TEXAS AGRICULTURAL EXPERIMENT STATION A. B. CONNER. DIRECTOR College Station, Texas BULLETIN NO. 628 SOIL FUMIGATION FOR PLANT DISEASE CONTROL G. H. GODFREY AND P. A. YOUNG Division of Plant Pathology and Physiology AGRICULTURAL AND MECHANICAL COLLEGE OF TEXAS T. O. WALTON, President APRIL 1943 '7 ‘l “HEW 4) 4L T1,‘? I313 Q“!- K I5\ b B32-343-4M-L180 The control of soil-borne plant diseases by soil fumigation has been shown to be efiicient and practicable in greenhouses, seed beds, cold frames, small gardens, and; for certain crops in the field. Any method of soil sterilization is expensive. Therefore, soil fumigation is ordinarily used only for those crops that are valuable enough to justify a fairly high cost of production. d ‘ The root-knot nematode, root-lesion nematode, tomato wilt fungus, southern blight fungus, damping-off fungi, and weeds usually were con- trolled in soils fumigated with chloropicrin at rates of 2.5 to 4 milliliters per cubic foot (400 to 600 pounds per acre). The root-knot nematode generally was controlled in soil fumigated with carbon disulphide at rates of 1000 to 3000 pounds per acre; with methyl bromide at 165 to 300 pounds per acre; and with pentachlorethane or tetrachlorethane at 2000 pounds per acre. Less satisfactory results were secured vfilth xylene, ethylene dichloride, sodium cyanide and formaldehyde. Paper impregnated with hoof-and-horn glue, casein glue, or vegetable paste, and adequately sealed at the edges, was most satisfactory for con- fining chloropicrin and carbon disulphide in the soil. Ilowever, good results were secured with these chemicals when the fumigated soil was covered with Sisalkraft, asphalt-coated paper, or when the surface of the soil was kept wet. Low concentrations of the fumigants were effec- tive when the fumoigants were tightly confined in the soil. Soil fumigation boxes, made gas-tight by glueing the boards together and sealing the cover, were very effective for confining fumigants, to kill plant-disease fungi and other pests in potting soils. Detailed directions for the fumigation method of soil sterilization are given together with an outline of the necessary precautions. CONTENTS Page introduction - 5 Experiments at Weslaco __ _- 11 Fumigation of Soil in Gas-tight Containers ________________________________ __ 11 Fumigation of Outdoor Plots 16 Experiments at Jacksonville 20 Fumigation of Field Plots -- 20 Control of Damping-off Parasites 26 Factors Influencing the Effectiveness of Soil Fumigation __________________ __ 27 cost of Soil Fumigation 32 Practical Uses" for Soil Fumigation 33 General Directions 33 Precautions 35 Physical Properties of Soil Fumigants 36 Dosages and Conversion Figures 36 Literature Cited 38 SOIL FUMIGATION FOR PLANT-DISEASE CONTROL“ G. H. Godfrey and P. A. Young, Plant Pathologists Division of Plant. Pathology and Physiology Under natural conditions, soil that is used for growing cultivated plants is inhabited by many forms of plant and animal life. Most of these are beneficial, such as the nitrifying bacteria, humus-decaying or- ganisms, and earthworms. In the same soil destructive organisms are also commonly present including certain fungi, bacteria, and nematodes that cause plant diseases. Undesirable weeds are also frequently present. This bulletin deals with soil fumigation designed especially to control these disease-producing organisms that infest the soil. It is based on experi- ments conducted in the last seven years at the Tomato Disease Laboratory at Jacksonville, Texas, and in the last four years at the Lower Rio Grande Valley substation at Weslaco, Texas. Also, a compilation is presented of data gathered from all available sources on preferred methods of soil sterilization by fumigation. Soil sterilization for the control of various soil-borne organisms detri- mental to plants has been practiced for many years. The root-knot nema- tode, Heterodera marional (Cornu) Goodey, probably is the most destruc- tive soil-inhabiting plant pest that has been combated in this way (3, 68) b. In