root development due to its density and possibly se- 2B Irrigation Schedule vember. This schedule will supply water at the firs vere cracking. Hardpan, dense layers, or high water: table conditions in coarse-textured soils are unfavor- able for root development and plant growth, thus re- ducing the soil moisture supply available for plant‘, use. a Soil salinity may increase "water use by plants by! making it necessary for growers fto irrigate more fre-f quently. Soils containing relatively high salt contents are not recommended for citrus production, because‘ citrus trees are extremely susceptible to salt. g Since rainfall amounts and frequencies are less and evapotranspiration greater in the western end of‘ the Valley, the interval between irrigation possibly? should be closer to 2O to 3O days for citrus growing, in that area. Irrigations are not recommended in; December and January. About 5 to '7 irrigations are, needed to properly irrigate citrus during an averag; year. This is assuming that rains in May and June will supply enough water for one irrigation and rainsi in late August and September will supply enougf water for one irrigation. A minimum of three irrigations is estimated being necessary to keep citrus trees alive. These irri gations should be applied in February, July, and N0, sign of flushing in the spring, water in July durin the peak moisture demand period, and water in N" vember to prevent excessive defoliations as a resul of mesophyll collapse during the fall and winte months. Each irrigation should supply enough water t bring the soil to field capacity to a depth of 5 to feet. Occasional over-irrigation to leach accumulat salt is desirable. PRUNING p, Bearing citrus trees in the Valley require littl pruning. Generally all that is necessary is remov of dead limbs, headback or remove water sprouts, an trim low hanging limbs to a height so that tracto equipment will not injure the tree. ‘l Close-planted groves may require hedging aftel they reach l5 to 2-0 years of age. Cutting back t‘ tops of old trees will open up the center so that lig ' penetrates and starts new wood growth and fruit pr duction. i‘ Where pruning is necessary, all cuts should _ ' made flush with a limb or shoot and no stubs allow t0 remain in the tree for wood rots and insects enter. The tools used should be sharp and in good condition. All cuts of 1/2 inch or larger should be covered with a good pruning compound to prevent drying, gand entrance of disease and insects. i Do not prune heavy in the middle of the summer because of sunburn damage to wood that has not been . fexposed to the direct rays of the sun. Pruning should inot take place late in the fall because it will start the itrees growing and make them cold tender. Probably ithe best time of the year to prune is early in the spring or during September and early Uctober, CHEMICAL WEED CONTROL g The use of chemicals for the control of weeds and igrasses in citrus groves has become important in Cali- "Tifgfornia and Arizona during recent years. Experiments iwith various herbicides are underway in Texas, and while short-term results appear promising, not enough time has elapsed to determine comparative costs and long term residual effects on soils and trees. Fig. 8. Pruning wounds larger than 1/2 inch in diameter should be protected with a weatherproof antiseptic paint of the asphaltom-carbolineum type. As research information on chemical Weed con- trol in Valley citrus groves becomes available, specific recommendations will be released through appropriate channels. 27 AGRICULTURAL METEOROLOGY FOR CITRUS PRODUCTION Donald l. H addoclf‘ MINIMUM TEMPERATURE FORECAST PROGRAM A minimum temperature forecast for fruit and vegetable interests for various locations throughout the Valley is prepared during the critical season by the Weather Bureau, and is issued daily at regular intervals. i Agricultural and public forecasts are also issued several times daily throughout the year. These fore- casts contain information on maximum temperatures, cloud cover, wind direction and speed, rainfall cover- age and amounts, dew intensity, and minimum tem- peratures for the next 36 hours, plus a general outlook for an additional 24 hours. All weather forecasts and summaries are trans- mitted over the Weather Bureau teletype circuit. Tele- vision and most radio stations throughout the Valley are connected to this agricultural-meteorology circuit and thus can broadcast complete and up-to-date weath- er information. AGRICULTURAL INTERPRETATION OF WEATHER FORECASTS Farm weather summaries point out the effect that the predicted weather will have upon current cit- rus operations. The Weather Bureau Agricultural Service Office, the Texas Agricultural Extension Serv- ice, and the Texas Agricultural Experiment Station at Weslaco jointly prepare these farm advisories based upon the 36-hour specific weather forecast, and the more general 5- and 30-day weather outlooks. These summaries are issued on the agricultural-meteorology circuit, Monday through Friday, year around. THERMOMET ER CALIBRATION AND EXPOSURE An accurate thermometer and a good instrument shelter are two valuable aids in cold protection of citrus. A small, economical and easily constructed shelter is needed by each citrus grower in the Valley as part of his standard grove equipment. By knowing the representative air temperature near his trees, a ‘Advisory agricultural meteorologist, Weather Bureau Agricultural Service Office, iVeslaco, Texas. 28 ' from direct exposure to the sky. shelter should be installed adjacent to the grove o‘ grower can properly decide when to begin or termi- nate cold protection operations. A high-quality thermometer is worth the extra. cost, when used in protecting high value crops siiclr as citrus. A temperature difference of only one or; two degrees within a critical temperature range for citrus may determine whether freeze losses will be great or small. l ’ Before the cold season begins, thermometers“ should be calibrated for accuracy. This is necessary since damage to the thermometers may occur during. summer storage or in field use. An accurate thermometer must be expo properly if it is to correctly indicate the representativ air temperature in the immediate vicinity. The popuj lar mercury or alcohol-in-glass thermometer will mea _ ure the desired air temperature only when the ther mometer has a free circulation of air and is protect An instrument shelter made of wood with loud vered walls, double roof with an air space, and sma holes in the bottom will meet the foregoing qualifica: tions. It should be painted white and be large enoug that the thermometer, especially the bulb, will be =3 least 3 to 4 inches from the inside walls, and 6 to 2i inches or more from the ceiling and bottom. -. between tree rows, firmly anchored so the wind w' not shake the shelter, and at a height so the sensin element of the thermometer is 5 feet above the groun * Forecast and observed temperatures for various locj tions throughout the Valley are at the standard 5-fo’ height. A grower can determine the general temper ture difference between his location and the near ~ forecast station, and thus obtain a forecast for 11' own grove. INTER VAL BETWEEN FREEZES Severe freezes occurred in the Lower Rio Gran - Valley in 1930, 1949, 1951, and 1962. These perio . of unusually low temperatures caused freeze dama to the citrus trees, as well as the fruit. Low temper tures observed throughout the Valley during the la a three severe freezes are summarized in Table 6. 6- iXtreme minimum temPw-‘Itvres ebserved during ing t0 the 63 percent level. For a given temperature, three severe free!” there is a 5O percent chance (10070 —— 50% : 50%) Jun. 2941 Jan. 29- Jan. 9J2 of occurrence, within the indicated interval as well as 1949 Fig? 1961 a. 55) percent chance of non-occurrence within this pe- rio . g‘; 19 Values listed at the 63 percent probability level 19 20 of occurrence are the average return periods (number g5 12; 16 of years) because of the peculiarity of the extreme 13 value distribution. This particular level indicates iii“ Tzfczdens l? A $2 l4 that there is a 63 percent chance that a given tem- 7 14 perature will occur within the listed interval. It also 6 NE 21 a l4 means that there is only a 37 percent chance (10070 —— .. g0 19 63%:37%) that this same temperature will not _ nos 2t 20 13 occur within the interval. 22 l9 17 ,l l8 l6 1° For example, at Weslaco there is a 50 percent t, i 7 s 2° 19 chance that a temperature of 20 degrees or lower will ZONN 2o l3 occur before 10 years. This also means that there is *1 A“, 2o 2o 12 a 50 percent chance (100 7v — 50 "/0 I 50%) that this 1 21 i9 same temperature threshold will not occur within 10 ' Texas 2t 19 t4 - ville 2o ‘I 9 i 4 Years- diasciiw l7 :3 lo There is a 63 percent chance that this same tem- §97NE 15 perature will occur within 15 years and only a 37 ‘L: l; a percent chance (100% —63% 237%) that it will 1r Farms 12 not occur before 15 years. on it‘; l; 16 The extreme minimum temperature of 20 degrees V. . 