the South. the root-knot disease caused by this nematode affects a wide range of plants in fields, nurseries, and home gardens (68). In northern states, it is serious in greenhouses. The root-lesion (or meadow) nematode, Pratylenchus pratensis (de Man) Filipjev, has been reported on an increas- ingly wide range of plants, and occasionally is serious in nurseries and fields. The bulb and stem nematode, Ditylenchus‘ dipsaci (Kuehn) Filipjev, also is largely soil-borne, and several other species of nematodes cause dam- age to special crops. The sugar beet nematode (Heterodera schachtii) is an- other destructive species. Many kinds of soil fungi and bacteria cause root decay, wilting, and other signs of plant injury; others cause damping-off of seedlings and young plants. Persistent weeds such as nutgrass (Oyperus rotundus L.), Johnson grass (Sorghum halepense (L.) Pers.), crab grass (Digitaria sanguinalis (L. Sco.p.), and others, many of which have deep underground stems and rhizomes that make weeding by cultivation ineffec- tive, are frequent obstacles in plant culture. All of these many soil-inhabit- ing pests may be controlled by soil fumigation. Various methods of soil sterilization have been used. Heat was probably first employed by burning brush or straw on the surface of the ground. As this treatment did not penetrate dee-ply enough into the soil, methods of applying live steam and drenching with hot water were developed and “Acknowledgment is made of certain materials and applicators that were furnished for this work by Innis, Speiden & Co., New York City; Free- port Sulphur Co., New York City; R. & H. Chemicals Dept. 0f E. I. du Pont de Nemours Co., Niagara Falls, N. Y.; Dow Chemical Co., Midland, Mich.; and Thompson-Hayward Chemical Co., Houston, Texas. bNumbers in parentheses refer to Literature Cited. 6 BULLETIN NO. 628, TEXAS AGRICULTURAL EXPERIMENT ‘STATION “both of these methods are still used commercially (44, 56, 30). Recently, heat sterilization by means of a system of electrically heated units in the plant bed or soil box has been developed. Liquid soil drenches with disinfecting chemicals such as mercuric chlo- ride, formaldehyde, various organic mercury compounds, cyanide compounds, sulphuric and other acids, and emulsions of chloropicrin and carbon disul- phide, all have merit for specific purposes and are used extensively (25). Figure 1. Tomato root-knot caused by the nematode, Heterg- dem martoni. Large fleshy galls of this type are frequently produced by the root-knot nema- tode on many herbaceous plants, such as beans, to- matoes and watermelons, All soil drench treatments require large amounts of Water, however. lair" adequate distribution of the sterilizing agent through the soil, and a long period for drying of the soil is required before planting. Surface applications of cuprous oxide, organic mercury compounds, and zinc oxide to the soil are used to control damping-off fungi (70). Soil sterilization by ‘fumigation has a distinct advantage in that only relatively small quantities of the chemical need be applied, as contact with the soil organisms is obtained by diffusioniof the gas. Furthermore the soil may be planted within a few days after treatment. Hydrocyanic- acid gas, carbon disulphide, chloropicrin, ethylene dichloride, methyl bromide, and paradichlorobenzene are the most important chemicals used for soil fumigation. All of these compounds produce gas immediately after application, and penetrate the soil thoroughly from the points of application (18, 22, 52). Much of theearly literature on chloropicrin was concerned with its manufacture, physico-chemical properties, and possible uses for plant pest control (32, 47, and 48). Chloropicrin did not become valuable as a soil fumigant until adequate methods were developed for confining it ‘SOIL FUMIGATION FOR PLANT DISEASE CONTROL 7 in the soil. The most efficient practical method found consisted in covering of the soil with paper bearing a gas-confining coating of hoof- and-horn glue, casein glue, or vegetable