0r lower was selected because this particular tempera- IOWeTS eall use the probability data 0f eXtfeme _ ture threshold summarizes those that occurred at Wes- tum temperatures ill Table 7, a5 a useful t00t in laco in the severe freezes of 194-9, 1951, and 1962, A _' g upon the establishment or rehabilitation of which were 20, 19, and 16 degrees, respectively. _ groves. This table indicates the number of years ,, between freezes of different severity for the TEMPERA TURE-DURA TION j» 63 percent probability levels of occurrence for RELATIONSHIPS IN SEVERE FREEZES Valley stations‘ Duration of freezing temperatures is one of the A e number of years listed at the 50 percent level many factors related to freeze damage of citrus trees t‘ more useful to growers than those correspond- and fruit. The duration in hours of some tempera- ‘liable 7. Interval (number of years) between freezes of different severity for two probability levels of occurrence. Elie Grande Probability Winter extreme minimum temperature (°F.) equal to or lower than stations level 2s 26 24 22 2i 2o i9 ‘I8 i6 h} N GI O i ille* so% i 2 a s 7 i2 is i9 24 32 i’ 63% 2 a 4 1 ii 17 22 2a as 46 ‘tni 50% i i 2 a 4 7 9 ii i4 ‘t7 2a 63% i 2 a 4 6 io i2 i6 2o 2s 4o so% i i 2 2 4 6 a io i4 t8 so , 63% i 2 2 a s 9 ii is 2o 26 44 ville so % i i i 2 a 6 a i 2 ' is 22 a9 j. 63% i i 2 a s 9 i2 t7 22 a2 s6 A a. City* .,. .so% i 1 t 2 a 4 s 1 a ii 17 ‘ .5 63% i i 2 a 4 6 a io i2 is 2s jValues listed at the 63% probability level of occurrence are the average return periods (number of years) because of the peculiarity of the extreme value distribution. Temperatures are for the five-foot height. Temperature data used in this study ‘were for the 30-year period, November through March, 1933-34 through 1962-63; except for Raymondville, which was for the iWZ-t-year period, 1939-40 through 1962-63. dlity data were obtained from Webb (i963). 29 Table 8. Temperature-duration relationship for severe freezes (1949, 1951, and 1962) in the Lower Rio Grcmde Valley of Texas. Temperature (° F.) [izzaflsii Extreme minimum +0 l 0.9 Extreme minimum +1 2.5 Extreme minimum +2 4.1 Extreme minimum +3 5.7 Extreme minimum +4 7.3 Extreme minimum +5 8.9 Extreme minimum +6 10.5 Data in this table were obtained by a linear regression analysis of 366 duration observations of freezing temperatures ranging from the extreme minimum temperature to 6 degrees higher. Temperature records used were from 15 climatological stations in the Valley in the 1949 severe freeze, 2O in 1951, and 18 in 1962. The correlation coefficient is +.78. The regression equation is y : .9 + 1.6x where y is the duration in hours and x is the difference between the observed and the extreme minimum temperature. 30 tures in reference to the extreme minimum for the severe freezes of 194-9, 195-1, and 1962 is shown inf Table 8. The relationship found in these three freezes is only a general guide in estimating duration of thl critical temperatures in future freezes because eaclf freeze and related cold air mass‘, differs greatly front another. But even a rough estimate will be valuabl when growers are deciding on how many hours of col protection equipment will be needed during the night For example, suppose that the forecast minim temperature for a given night is 24~ degrees, andf - grower plans to light his heaters to maintain a grov temperature of 28 degrees. Thus, he would light th t. when the temperature had lowered to 4 degrees highe j than the expected minimum (280 — 24°:4~°). B using Table 8, a duration of 7.3 hours was found fof a temperature of the minimum plus 4 degrees. As rough estimate, this means that sufficient fuel oil i; labor will be needed for approximately 7 to 8 hour in protecting the citrus during the night. l tow‘ , only in leaves and fruit. observed in leaves and green wood. COLD PROTECTION Roger Young, Price Hobgood, Norman Maxwell and Don Hadd0ck* PHYSIOLOGICAL ASPECTS OF FREEZE INJURY TO CITRUS Curiously enough, after every severe freeze in a citrus growing area, several trees 0r groves of trees survive the freeze much better than most in the same area. There are logical explanations for the behavior of these more hardy trees in some cases, but in many cases there is no logical explanation. The lack of an obvious answer for this added hardiness indicates 1) the need for a better understanding of the physi- ology of citrus cold hardiness, and (2) with a better understanding the solution of the freeze problem becomes more definite. lt is reasonable to expect that the freeze problem in citrus can be solved since trees have been noted to i survive very severe freezes with little injury. _ Plant Responses During Freezes The actual process of freezing in citrus tree parts is not clearly understood. But it is recognized that as water turns to ice within the tissues, heat is liberated and the tissue temperature rises to its freezing point. ln Figure 1, the rise in temperature of a grapefruit leaf indicated that freezing had begun. At that time “water-soaking” became apparent in the leaf. The “water-soaking” was manifested as darker green areas 0n the tops and bottoms of the leaves. As the leaf temperature increased, the “water-soaking” became more general. The true freezing temperature occurred at 25.1 degrees F., at which point the leaf was com- pletely “water-soaked.” This freezing point repre- -sented an increase in the leaf temperature of 3.1 de- grees F. from the minimum “under-cooling” tempera- ture of 22.0 degrees F. The temperature of the leaf subsequently decreased, indicating that most of the solute within the leaf was frozen and that no further heat of crystallization was being released. This rise in tissue temperature as freezing progresses is char- actéristic in all tissues but in citrus has been recorded “Water-soaking” has been Figure 9 summarizes the temperature changes in a leaf on a plant which did not freeze. No tempera- ‘Respectively, physiologist, USDA-CRD, Weslaco; head, Department of Agricultural Engineering, Texas A&M University, College Station; associate horticulturist. Texas Agricultural Experiment Substation 15, Weslaco; advisory agricultural meteorologist, Weather Bureau Agricultural Service Office, Weslaco. ture increase was noted, but rather a gradual decrease occurred. Leaves on this plant did not “water-soak” and remained uninjured. This plant exhibited what is commonly referred to as “supercooling.” Super- cooling is the process where tissues cool below their freezing point without freezing. Citrus tissues super- cool before freezing. The amount and length of super- cooling include rate of cooling, wind velocity, air tem- perature, and humidity and tissue condition. The fact that citrus tissues do supercool often is a factor in the amount of freeze injury incurred during a freeze. Tissues which remain supercooled longer before freezing generally sustain less injury since the time in the frozen state will be reduced. Freeze injury in tissues occurs subsequent to ice formation. However, injury may result not only from ice formation, but may be related to the time the tissue is in the frozen state, the severity of the freezing temperature, and thawing conditions. ln many freez- es, leaves often recover after being completely “water- soaked” or frozen, which suggests that the formation of ice is not the sole cause of freeze injury. ~r as 32" 3| so 29 2a- 21 2s 25 ~ 24- 2a- 22 2| -~ 2o- ‘9 l l l l l I l l ‘l o 2o 4o so a0‘ ! '\ (A) FROZEN LEAF -_‘ SUPERCOOLED LEAF l l l 100 MIN. Fig. 9. Temperature changes of leaves during freezing and supercooling. 31 7(1_ LEAF TEMPERATURE 5O I. _ ----------------- -- zxroazo T0 sxv <<<<<< -- manor: CAIIOPV ' R a LEAVES WATER SOAKED °F TWIG 8 TRUNK TEMPERATURE 7O 6O 50 i} U TVIIBS WATER SOAKED TVIIGS SPLIT 40 °F FRUIT TEMPERATURE a icz cnvsnrs b uczo rum c FROIIN soun Nil QFI IT. ILII OPII IT. IAN ‘PM MT. Ill! Figure ‘IO. Temeruiures of cir (7-foot level), leaf, twig, trunk and fruit tissue recorded during the ‘I962 freeze in a 30-year-old Valenica orange tree in a grove 2 miles northwest of Monte Alto, Texas. Tree tissue responses. during a freeze (1962) are shown in Figure 10. Leaves 0n the outer surface of the canopy of the tree were warmed 6 degrees F. above air temperature by the sun’s radiation during the day and cooled O to 4 degrees below air tempera- ture by radiation to the clear sky during the night. The first two nights, where skies were overcast and the Winds strong, exposed leaves did not cool below air temperature. The last night, where skies were clear and winds slight, leaves cooled 3 to 4 degrees below air temperature. Leaves inside the tree canopy were at air temperature or 1 to 2 degrees warmer than air during the entire freeze period. Twig tem- peratures followed air temperatures during the entire freeze. Fruit and trunk temperatures were warmer than air temperatures during the freeze. Because of the size of these tissues, considerable heat was present 32 jured fruit may either show injury several mon If and more time was required to remove it. Fruit an‘ trunk temperatures never reached air temperature but critical temperatures were reached in the fruit. .; Temperature changes in the various tissues dur‘ ing the 1962 freeze were typical for both a blowing type freeze and a radiation-typejireeze. i Freeze Injury Freeze injury in tissues manifests itself in man ways. In fruit, injury may appear as dry segme - and separated cell walls (Figure 12). In some if cell breakdown and crystal and gum deposits beco if apparent. In sweet oranges, hesperidin crystals ‘i appear in the segment membranes several days aft, freezing. Hesperidin crystals are good indicators u freeze injury. Injury is generally greater in the ste end of the fruit, and smaller fruit sustain more inju I than larger fruit. If fruit injury is severe enoug quality will be affected. Decreases in acid and jui‘ usually occur in frozen fruit, and often severely frozi fruit will show a decrease in sugar content. 1:. severity of freeze injury and the climatic conditio following the freeze generally determine the rate v change in fruit quality. Severely frozen fruit, as ' the 1962 freeze, may show large changes in fruit qu A ity within 2 to 5 days after the freeze. Slightly l‘ after the freeze, or may show hesperidin crystals .11 off-flavors immediately after the freeze and later r: cover good flavor. Cool weather generally slows t, breakdown of frozen fruit whereas warm weath enhances it. Severely frozen fruit usually fall fr the tree 7 to 14- days after freezing; some cases whe the fruit stems are killed, the fruit may remain on g; tree for longer periods. Following the severe ‘i: freeze in Texas, fruit drop was heavy within 7 (la: after freezing. Freeze injury to leaves may include all or o I part of the leaf. Generally, leaves killed by freez", curl, dry, and abscise within a week. Partially inju H leaves may remain on the tree, while those which :- damaged more than 5O percent usually drop. Leav killed by freezing need not turn brown after dryi u Often they remain light_ green even though (let Where weather conditions are cool and wet followi the freeze, dead leaves may uncurl and appear no for several days. _As with fruit, weather conditi ' often control the rate of leaf drying and abscissij In severe freezes where terminal wood is killed, lea _ may. remain on the tree for several weeks. Howev. dead leaves remaining on a tree for 2 weeks or lon indicate only that the terminals subtending the lea killed and does not indicate the severity of in- ury t0 larger wood. I, F reeze-inj ury to terminal wood, 1/16 t0 1/ 4 inch jameter, is usually manifested by browning and dry- I g of the injured areas. When injury is severe, white- i] ckling may be noticeable in the green wood several 'urs after thawing. As injury symptoms develop, w the entire twig becomes grayish-white, followed browning in later stages. Injury to terminals may ,0 involve splitting of the bark without killing the ; Tues. F reeze-injury symptoms on terminals usually fully developed 1O to 14 days after freezing, iirhough cool weather may prolong full development injury symptoms. Injury to wood larger than M; inch is difficult _l assess several days after a freeze. Three weeks to eral months may be necessary before full injury is ilized. Weather conditions and severity of injury ermine the time required for complete injury de- "lopment. Wood need not have split to be killed, it splitting often occurs. Bark splitting usually will i?» ur in weak areas of the limbs, particularly in the ditches. Splitting may occur either when the tissues e frozen or after the tissues have thawed and begun 'ng. Tissues split while frozen because of irregu- expansion or contraction during freezing. These 'ts may heal. Splits which occur as the tissues dry ause of tissue contraction usually will not heal, ough callus formation underneath the dead bark cover the area wliere the split occurred. Freeze 'ury on large limbs and trunks may appear as dead as or “freeze cankers,” ranging in size from an i. h in diameter to an area several feet long which 4' encompass half or more of the circumference of Fig. 11. Freeze lesions (arrows) on large limbs and trunk of a Red Blush grapefruit tree. a limb or trunk (Figure 11). These dead areas or “freeze cankers” may be large enough to render a limb or even a tree worthless. Bark examination on large wood following a freeze may indicate the severity of freeze injury to the wood. Visual examination several days after a freeze usually does not reveal freeze injury, and one should wait a week or more before attempting bark examina- tions. Even then it is difficult to definitely detect freeze injury. F reeze-injured bark usually is dry and takes on a blotchy, olive green color in contrast to the normally straw colored bark. Dryness alone, Fig. l2. Freeze injury to large and small grapefruit (top to bottom) in stem end, middle, and stylar end of fruit. 33 however, does not indicate freeze injury since normal bark may at certain times be dry. The wood proper may also be off-colored following injury by freezing. ‘Critical Tissue Temperatures The critical temperatures of various tissues deter- mines to a large extent the injury which may result from exposure to freezing temperatures. Tissue age, type, condition, and other factors, such as climate and nutrition, influence the freezing point of citrus tissues. For this reason, specifying certain critical temperatures for various tissues is difficult since con- ditions vary considerably. From experience, one can suggest certain temperature ranges for various Va- lencia orange, Red grapefruit, and Satsuma mandarin tissues to which exposure may cause injury (Table 9). These temperature ranges are suggested for nongrow- ing, healthy bearing trees during mid-winter. Trees weakened by various factors or in a growing condi- tion may be injured by warmer temperatures. On winter hardened grapefruit or orange trees during a normal winter, wood l/i to 2 inches usually is killed by minimum temperatures ranging between 22 and 16 degrees F. Red grapefruit and Valencia orange tree limbs 2 inches in diameter have been killed by minimum temperatures of 18 degrees F. Some Red grapefruit and Valencia orange trees had little wood larger than 1/2 inch injured from exposure to 12 degrees F. in 1962. Mandarins have a several de- gree colder temperature range for the same size wood while lemons and limes have a several degrees warmer temperature range. Many mandarins had no injury to wood 1/16 to 1/1 inch following exposure to 1O to 12 degrees F. in 1962. The severity and duration of temperatures often dictates the eventual tree tissue types which will be injured. The type of freeze also affects the amount of freeze injury to citrus trees. Blowing freezes tend to Table 9. Suggested critical temperature ranges for various tissues of oranges, grapefruit, and tangerines grown in the Rio Grande Valley based on observations during natural and artificial freezes. Tissue Valencia iked Suisumu grapefruit Mandarin Bloom 29°-30° 29°-30° 29°-30° Small green fruit 28°-29° 28°-30° 29°-30° Half ripe fruit 27°-28° 26°-28° 29°-30° Ripe fruit 25°-27° 24°-26° 28°-30° Tender leaves 26°-28° 26°-28° 25°-27° Mature leaves‘ 23°-25° 23°-25° 20°-23° Tender twigs 23°-25° 23°-25° 22°-24° Mature twigs‘ 2i °-23° 2'l°-23° I6°-19° ‘Mature leaves and twips assume also winter hardened tissues. Mature leaves and twigs unhardenei would be injured at warmer temperatures. 34 remove much of the stored heat and moisture in the A tree and ground. When enough heat is removed, even - large wood may be cooled to the critical range and 1 injury may result. This occurred in both the 1951i Severe desiccation or l. water loss also may be adverse in that more injury t may result. In the 1962 Florida freeze, trees in the direct path of the wind were injured more severely l than protected trees although the exposure tempera- and 1962 freezes in Texas. tures were similar. In calm freezes, loss of heat and moisture from 1 the tree and soil is not as rapid. Consequently, wood and fruit may not cool to as low a temperature and _. injury may be less or may not occur. The tempera- 1' ture of leaves and twigs, regardless of the type of ‘ freeze, usually remains similar to air temperatures , since they have very little stored heat. The occurrence of freeze injury to citrus in the I simplest terms is influenced by a relationship between 1 (1) the critical tissue temperature, (2) the severity ‘j ~ and duration of freeze temperatures, (3) the amount of stored heat, and (4) the presence or absence of wind during the freeze. Dormancy and Cold Hardiness Growth of citrus is characterized by intermit- , tent growth interrupted by periods of no visible out- j ward activity. These periods of growth and nongrowth j often occur during more or less constant environ-5 mental conditions. In the Valley, usually four growth I flushes occur followed by periods of growth interf ruptions. These flushes occur in early spring, early s summer, early fall, and late fall. When winter night 1 temperatures fall below 55 and 5O degrees F ., mini- mum temperatures for growth of oranges and grape- fruit respectively, no flush of shoot growth occurs, a and all buds remain dormant. If, however, night y temperatures warmer than 55 or 5O degrees occur during the winter or early spring for a period of 5 ' to 10 days, bud growth may begin. Cambial activity ,5 in twigs and wood and root growth is almost continu- ' ous the year around, although in the winter, twig - cambial activity usually ceases and wood cambial ac- l tivity and root growth lessen. In periods of nonbud growth during the growing season, buds are dormant. This dormancy is not climate-induced but results from certain physiological f] conditions, probably of hormonal nature, within the tree. This type of bud dormancy, more properly re- ferred to as bud inhibition, often occurs during pe? riods where environmental conditions are favorable? for growth. In the winter, bud dormancy is tempera- ture-induced, and the tree tissues that become dormant depend on the microclimate of the tree during the told period. Closely correlated with tree dormancy is tree cold hardiness. The more dormant a tree is the more cold hardy it is. Actively growing trees are very cold sensitive; thus, conditions during the winter which induce growth will reduce the