paste (17, 23, 72). A cheaper, easier but less dependable method was the use of the wet-soil seal (water seal) for confining gases that are insoluble in water (72). Soil fumigation with chloropicrin has been especially useful in con- trolling root-knot nematodes (18, 45, 58, 71, 72, 73). However, nema- todes that are very deep in the soil may escape injury by fumigation (66). Although the plow-sole layer may limit gas penetration (65), many sandy fields" do not have a plow sole. Fumigation is used extensively to control nematode-s in greenhouses (30, 44, 46, 53), but living plants must be ' removed from the greenhouse before fumigation, as chloropicrin kills green vegetation (16, 19). In the field, certain crops are so vaFuable that they justify fumigation of acres of land. Increased yields of several farm crops resulted from chloropicrin fumigation of soil in fields (23, 18, 34, 28, 29, 39, 43, 50). Thousands of acres" of pineapple land were fumigated with chloropicrin in Hawaii, often by injections through mulch paper (33, 34). The fumi- gation of small spots in fields for setting widely spaced plants has been done in some cases, and this method has commercial possibilities (43). Spot treatments were adequate in controlling root knot in a field of watermelons in Georgia (63). - . In addition to killing fungi and other pests, chloropicrin has been known to kill some of the nitrifying bacteria in the soil, but there was also some increase in ammonia nitrogen in the soil after fumigation (51, 54, 55). Chloropicrin fumigation is desirable because it results in partial sterilization of the soil and permits good growth of plants immediately after the gas has evaporated (51). Fumigation is therefore superior to ordinary heat sterilization of soil which kills all of the beneficial organ- isms and brings about changes in the soil that make it unfavorable for the growth of plants. ' ' Besides killing root-knot nematodes, chloropicrin fumigation is also effective against the root-lesion or me-adow nematode (21) and the bulb nematode (6, 7, 8). As a further aid in plant disease control, this fumi~ gant often controlled species of fungi in many experiments (19, 70, 71, 72, 2, 13, 9, 12, 35, 36, 38, 42). Since weed control* is a main item of" expense in farming it was of value to learn that chloropicrin killed most of the seeds, bulbs, and underground root-stocks of weeds (20, 71, 40, 69). In addition, chloropicrin also controlled insects and centipedes that dam- age certain plants (41, 42, 49). Successful soil fumigation depends on proper conditions among which temperature and moisture are important. Soil temperatures continuously above 68° F., and soil moisture of 10 to 3O percent (depending on soil type, fumigant, and kind of pests present) are optimum conditions for control of soil organisms by chloropicrin (40, 51). Dosages have been fWeed control by soil fumigation is discussed in the following book that arrived after this bulletin was completed: Robbins, W. W., A. S. Crafts, and R. N. Raynor. Weed Control. McGraw-Hill Book Co., New York. 1942, l 8 \ BULLETIN NO. 6218, TEX-As AGRICULTURAL EXPERIMENT STATION Figure 2. Rose roots with galls caused by the root-knot nematode. tode galls such as these usually occur on woody plants (also on cotton). Small nema- calculated and spacing of injection holes de-termined for various conditions (72, 57, 6, 46, 61). The methods of soil fumigation are readily adapted for practical use in many places (24, 34, 19, 20, 72). as“, W. SOIL FUMIGATION FOR. PLANT‘ DISEASE CONTROL Figure 3. Boxwood plant (above) showing effects of root- lesion nematode, Pratylenclz/us pratensis, Note tufted condition of small rootlets. Below, separate rootlets from plant at top showing lesion type of injury and bunchy condition of the smaller roots caused by this nematode. (Specimen from Smith County, Texas, 1939.) 10 BULLETIN NO‘. 