tree’s cold hardiness as a was clearly pointed out in the 1951 freeze in Texas. Buds on trees which were defoliated by a frost which occurred in December 1950, were actively growing at the time of the freeze in January. Injury to those trees was severe and many large trees were killed. Trees not defoliated in December 1950 were not as severely injured in the freeze. Climate has been shown to influence greatly the cold hardiness of citrus. In the Valley during the winter, ambient air temperatures at night usually do not fall much below 5O degrees F. for extended pe- riods. Under these conditions, dormancy may be imposed on buds and twig cambium, but not on large wood cambium or roots. Citrus trees in the Valley usually acquire about 3 to 4~ degrees cold hardiness during the winter. On the other hand, in California where ambient air temperatures at night fall below 4-0 degrees every night, roots, buds, and cambium of the entire tree may become dormant. Trees in Cali- fornia during the winter may acquire up to 10 degrees cold hardiness. Cool temperatures during the winter are the major climatic factor responsible for the induction of cold hardiness in citrus. Ten-year-old grapefruit trees exposed to artificial temperatures of 23 degrees F. for 4r hours during various times in the 1960-61 winter, sustained varying degrees of injury (Table 10). Trees in November, before exposure to any cool winter temperatures, sustained the greatest tree and ,0 fruit injury. These trees were not dormant. Trees in December after 2 weeks of cool temperatures, showed much less tree injury and less fruit injury. In January, after 6 weeks of cool temperatures, very f little tree injury occurred and much less fruit injury was found. Buds and small wood cambium were i. dorrriant on trees in January. In February, 10 days ‘ of warm weather above 50 degrees F. occurred and buds began to grow. Trees sustained more injury g than in January although fruit were only slightly l injured. Thus, large changes in tree cold hardiness may occur during the winter and are brought about by changes in temperature. Although temperature is the major climatic fac- or influencing tree cold hardiness, soil moisture stress is also important. Trees slightly on the dry side be- come more dormant and are more cold hardy than those with adequate moisture. However, if trees are allowed to become too dry, cold hardiness may be affected adversely. Withholding moisture during the winter may induce more cold hardiness, but this prac- tice is risky in the Valley. Winter rains on dry trees may supply enough moisture to induce bud growth which would greatly reduce the tree’s cold hardiness. The lack of winter rains and low humidity during the winter also favors cold hardening. These conditions, however, are usually less effective than withholding irrigation water. I Varieties and Rootstocks Following natural freezes, rather large differences in the cold hardiness of various citrus varieties have been noted. These differences can be explained in part by the degree of dormancy which the particular variety exhibits during the winter. Those varieties which stop growth at a higher night temperature usually exhibit more winter dormancy. Following the 1962 freeze in Texas, freeze-injury records indicated that some varieties were much more cold hardy than others. Most mandarins, such as the Clementine and Kara, were very cold hardy; however, the Dancy and the Murcott were very cold sensitive. Washington navel, Texas navel, Jaffa, Parson Brown, and Hamlin oranges, although less hardy than Clemen- tine mandarin, were the most hardy of the sweet oranges. Valencia orange followed navels in hardi- ness while Marrs and Pineapple oranges were the least hardy of the sweet oranges. Temple orange was variable in hardiness. Both Red and white grape- fruit and Orlando and Minneola tangelos were slightly less hardy than the sweet oranges; Mexican limes, Eureka lemons, and Meyer lemons were the least hardy of the citrus varieties. Meyer lemon was the most hardy of the lemons and limes. Table l0. lniury to Red Blush grapefruit trees and fruit after exposure to 23° F. for 4 hours in November and December, ‘I960, and January and February, 1961. Bark nee lnlwY Number of . . o/ o/ segments Dam sjlsigtllg defbli- twig mlwed ation I injury P" "u" November 2.4 86 98 4.9 December 1.8 40 ‘l2 4.2 January 0.9 l4 0 2.2 Februaryz 3.0 39 ‘l ‘l 0.8 1O z bark does not peel; l : bark barely peels; 2 = bark peels easily; 3 = bark peels easily and is moist. “Bud growth had started and buds were V4" to l/z" long. 35 Fig. ‘l3. Freeze iniury to and subsequent recovery of 8-year-old trees of (A) Red Blush grapefruit on Cleo, (B) Red Blush grapefruit on sour, (C) Valencia orange on Cleo, and (D) Valencia orange on sour. Pictures were made in April 1963. - Under Valley conditions, mandarins or varieties with mandarin or tangerine parentage generally are the most cold hardy of the commercial varieties. Limes and lemons, because of mild Winters in the Valley, usually grow the year around and during the winter are very cold sensitive. Large plantings of limes or lemons are not recommended for the Valley. Rootstocks also have a pronounced effect on the cold hardiness of the tree. Many rootstocks have been tested under Valley conditions and their cold hardi- ness behavior observed. In most plantings Cleopatra mandarin rootstock induced more cold hardiness to the top than did sour orange (Figure 13). These two rootstocks are commonly used in the Valley. Of the two, sour orange generally is preferred because it produces better fruit size, and the trees are less susceptible to iron chlorosis. MECHANICAL COLD PROTECTION Engineering Considerations Grove heaters provide the most widely-used meth- od of freeze protection. They heat the air immedi- ately around the flame and immediately around the surfaces of the heater. This air in turn is dispersed by thermal movements or Wind movements, and tends to warm the atmosphere in the immediate vicinity. Since warm air tends to rise, the heat from thermal convection will move upward and quickly out of the grove unless wind movement is sufficient to disperse it. among the trees, or unless this heat is applied under the canopy of the trees, where its upward movement is restricted. High winds tend to move added heat out of the orchard very rapidly, making it virtually impossible to change the temperature appreciably. The heater has a second means of supplying heat to the plants in that it radiates heat outwardly and 36 this heat is absorbed only by the mass of the plant, soil or sky. In many cases, such as on clear, cold, l still nights, this radiant energy can effectively prevent‘. excessive freeze damage. Young trees are best protected through the usej of small heaters and by taking advantage of the con-E vective heat that will filter up through the center of the tree. This method requires at least one heater per tree. When. heaters are placed in this fashion; ' the radiant energy available normally is received only; . by the trunk, wood, and foliage in the near vicinity of the heat source. The convective heat is carried - through the tree and can be retained in the orchard for effective use a little longer than it could if it were out in the space between trees. i Where the grove is composed of older trees, with wide canopies, fewer heaters often can be used satis-i; factorily by placing them in the space between trees,i so that any wind movement will tend to carry the Fig. 14. The University return stack heater is a popul, unit for grove heating since it burns with a clean flame and cal: be used at variable burning rates. i oss the grove. Heaters used in this fashion are large return stack heaters that give off able radiant energy as well as convective The radiant energy will then be transferred surrounding the heater to help protect them. _iant energy is especially effective on cold, ghts. The return stack heater is a rather pop- It for use in spaces between trees since it is flavailable, effective, burns with a clean flame, ‘l be used at variable burning rates. On a still “th winds of from 1/2 t0 1 miles per hour, ately 9 million BTUs of heat were necessary ain a 1O degree temperature response in the hen there was very little inversion. This of heat would require 65 to 7O gallons of fuel ‘T1011! per acre, and would indicate the need for return stack heaters per acre to maintain this ‘ee rise. A lower temperature response should p, ted with a reduced number of heaters or rate. gyjall areas and isolated groves are more diffi- -heat successfully since there is a definite stack ‘sicreated by the rising of the heated air and 0m the heaters. Cold air is drawn in from of such an isolated area and may reduce the ‘x to heat for as much as 1O to 15 rows from e. Higher rates of fuel input per acre cause -- grove stack action and indraft; therefore, _ ortant to operate the heaters, especially those ‘tithe plot, at the lowest rates which will give e protection. Border heaters should be dis- over the first two or three rows rather than ‘ ated on the outside of the grove. A good ‘ is to use one heater per tree on borders. ‘_ us in border heating. The greatest concen- lof border heaters should be on ‘the up-wind Often heaters in combination with wind ma- ‘give extra response because the convective heat ‘i heaters can be pushed back into the grove ectively. Table 11 gives results obtained from hard heating test conducted on avocados during adiant freeze. In that test with 7O University Stack heaters per acre burning at a rate of 2- of fuel per hour, temperatures in the heated re raised 5 to 9 degrees above the nonheated ln Arizona, 4-5 Jumbo Cone heaters per acre ‘ fuel at 1 gallon per hour raised grove tem- ‘ 3 to 5 degifiees F. during several radiant _ es with high winds dissipate heat rapidly ye grove and conventional heaters are less ef- ‘ Heaters which emit more radiant heat are i with high radiant output are especially ad- Table 11. Temperatures in degrees F. at various locations in heated and non-heated areas in an orchard healing test at Rio Farms, Inc., Texas, February 11, 1955. Temperatures ° F. at ‘ “cum”: The" indicated time o. an mometer description No.2 2 3 4 5 6 OJTI. a.m. a.m. GJII. a.m. Heated area‘ 1 East side 1 36 34 35 34 34 Inside heated area 2 37 35 36 34 35 2 Inside heated 1 37 35 36 34 36 area 2 38 35 36 35 35 3 Inside heated 1 37 35 36 30 31 area 2 37 37 38 32 32 4 North ‘ 1 35 35 36 . 33 33 ‘ perimeter 2 38 36 36 33 33 5 South perimeter 1 32 31 31 28 28 Outside heated area 2 32 31 31 28 29 6 Inside 1 36 36 36 32 34 tree 2 37 36 36 32 34 7 West side 1 39 38 37 32 32 2 inside heated area Non-heated areas 1 East side 1 31 30 30 28 28 2 31 30 3O 28 28 2 In the open 1 28 28 29 25 27 not near any trees 2 3O 28 28 26 26 3 North 1 31 31 31 27 28 perimeter 2 31 3O 31 27 28 4 Center of 1 plot 2 30 3O 31 27 27 5 West side 1 33 31 3O 26 26 of plot 2 32 31 31 27 27 lProtected on north, south, and east sides by windbreak and heated wtih 7O University Return Stack heaters per acre. Very slight wind drift from N.E. to S.W. zThermometer No. 1 was located 3 inches above ground and thermometer No. 2 was located 3 feet above ground. 3Nonheated area was adiacent and west of the heated area and was not separated from it by a windbreak. There was a wind- break on the north and south sides of the nonheated area but not on the west. more effective in freezes with high winds. Radiant heat is not affected by wind. The University Return Stack and the Jumbo Cone heaters emit more radiant heat than other conventional heaters. The cost per acre to equip a grove with heaters varies with the type of heater and the number per acre. The University Return Stack heater costs ap- proximately $8, the Jumbo Cone, $7, and others some- what less. Assuming one uses the University Return Stack heater at 6O per acre, one might expect to incur the following initial costs on a ILO-acre grove: 6O heaters per acre, $8.00 ea ..... "$480.00 9 gallons fuel, $0.12 per gal. X 60 .............. _. 64.80 $544.80 X 40 = $21,792.00 12,000 gallon storage tank erected .......................... .. 1,500.00 One SOD-gallon oil cart with pump .......................... .. 575.00 5 thermometers, $5.00 ea ....................................... ._ 25.00 1O lighting torches, $6.00 ea ................................... _. 60.00 $23,952.00 37 The initial investment in this grove would average about $599 per acre. In case more heaters were used per acre, a higher initial investment would be neces- sary. Annual maintenance costs, fuel and labor costs during freezes, and depreciation have not been includ- ed. Maintenance costs plus fuel and labor costs may total $50 per acre per year. Windmachines The use 0f windmachines in Texas has not been fully evaluated. They are effective only in freezes where a strong temperature inversion up to 50 feet exists and the wind velocity is less than 3 miles per hour. In California and Arizona where the use of windmachines is recommended, freezes are still, radi- ant freezes with strong inversion air layers generally about 5O feet above the ground. windmachines mix this warmer air with the cold air close to the ground surface and thereby raise the temperature in the or- chard. In Texas, less is known about temperature inver- sions during freezes. The inversion was measured at 20 and 40-foot levels during the entire 1962 freeze at Monte Alto. No inversion existed during the first two windy days and nights and only an 8 degree F. inversion existed at 40 feet on the last night where a still radiant-type freeze occurred. Under those condi- tions, one could expect no more than a 6 degree F. Table 12. Occurrence of temperature inversions‘ during cold nights at various Valley locations. Station and Minimum High. Number nights with indi- winter temperature . w Hie cated degrees of inversion s,‘ v I inversion o o a o o o =99=9H l 9 91 meusumd o -4 s -10 11 -1s Horlingenzz 1931-32 30-40 98' 8 2 0 1932-33 28-40 98' 9 0 0 1933-34 35-40 9a’ 3 o o Engleman Gardenszz 1934-35 24-37 49' 5 1 0 1935-36 25-38 49' 12 3 0 Weslacms 1960-61 34-39 26' s o 1961-62 13-32 26' 3 1 o 1962-63 ' 24-33 26' 6 o o Monte Altoz‘ 1961-62 12-23 40' 2 1 o 20' 2 1 0 ‘Temperature inversions are defined as differences in temperature between that at 5 feet and at indicated height. No attempt was made to measure height at which a maximum inversion occurred. zData taken from a "Summary of fruit frost data obtained in Lower Rio Grande Valley district during five seasons beginning with the winter of 1931-32." Esek S. Nichols, U. S. Dept. of Agriculture, Fruit and Vegetable Frost Service of the Weather Bureau, Harlingen, Texas. “Data from Texas Agricultural Experiment Station, Weslaco, Texas. ‘Data from Weather Station belonging to U. S. Department of Agriculture, ARS, CRD, Weslaco, Texas. 38 (three-fourths the temperature inversion) rise in tem- perature with the most effective windmachine. Dat in Table 12 indicate that most freezes between 193 and 1937 had 0 to 4~ degrees of inversion existing a levels between 26 and 419 feet. In the 1962 Texas freeze, jseveral groves in th Valley were equipped with windmachines and sever had both windmachines and heaters. Qbservatio following the freeze indicated that in one grove, t? windmachine alone had no effect; one with heat ‘l and a windmachine had little or no effect, anotlii with 18 heaters per acre and a windmachine rais the grove temperature 3 to 4» degrees F. i The use of windmachines alone under Valley c011: ditions appears, in most instances, not to be a practi cal method for freeze protection. General informl tion indicates that temperature inversions at the eff a tive level are weak and inconsistent. The use of both heaters and windmachines J pends largely on the temperature and wind conditio in each grove. With a strong temperature inversio’ and no wind, both heaters and windmachines woul‘ be effective. With a weak inversion, windmachin would be of value only in distributing heat from t‘ i heaters more uniformly throughout the grove. i; light winds are present, which generally is the case most radiant freezes in the Valley, the value of a win , machine would be nil. i‘ Cost of windmachines varies with the size 1 machine and the acreage to be covered. Small :._ chines which are not too effective average betwi $1500 to $2500. Larger machines equipped with tw 100 H.P. engines which may cover 10 acres adequa ly, under optimum conditions, costs $5000 to ullf Initial costs per acre run between $500 to $600 and do not include annual maintenance costs, fuel labor costs during freezes, and depreciation. Irrigation Water has been applied on crops to prevent fre '1 damage many times in the Valley. Water is warm than air and releases heat. As water freezes, heat again released. Research in other areas shows t’ water cooled from 65- to 32 degrees F. will change 1i air temperature 2.5 to 4» degrees F. with an initial =" temperature of 26 degrees. This could be a real a in a freeze of short duration or if the temperature A not too low. ‘i In Arizona and California, flooding orchards r- water for freeze protection has been used. General] one can expect 1 to 4 degrees F. rise in grove te ‘ture from flooding depending on the type of - and distance from the water source. gjFlooding an orchard with water would not be Tipractical in the Valley. Due to the methods used ' 'tributing water through canals by the districts, fa few growers could be serviced in any one night. at the temperatures at which water gives pro- i 'on, mature citrus trees probably would not receive pith cold injury, provided the trees were not in a 'wth flush. If the freeze were more than one night it ration, the soil would become waterlogged and in- fry to the trees could result. Sprinkler irrigation for cold protection does not seem to be a practice to recommend to Valley citrus growers for several reasons. As with flooding, most itrus growers in the Valley do not have a large supply f water they can use on short notice. A lot of the cifreezes in the Valley are accompanied by high winds hich would reduce the effectiveness of the sprinklers. On an evergreen tree like citrus, it would be difficult o get even distribution of the water on the tree, and f the freeze lasted very long the weight of ice on the imbs would cause as much or more damage from reakage as the freeze would have done. The cost of sprinklers would vary depending on e number used per acre and the type of materials sed for water distribution. Generally, a system ade- uate for both frost protection and irrigation will run etween $600 and $1000 per acre initial investment. ultural Practices g Under Texas climatic conditions any cultural eatment to a citrus grove in the fall should be done ith the thought of how it will affect dormancy and, onsequently, the cold hardiness of the tree. Young trees up through 3 to 4- years of age can - give-n winter protection by banking the trunks with ash-free