628, TEXAS AGRICULTURAL EXPERIMENT STATION Sometimes fumigants are mixed before application. Root-knot and bulb nematodes were controlled by a mixture of chloropicrin with ethylene dichloride, while ethylene dichloride alone was sometimes ineffective (7, 8, 45, 58). In some tests, gasoline appeared to help chloropicrin in controlling nematodes (72). Soil fumigation with ethylene dichloride or paradichlorobenzene is the usual method of controlling peach tree borers in orchards in the eastern half of the United States (52, 14). Carbon disulphide (locally called “highlife”) is very useful as a fumi- gant for controlling root-knot nematodes in the soil, but it was ineffec- tive against weeds, the Fusarium-wilt fungus, and damping-off fungi (24, 25, 60, 68, 58, 59, 70, 71, 72, 73). However this chemical con- trolled Armillaria root rot (64) and. meadow nematodes (59, 60). Par- tial control of nematodes was obtained by evolving carbon disulphide gas" from potassium xanthate (11). In most tests, calcium cyanamid was unsatisfactory as a soil fumigant (31, 72, 56). However, a mixture of chloropicrin and cyanamid gave best results in killing fungi, nematodes, and weeds in tobacco seed- beds (5). Z ' Hydrogen cyanide derived from calcium cyanide apparently was the first soil fumigant used in America to control pests in the soil (67, 4, 10, 42). The methodof liberating this gas in the soil was improved by combining solutions of sodium cyanide and ammonium sulphate in the soil (68, 72). This method is in use for controlling nematodes in the field in Florida (68). Methyl bromide is the most promising soil fumigant used in recent experiments in'which it showed special advantages (22, 61, 62, 58, 26, 27, 49, 15). It is toxic in low concentrations, and penetrates the soil thoroughly because of its low boiling point, high vapor pressure, and low molecular weight. However, liquid methyl bromide boils at 40° F. so special equipment is necessary for handling this chemical. It is in com- mercial use (26). When properly confined, formaldehyde. controls soil ‘pests satisfactorily and has been used extensively in greenhouses (25). However, it failed to control root-knot nematodes in the field (56, 72). Certain fungi were controlled by formaldehyde in carbon disulphide emulsion (25). Tetrachlorethane and pentachlorethane controlled soil disease-producing organisms in some experiments, but these chemicals were slow to evap- orate from the soil, due to their low vapor pressures and high molecular weights ( 1, 13, 74). However, tetrachlorethane was ineffective against root-knot nematodes according to one report (37). Dichloroethylether had some value as a nematocide (26), and orthodichlorobenzene killed alfalfa snout beetles (49). SOIL FUMIGATION FOR PLANT DISEASE CONTROL 11 EXPERIMENTS AT WESLACO Fumigation of Soil in Gas-Tight Containers In this series of tests, special methods were used as follows: The soil was Victoria fine sandy loam infested either naturally or artificially with root-knot nematodes, and was loose and fairly dry, containing usually about 8 to 10 percent moisture by weight on an air-dry basis. Containers were Cyanogas drums 22 inches high and 14 inches in diameter, contain- ing 2 cubic feet of space. Unless otherwise specified, the chemicals were injected at two depths, equidistant from each other and fromthe top and bottom of the drum. This placement of the fumigant required diffusion of only 10 inches to reach all parts of the soil mass. After injection, the drums were sealed immediately with glue-coated Kraft duplex paper. Sealing was efficient in every case, as manifested by the strong odor of gas when the covers were removed, usually 4 days after treatment. The soil was ventilated for two or three days. Soil samples were placed in flower pots for the setting of indicator plants——usually tomato plants that had been started in sterilized soil. Data on the condition of the indicator-plant roots were take-n after 30 days of growth, or later if tem- peratures were continuously low. In the tables‘ (1, 2, 3, and 4), number ratings are given for degrees of nematode infestation, ranging from 1 (slight) to 6 (extremely heavy). A test with