soil in November. Banks should be up by ‘ e first of December although it is very seldom that I trunk-injuring freeze occurs before the tenth of Jecember. The larger the bank the more wood is irotcgfcted in case of a freeze. Some growers bank 1 trees in progressive stages of grove maturity. establishment, $709; development, $1000; yo A grove, $1500; and mature grove, $2000. i An additional charge is the annual interest t) operating expenses. This charge is calculated =5 percent of one-half of the annual operating expe (Table 14.). Table 15. Summary table of production costs per acre. (116 trees/acre, 1963 prices) ‘ Devel0p- Young Motu ' Establish- » ment rove gro _, Cost ment 24 gfim 1H,. l“ Year years years years; Operating i expenses $115.81 $106.90 $137.95 $175.1' Interest on ' T operating 1 expenses 3.47 y 3.21 4.14 5.1 Operating l labor and _{ management 12.00 12.00 12.00 12. Production costs excluding interest on investment 131.28 122.11 154.09 Interest on value of land and trees 42.54 60.00 90.00 Total costs $173.82 $182.11 $244.09 if URN TO OPERA TOR LABOR AND A MANAGEMENT l. fee of $12 per acre per year for supervision is I~~ by grove care companies. While this amount l. included as the return to operator labor and lment, it would be a direct cost to absentee iwners instead of a return. Any additional re- l ove specified costs and returns can be consid- ». profit on the investment or additional return ' agement. ST OF REHABILITA TING FREEZE- ‘ DAMAGED GROVES i» cost of rehabilitating one acre of citrus de- ion several factors. The cost of removal and #- ent of dead trees varies from $3 to $4 per r 4-t0-7 and 10-to-15 year old trees, respectively. t per acre varies with the number of dead trees e. e cost of pruning varies from 60 cents to 80 ‘rtree depending on tree age and amount of required. The cost per acre varies with the ' ber of trees per acre and the number of dead acre. YIELDS 'eld of fruit varies by variety, tree age, and of trees per acre. Grapefruit, early and mid- l orange trees begin to bear at 3 years of age i‘; te season oranges start bearing at 4~ years of _] lelds increase steadily until the trees reach 12 years of age. Table 16 contains estimated an- folds for the different species. PRICES liice estimation is perhaps the most difficult, pportant, part of costs and returns analysis. ylimation should be based on expected produc- Table 16. Yield per acre for Texas citrus. " ' (116 trees per acre) Grape- rfiiudrzegggn Late-season fruit orange orange (tons/acre) (ions/acre) (tons/acre) 1 0.7 0 2.5 1.2 0.4 6.5 ,_ , 3.0 1.3 _ 10.0 5.5 3.0 16.0 l‘ 8.0 5.0 18.0 10.0 7.0 19.0 12.0 9.0 20.0 14.0 10.0 21 .0 1 5.0 1 1 .0 22.0 16.0 12.0 Table 17. Returns per acre of grove. Receipts by species} Grape- Early and Late-season Gross Tree fruit mid-season orange receipts‘ age (‘/3 acre) ‘ii/insane,’ (‘/3 acre) (per acre) -— -— -— — dollars -— —— -— - 3 8.42 8.56 0.00 16.98 4 21.05 14.68 6.07 41.80 5 54.73 36.70 19.72 111.15 6 84.20 67.28 45.50 196.98 7 134.72 97.86 75.84 308.42 8 151.56 122.32 106.18 380.06 9 159.98 146.79 136.52 443.29 10 168.40 _ 171.25 151.69 491.34 11 176.82 183.48 166.86 ' 527.16 12 ('1') 185.24 195.72 182.03 562.99 lAssumes 116 trees per acre and the prices and yields developed in previous paragraphs of this section. tion in other areas and expected changes in consumer preferences. Prediction of this nature is beyond the scope of this study, but these factors should be kept in mind when evaluating the returns computed in this section. Average prices for past seasons are used in the estimation of returns. The average on-tree price for Texas oranges from 1954- through 1962 was $1.79 per box or $39.78 per ton based on a value of 30 million, 515 thousand dol- lars for 17 million and 4-0 thousand boxes. The volume of early and mid-season oranges exceeded the volume of late-season oranges by 80 percent during this period. At the same time, the on-tree price in Florida for late-season oranges was 24 percent higher than the on-tree price for early and mid-season oranges. Assuming that this factor was the same in Texas and that Texas orange production was divided evenly between early and late-season oranges, the estimated price is $36.70 per ton for early and mid-season oranges and $45.50 per ton for late-season oranges. The price for Texas grapefruit, during the same time period, was $1.01 per box or $25.35 per ton with a value of 29 million, 614 thousand dollars for 29 million, 34-0 thousand boxes. RETURN FR OM CI TR US Gross receipts from an acre consisting of one- third grapefruit, one-third early and mid-season orange, and one-third late-season orange are shown in Table 17. Net returns from an acre of citrus are shown in the following table. The returns in Table 18 are based on the costs and returns developed in previous paragraphs. 51 Table 18. Costs and returns per acre of grove. Column 1 2 3 4 5 Production costs Return to Interest Gross excluding investment on Profit Tree receipts interest in land investment (Column age, from on and trees in land 3 yeqrs Table inveflment (Column and trees minus 17 from 1 from 4) Table 15 minus 2) Table 14 ----—dollars-——-—-—-— 1 0.00 131.28 —131.28 42.54 —-173 94 2 0.00 122.11 —122.11 60.00 —182 11 3 16.98 122.11 —105.13 60.00 —165 13 4 41.80 122.11 — 80.31 60 00 —140 31 5 111.15 154.09 — 42.94 90.00 ——1 32 94 6 196.98 154.09 42.89 90.00 — 47.11 7 308.42 154.09 154.33 90.00 64.00 8 380.06 154.09 225.97 90.00 135.97 9 443.29 154.09 289.20 90.00 199.20 10 491.34 154.09 337.25 90.00 247.25 11 527.16 192.35 334.81 120.00 214.81 12 ('l‘) 562.99 192.35 370.64 120.00 250.64 CAPITAL REQUIREMENTS FOR GR O VE DE VEL OPMEN T 1t takes a large amount of capital t0 establish and develop a citrus grove. Annual operating ex- penses and annual interest charges on investment in land and trees accumulate during the early years of grove life. In each of the first 6 years, costs of pro- duction exceed returns. Capital employed in the grove is at a peak after the sixth year. The grove owner has employed approximately $1,550 in develop- ing the grove to this point. In succeeding years, in- come from the grove exceeds annual production costs and interest charges on investment. After the 12th year, income from the grove has paid for development of the grove plus a return on investment in land. Qne additional year of production is required for paym of the initial cost of land. The annual costs, retu and capital requirements are estimated for the first t years of grove life in Table 19. f GROVE VALUE ESTIMATION The value of a resource‘ (fr combination of _ sources, such as a citrus grove, is based normally the net income which the resource can be expected e produce during the remainder of its effective life . the salvage value of the resource at the end of its l_ The stream of annual net incomes and salvage val of the resources are discounted back to the pre to estimate current or present value. Discounting a process by which incomes received at some fut g time are converted to present values. For examp the present value of $25 ten years from now, in v form of a United States Government bond, is $16. This is obtained by discounting the $25 future ret -, at a 4 percent rate of interest. The discount r: which is used can be an expected rate of return or I interest rate charged on borrowed money. A procedure referred to as present value estb tion, can be used for estimating the value (at prese of a citrus This procedure estimates i: amount of money which could be paid for a grove i present and still receive a reasonable return on c)’ tal invested. Obviously, the procedure cannot J2 the future; however, it can be used to obtain estima for different situations which may be expected to s’ in the future. For example, present grove values v7 be estimated for various fruit prices, fruit yields a‘) grove. levels of freeze damage. Table 19. Estimated costs, returns and capital investment in one acre of grove. Column No. 2 3 4 5 6 7 ‘mares. Annual Additional . I production Total Gross capital Grove Capital o." cuplw costs costs returns required in age’ invested mvesled excluding (Column 3 (From succeeding Years lcolo‘ 2 x interest plus Col. 4) Table 17) year (Col. 5 ' 6 Al (From Table 15) minus Col. 6) 1 _ _ _ _ _ _ — — — -——dollars——————--———-——-- 1 709.00 42.54 131.28 173.82 0.00 173.82 2 882.82 52.97 122.11 175.08 . 0.00 175.08 3 1057.90 63.47 122.11 185.58 16.98 168.60 4 1226.50 73.59 122.11 195.70 41.80 153.90 5 1380.40 82.82 154.09 236.91 111.15 125.76 6 1506.16 90.37 154.09 244.46 196.98 47.48 7 1553.64 93.22 154.09 247.31 308.42 — 61.11 8 1492.53 89.55 154.09 243.64 380.06 —136.42 9 1356.11 81.37 154.09 235.46 443.29 ——207.83 10 1148.28 68.90 154.09 222.99 491.34 —-268.35 11 879.93 52.80 192.35 245.15 527.16 —282.01 12 597.92 35.88 192.35 228.23 562.99 —334.76 13 263.16 15.79 192.35 208.14 562.99 —354.85 14 —— 91.69 52 mated t0 be $709 per acre. Table 20. Estimated present value of investment in T-acre mixed orchard} Greve ege Tree age at time of freeze of present (years after establishment) (years) 8 10 ‘I2 14 18 0 (at establishment) 58.78 372.32 701.91 1025.93 1571.13 4 559.72 955.21 1370.66 1779.85 2468.11 8 818.85 1343.82 1888.99 3728.31 1% grapefruit, V3 early and mid-season orange, V; late-season orange. Rate of return : 6 per cent. Salvage value z $450 per acre. Lost l/a of the grapefruit crop and 3/4 of late-season orange crop through freeze damage in last year. Present values were estimated by this procedure 7 for groves of various ages and life expectancies. ln Table 20, a 6 percent rate of return, salvage value of $450 per acre and the annual returns shown in column 3, Table 18, were used in estimating present values. ln this case, the grove was assumed to be frozen out at various ages. The $450 salvage value is calculated by subtracting $50 per acre, for removing dead trees, from the $500 per acre land value. The price of land was assumed to remain constant. Since the grove was assumed to be frozen out, one-third of the grape- fruit crop and three-fourths of the late-season orange crop was assumed to be lost through freeze damage in the last year of ownership. Earlier, the cost of establishing a grove was esti- y In Table 20, present l value at the establishment of a grove which is expected to be frozen out at 12 years of age, is $701.91. A comparison of these figures shows that the returns from this grove would be $7.09 ($709.00—$701.91) per acre short of yielding the owner of the grove a 6 percent return on his investment. In other words, a citrus grove which is frozen out before the twelfth year would not justify its establishment. The resale or salvage value is important in esti- l mating a grove’s present value. When the grove can ~ be sold before a severe freeze has damaged the trees, l, the resale value of the grove should be much higher l, thanlthe $450 per acre salvage value used above. a Estimates of the resale values which could be expected are as follows: Tree age a Resale value (years) ($/acre) 6 1 soo 8 1700 10 ‘I900 12-18 2000 1f these resale values are received when the groves are sold, the present value estimates for these groves would be much higher than the estimates in Table 20. Present value estimates shown in Table 21 are based on high resale values, a 6 percent rate of return and the annual returns shown in column 3, Table 18. Crop losses in the last year are not includ- ed in these estimates, since no freeze damage is assumed. The present value of a grove at establishment is $671.58/ acre if sold after the 6th year and $9'24.62/ acre if sold after the 8th year. A rough estimate of the present value of the grove if sold after the 7th year is $800/ acre. Since the cost of land, trees and planting is $709/ acre, the grove is expected to yield a 6 percent return on investment plus $91/ acre profit if sold after the 7th year. The $91/ acre profit is for the full 7-year period, not on an annual basis. ln comparison, a grove which is killed by a freeze and sold for $450/ acre is expected to yield a 6 percent return on investment only if the freeze is incurred in the twelfth year or later. COLD PROTECTION VALUE ESTIMATION The annual payment for cold protection which can be economically ustified is determined chiefly by two factors—effectiveness of the cold protection sys- tern, and annual costs and returns from the grove. Effectiveness is an important factor in determin- ing value of a cold protection system. Costs and re- turns from the grove also have a substantial effect on the value of a cold protection system. During a period when freezes keep U. S. production at relatively low levels, fruit prices will be high and the cold pro- tection system will be more valuable than in periods when fruit prices are low. Numerous procedures may be used in estimating the maximum allowance for cold protection. The most practical procedure is to allow profit or income in excess of a reasonable return to be used for cold protection. Profit, in this case, should be estimated for a series of years rather than an individual year. Present value estimates may be used in estimating this profit. For example, the present value of a grove at establishment is $2184.55 if the grove is sold for $2,000 in its eighteenth year. This present value is shown in Table 21, and is based on the costs and returns developed in this section. The cost of land, trees, and planting is $709; therefore, $1475.55 ($2184.55—$709) is discounted profit in excess of 6 percent return on the capital invested. It is assumed that effective cold protection will be started in the 53 Table 21. Present value estimates for land and trees in one-acre of grove} (High resale value.) P"°5°"l "99 Grove age at time of resale of grove (Yew, 5 a 1o 12 14 15 o 571.58 924.52 1279.31 1577.41 1303.52 2014.45 (establishment) 2474.57 2757.92 $3049.55 4 1332.15 1553.34 2100.15 2733.25 3135.40 ‘ 3435.07 a 2253.25 2455.15 2331.15 12 - 15 lResale values: 6 years of age—$l500/acre. 8 years of age— l700/acre. l0 years of age—— l900/acre. 12-18 years of age——— QOOO/acre. Rate of return : é percent. V; grapefruit, V; early and mid-season orange, and V; late-season orange. fifth year and that the 14- annual payments for the REFERENCES fifth through eighteenth years will be of equal value. The maximum annual payment which could be justi- fied is $200.30 per acre. The discounted sum of 14~ ADAMS, R. L., Protecting Citrus Groves from Frosl Costs and Benefits t0 Growers. Calif. Agr. Exp. j‘ annual payments of $200.30 per acre is $1475.55. BuL 730' ., The cold protection systems must be able to save both ADVANCES IN AGRONOMY, Vol. 11, Edited by A. A v trees and fruit in order to justify $200.30 per acre Norman. 1959. i er ear a merit. , if p y p y BATCHELOR, L. D. AND H. J. WEBBER, Editors, Maximum annual payments for 601d prntaction Citrus Industry, Vol. 2, Production of the Crop. '0} can be estimated by the same procedure for other . . _ d levels of return. For example, the grove owner may BLOODWORTH’ M‘ E" some Pnnclples an Prac require a 6 percent return on investment plus $50 per in the Irrigation 0f Texas Soil. TcXaS Agri- EXP‘ . acre per year profit or return to management. In Bul‘ 937' 1959' l this ease he emlld affQYd t0 Pay aPPYQXimatBIY $130 BRANSON, R. E. AND T. PRATER, Vertical Integral Per aere Per Year fer effective eeld Preteetien- in Texas Agriculture—Citrus Fruits. Texas Costs and returns in this section were developed EXP‘ Sta‘ and Ext‘ Self‘ L488‘ 1960' to represent groves planted 116 trees per acre with BRANSON, R E“ CARTER PRICE AND H_ V_ C-OURTEN‘ one-third grapefruit, one-third early and mid-season Consumer panel Test of Texas Fortified Red G1. orange and one-third late-season orange trees. The fruit Juice_ Texas Agn Exp Sta Mp_481_ production costs are above the average costs for the Valley due to the assumed close spacing and improved practices. However, the expected yields are also above BROOKS, F. A., An Introduction to Physical Micr‘ matology, University of California at Davis, Syll the Valley average and more than offset the increase Ne- 397- 1960- 4 in production costs. A grove to which these costs and BUCK, WILBUR R AND H_ B_ SORENSEN, Mark realms apply must be kept m Pf°du°tl°n Wlthout Adjustments Made by the Texas Citrus Indust IIIHJOI‘ freeze damage through the thirteenth year after Freezes of 1949 and 195L Texas Agr_ Exp Sta_ establishment, if the owner is to pay for the land and 328 1959 trees and receive a 6 percent return on his investment. If freeze damage was not a problem, the grove owner BURDICK, EVERETTE, SYIVIIPtOmS 0f F T6916 Damag could expect an additional return from the increase Citrus Ffllit- 10l1r- Rio Grande Valley Hort. Soc in grove value. However, this return from grove value 117-120- 1951- increases would probably. be offset by high, annual THE CITRUS INDUSTRY, Vol. 2, Edited by L. D. Ba * payments for cold protection once an effective system 10f and H. JI Webber. 1948. is designed. At present, it appears that the grove l owner will be able to pay from $100 to $200 per acre CLEMMONS, W. ELTON, Grove Heating—A Meth I per year for effective cold protection, dependent upon Protecting Florida Citrus. The Citrus Industry 1 the rate of return required by the individual. (6) : 18,20,241. 1963. f‘ 54 OPER, WILLIAM C., Cold Hardiness in Citrus as :- lated t0 Dormancy, Proc. Florida State Hort. Soc. _' : 61-66. 1959. COOPER, WILLIAM C., AND BRUCE J. LIME, Quality of fa Grapefruit 0n Old-Line Grapefruit Varieties on . _yloporosis—and Exocortis-tolerant Rootstocks. Jour. _ '0 Grande Valley Hort. Soc. 14: 66-76. 1960. COOPER, WILLIAM C., AND EDWARD O. OLsoN, NoR- ~ l AN MAXWELL, AND ARTHUR SIIULL. Orchard Per- irmance of Young Trees ofRed Grapefruit 0n Vari- ‘us Rootstocks in Texas. Proc. Amer. Soc. Hort. i. 70; 213-222. 1957. p- oPER, WILLIAM C., AND AscENsIoN PEYNADo. I reening Citrus Rootstock Seedlings for Tolerance to ialcareous Soils. J our. Rio Grande Valley Hort. Soc. : 100-105. 1954. ' i-OPER, WILLIAM C., AND AscENsIoN PEYNADo, _‘ron Accumulation in Citrus as Influenced by Root- ‘ock. Jour. Rio Grande Valley Hort. Soc. 9: 86-94. 955. h OPER, WILLIAM C., AND AscENsIoN PEYNADo, l: oride and Boron Tolerance of Young-Line Citrus tees on Various Rootstocks. J our. Rio Grande Val- a Hm. Soc. 13. 39-96. 1959. p-JOOPER, WILLIAM C. AND AscENsIoN PEYNADo, Win- Temperatures of 3 Citrus Areas as Related to ormancy and Freeze Injury of Citrus Trees. Proc. ‘ er. Soc. Hort. Soc. 74.; 333-34-7. 1959. foPER, WILLIAM C., AscENsION PEYNADo, AND J. R. ‘CURB, Effects of 1961-62 Winter Freezes on Valencia I anges in Florida, Texas, and California. orida State Hort. Soc. 75; 32-33. 1962. DOPER, WILLIAM C., AND A. V. SHULL, Salt Toler- of and Accumulation of Sodium and Chloride i,- in Grapefruit on Various Rootstocks Grown in a (aturally Saline Soil. Jour. Rio Grande Valley Hort. i . 7; 107-117. 1953. oPER, WILLIAM C., SAM TAYLoE, AND NoRMAN 1 WELL, Preliminary Studies on Cold Hardiness in I trus as Related to Cambial Activity 8r Bud Growth. 3.15. Rio Grande Valley Hort. Soc. 9. 1-15. 1955. LcURTo, J. M., HALsTEAD, E. W., AND HALsTEAD, H. ., The Citrus Industry in the Lower Rio Grande Val- ‘ 0f Texas. Texas Dept. Agri. Bul. 75. 1923. JWCETT, HowARD S., Citrus Diseases and Their Con- Bl. 2nd ed. 656 pp. McGraw-Hill Book Company, c. New York. 1936. g I ND, W. H., The J oppa Orange. Jour. Rio Grande == ey Hort. Soc. 3: 63. 1948. Proc. FRIEND, W. H., History of the Meyer lemon in the Valley. J our. Rio Grande Valley Hort. Soc. 8: 32-33. 1954. F URR, J. R. AND W. W. ARMsTRoNc, JR., Breeding Citrus for Cold Hardiness. Proc. Florida State Hort. Soc. 72: 66-71. 1959. FURR, J. R., J. B. CARPENTER, AND A. H. HEWITT, Breeding New Varieties of Citrus Fruits and Root- stocks for the Southwest. Jour. Rio Grande Valley Hort. Soc. 17: 90-107. 1963. GERARD, C. J., BLooDvvoRTII, M. E. AND CowLEY, W. R., Effect of Tillage, Straw Mulches and Grass upon Soil-Moisture Losses and Soil Temperatures in the Lower Rio Grande Valley. Texas Agri. Exp. Sta. MP-382. 1959. GERARD, C. J. AND SLEETH, B., The Use of Tensiome- ters as an Aid in the Irrigation of Citrus Groves. J our. Rio Grande Valley Hort. Soc. 14: 47-52". 1960. HARRISON, D. S., AND S. E. DOWLING, Frost Protec- tion of Citrus with Sprinkler Irrigation. Paper pre- sented at 17th Annual Gulf Citrus Institute, Dade City, Florida. 1962. HILDRETH, R. J. AND ORToN, R. B., Freeze Probabili- ties in Texas. Texas Agr. Exp. Sta. and Ext. Ser. MP-657. 1963. HILcEMAN, R. H., AND L. H. HoWLAND, Report on the Frost Situation and Effect of a Wind Machine on Temperatures During 1954-55. stitute 1: 9-15. 1955. Arizona Citrus In- HILcEMAN, R. H. AND D. R. RoDNEY, Commercial Citrus Production in Arizona. Special Report No. 7, Agr. Exp. Sta. and Ext. Ser., Univ. of Arizona. 1961. HILGEMAN, R. H. AND L. H. HoWLAND, When to Irri- gate Citrus Trees. Progressive Agriculture in Ari- zona, College of Agriculture. University of Arizona. 1955. HUDsoN, A. C. AND H. B. SoRENsEN, Texas Wholesale Citrus Industry. Texas Agr. Exp. Sta. MP-336. 1959. HUME, HAROLD H., Citrus Fruits in Texas. Texas Dept. of Agri. Bul. 3. 1909. KEPNER, RoBERT A., The Principles of Orchard Heat- ing. Calif. Agr. Exp. Sta. Cir. 400. 1950. KLoTz, L. J. AND H. S. FAWcETT, Color Handbook of Citrus Diseases. Univ. of California Press. 1941. KN0RR, L. C., R. F. SUIT, AND E. P. DUCHARME, _Handbook of Citrus Diseases in Florida. Florida Agr. Exp. Sta. Bul. 587. 1957. 55 KREzDoRN, A. H., AND NoRMAN MAXWELL, Fruit Qual- ity Studies of Eight Strains of Red-Fleshed Grapefruit on Two Rootstocks. Jour. Rio Grande Valley Hort. Soc. 13: 54-58. 1959. KREZDORN, A. H., AND R. F. CAIN, Internal Quality of Texas Grapefruit. Jour. Rio Grande Valley Hort. Soc. 6: 48-52. 1952. LIME, BRucE 1., AND DoNALD M. TUCKER, Seasonal Variation in Texas Hamlin and Marrs Orange Juice, 1961-62. Jour. Rio Grande Valley Hort. Soc. 16: 78-82. 1962. MANELY, WILLIAM T., W. FRED CHAPMAN, JR., GEoRcE L. CAPEL AND H. B. SoRENsEN, Competitive Practices in Marketing Florida and Texas Grapefruit. U. S. Dept. of Agr. Eco. Research Ser. 1963. MARTSOLF, I. DAVID, The Protective Value of Wind Machines. The Citrus Industry. 4-4- (6): 29-30. 1963. MAXWELL, NORMAN P., Citrus Varieties in the Rio Grande Valley of Texas. Jour. Rio Grande Valley Hort. Soc. 3: 63-67. 194-8. MAXWELL, NoRMAN P., Production of Red Grapefruit Trees Through Nine Years of age in the Lower Rio Grande Valley. Jour. Rio Grande Valley Hort. Soc. 15: 4-1-43. 1961. MAXWELL, NoRMAN P., AND GEoRcE OTEY, Orchard Heating Test in the Lower Rio Grande Valley of Texas. Texas Avocado Soc. Yearbook. 31-32. 1954. NICHOLS, ESEK S., Summary of Fruit-Frost Data Ob- tained in the Lower Rio Grande Valley District Dur- ing Five Seasons Beginning with the Winter of 1931- 32. U. S. Dept. of Agr. Fruit and Vegetable Frost Ser. of the Weather Bureau, Lower Rio Grande Area, Harlingen, Texas. 1937. OLSON, E. O., The Marrs Orange, a Navel-Orange- Sport Variety Popular in Texas. Jour. Rio Grande Valley Hort. Soc. 17: 80-85. 1963. OLsoN, E. 0., W. C. CooPER, N. MAXWELL, AND A. V. SHULL, Survival Size and Yield of Xyloporosis- and Exocortis-Infected Old-Line Red Grapefruit Trees on 100 Rootstocks. Jour. Rio Grande Valley Hort. Soc. 16: 44-51. 1962. OLsoN, E. O. AND BRUcE J. LIME, The Parson Brown Compared with the Pineapple Orange in Texas. J our. Rio Grande Valley Hort. Soc. 17: 86-89. 1963. OLsoN, E. 0., AND A. V. SHULL, Size and Yield of 12- year old Valencia Orange Trees on Various Root- stocks in Presence or Absence of Exocortis and Xylo- porosis Viruses Jour. R10 Grande Valley Hort, Soc. 16: 40-43. 1962. 56 l ORToN, R. B., Letter correspondence on Freeze Pro‘ abilities for the Lower Rio Grande Valley, Septem 11, 1963. A PEYNADo, A., RocER YoUNc, AND WILLIAM C. CooPE A Comparison of Three Major Texas Freezes and’ Description of Tissue Temperatures of Valencia T .1 Parts During the January 9 to 1962 Freeze. J0 Rio Grande Valley Hort. Soc. 17: 15-23. 1963. i PRICE, W. C., Editor, Proceedings Second Confers Citrus Virologists. Univ. Fla. Press, Gainesville, _ 1961. ' RICHARDS, L. A. (Editor), The Diagnosis and I1 provement of Saline and Alkali Soils, USDA Ha i book 60, 1954». a SLEETH, BAILEY, AND E. O. OLsoN, Release of Te Virus-Indexed Citrus Budwood. Jour. Rio Gran Valley Hort. Soc. 15: 19-24~. 1961. I SORENSEN, H. B., Some Factors in Appraisal of Cit if Groves. J our. of the Rio Grande Valley Horticultu. Soc. 15: 32-4-0. 1961. 1 SORENSEN, H. B., Changes in the Cost of Pac ' Grapefruit, 1950-51 Season versus 1959-60 Seas‘ 1962 Jour. of the Rio Grande Valley Hort. Soc. E SORENSEN, H. B., Citrus V-A-L-U Computer. Te Agri. E-Xt. Ser. MP-653. 1963. SoRENsEN, H. B. AND C. K. BAKER, Methods and C of Handling Texas Citrus, 194-6-51. Texas Agr. l Sta. Bul. 771. 1953. SoRENsEN, H. B. AND E. E. BuRNs, Grapefruit Qu Deterioration in the Marketing System—Storage i Shelflife. Texas Agr. Exp. Sta. PR-2151. 1950, SoRENsEN, H. B., AND E. H. HAMMoND, Types f‘ Grapefruit Available in Texas Stores. Texas A Exp. Sta. PR-194~2. 1957. SoRENsEN, H. B., J. M. WARD AND L. H. HAMMO Sales Response of Pink and White Grapefruit Off in Bulk and Bags at Varying Prices. Texas Agri. E“ Sta. PR-1871. 1946. 5 THURMOND, R. V., How to Estimate Soil Moisture Feel. Leaflet No. 355. Texas Agr. Ext. Ser. 1 TURRELL, F. M., AND S. W. AUSTIN, Factors Influ ing Wind Machine Operation. The California Ci graph 4-6 (2) 1 38, 52, 53, 54-. 1960. ‘A I TURRELL, F. M., S. W. AUSTIN, AND R. L. PERRY, F Control. Western Fruit Grower. September. 21-: 1960. TURRELL, F. M., S. W. AUsTIN, AND R. L. PERRY, f’ perature Control in Orchards. Yearbook of the -~ Macadamia Soc. 6: 51-58. 1960. a ‘L, F. M., S. W. AUSTIN, AND R. L. PERRY, _ Heaters, and Wind Machines. The California Hph 46 (5): 154-160. 1961. , F. M., S. W. AUSTIN, AND R. L. PERRY, Pro- _ Orchards Against Cold. The California Citro- Q48 (11): 379, 397. 1963. o ITY OF ARIZONA, Agr. Ext. Ser. and Agr. Exp. iiChemical Weed Control Recommendations for pld Areas of Arizona Bul. A-1. 1962. ITY OF CALIFORNIA AGR. EXT. SER. Weed Recommendations. 1963. ,3 ITY OF FLoRIDA AGR. ExT. SER. Chemical fControl in Citrus Groves. Cir. 224. 1962. l C. K., Fitting of Climatological Extreme Val- Climatological Services Memorandum No. S. Weather Bureau. 1961. CARL, Varieties and S-trains of Citrus Origi- l in the Lower Rio Grande Valley of Texas. ZRio Grande Valley Hort. Soc. 7: 18-24. 1953. CE, J. M., Editor, Citrus Virus Diseases, Pro- First Citrus Conference, citrus virologists, fde, Calif., Univ. of Calif., Div. of Agri. a: Calif. 1959. S, WARREN R., Effectiveness of Freeze Protec- 44 (6): 3, 21, 22. 1963. A. C., Winter Extreme Minimum Temperature 'ility Graphs for Brownsville, Harlingen and frande City. Unpublished. 1963. during the Freeze of Dec., 1962. The Citrus l WOODRUFF, R. E., AND E. 0. OLsoN, Effects of Root- stocks on Physical Characteristics and Chemical Com- position of Fruit of Six Citrus Varieties in Texas. Jour. Rio Grande Valley Hort. Soc. 14-: 77-84. 1960. YOUNG, FLOYD D., Frost and the Prevention of Frost Damage. U. S. Dept. of Agr., Farmers’ Bul. No. 1588. 1947. YOUNG, F LOYD D., AND WAYNE E. HARMAN, Protecting the Citrus Orchard Against Frost. Chapt. 17. The Citrus Industry, Vol. 2, Production of the Crop, edited by Leon Dexter Batchelor and Hubert John Webber, Univ. of Calif. Press, Berkeley and Los An- geles. 1948. YOUNG, ROGER, Climate-cold Hardiness-citrus. Iour. Rio Grande Valley Hort. Soc. 17: 3-14. 1963. YOUNG, RocER, HERBERT DEAN, A. PEYNADo, AND JACK C. BAILEY, Effects of Winter Oil Spray on Cold Hardiness of Red Blush Grapefruit Trees. Jour. Rio Grande Valley Hort. Soc. 16: 7-10. 1962. YoUNc, RocER, AND ASCENSION PEYNADo, Effects of Artificial Freezing on Red Blush Grapefruit. Jour. Rio Grande Valley Hort. Soc. 15: 68-74. 1961. YoUNc, RocER, AND AscENsroN PEYNADo, Seasonal Changes in the Cold-hardiness of ten-year-old Red Blush Grapefruit Trees as Related to Dormancy and Temperature. Jour. Rio Grande Valley Hort. Soc. 15: 59-67. 1961. YoUNc, RocER, AND ASCENSION PEYNADo, Freeze In- jury to Citrus in the Rio Grande Valley in 1963 in Relation to 1961 and 1962 Freeze Injury. Jour. Rio Grande Valley Hort. Soc. 17: 43-54. 1963. 57 w‘ *- ACKNOWLEDGMENT This publication was compiled and edited by Norman P. Maxwell, associate horticulturist, Texas Agricul- tural Experiment Station, Weslaco; and Morris A. Bailey, area horticulturist, Texas Agricultural Exten- sion Service, Weslaco. -