chloropicrin, carbon disulphide and ethylene dichloride for root-knot control. This test was conducted in November, 1939. Starting with nematode-infested soil, supplementary inoculation was provided by mix- ing into the soil macerated infested tomato roots. Because the roots were not completely decayed, the efficiency of the fumigant treatment was de- creased, which explains the poor results shown by chloropicrin in Table 1. Under the conditions of this test, with a large population of root-knot nematodes and eggs protected by undecayed roots, chloropicrin at 400 pounds per acre was not as efficient.as carbon disulphide at 2000 pounds. The rate for ethlyene dichloride was high; but the fact that it was so efficient in nematode control was an important lead to further experimen- tation. V, Efiects of soil fumigants at different concentrations on root-knot and southern blight. The conditions "for this experiment, conducted in May and June, 1940, were similar to those for the previous experiment, except that the temperature was considerably higher, and root materials in the soil were well decayed. Dry sclerotia of the southern blight fungus, S. rolfsii, inclosed in cheese-cloth bags, were introduced into all of the cans. Re- sults are shown in Table 2. Under the conditions of this test, the lethal dosage of ethylene dichloride for nematodes was between 1000 and 2000 pounds per acre,‘and fon the dry sclerotia, more than 2000 pounds per acre. With carbon disulphide, 1000 pounds per acre killed all the nema- todes but this treatment did not kill 'the fungous sclerotia, while 2000 pounds per acre killed both. With chloropicrin, 500 pounds per acre killed the nematodes, but one sclerotium of the fungus survived in one of the two treated drums‘. Table 1. Effects of soil fumigation on root-knot nematodes and growth of tomato indicator plants in flower pots. Rate of Application Fresh weight of stems and 1eaves—0z. Nematode Severity)‘ Chemical Ml. per Lb. per Pot N0. Pot No. _ cu. ft. acre ft. 1 2‘ 3 ‘ 4 Av. 1 2 3 4 Av. None (cheek)_ ___________________________ __ ___ ___ 0.60 0.35‘ 0.70‘ i 0.90 0.64 0 6 6 6 6.0 Chloropicrin _____________________________ __ 2.5 400 1.05 1.12 1.05 0.95 1.04 4 3 5 3 3 8 Chloiro~picrin* _____________________________ __ 2L5 400 0'. 75 0. 80 1. 35 1 . 05 0.99 4 3 5 2 3 .5 Chloropierin emulsion _____________________ _-. _________ __ 2.51 400 0.75 1.05 01.70 1.10' 0.90 3 T 3' 4 4 3 5 Carbon disu]phide_ ____________________________ __ 10. 6‘ 2000 1.05 0.60 1.25 0.85 0.94 T 0 r 0 0 O 0 Ethylene , dichloride ______________________________ __ 50 6000 1.40 0.80 1.35 1.90 1.38 0' 0 0 0 O Vaporite’; ________________________________ __ 30 2722 _ 0.75 0.85 1.30 0.40 0.82 i 6 4 6 1 4 2 *This chloropicrin treatment. at the same rate as the first. was at onc point only, the center of the drum. ‘(Scale of severity from disease free (0') to extremely severe infection (61). IVaporite is an English preparation used for the control of soil pests, including nematodes; it contains 15.75 perrcnt naphthalene, and 16 per- cent creosote oils as active ingredients. ZI NOILViTlS LLNEIWIHHEIXEI TVHfLT/IHOIHBV SVXEKL ‘839 'ON' NLLEYITHEI SOIL FUMIGATION FOR PLANT DISEASE CONTROL 13 Table 2. Efiects of various soil fumigants on southern-blight sclerotia. and root-knot nematodes. ‘ Rate of application Survival Chemical Ml. per Lbs. per S.“ i H. _ cu. ft. A. ft. rolfsni limarionlt i None (check) _____________________________________________ __ — — 5 5 Ethylene dichloride _______________________________________________ _- 8.23 1000 3 f 1-5 ” _______________________________________________ __ 16.46 2000 2 i ” _______________________________________ _-_ ______ _- 32.92 4000 0 0 Carbon disulphide ______________________________________________ __ 8.29 1000 3 0 ” _ _____________________________________________ __i 16,57 2000 0 0 Ohloropicrin ______________________________________________ __l a. 17 500 o+tr.§ 0 ” ______________________________________________ __ 6.35 1000 0 ’ 0 tBased on growth of sclerotia in plate cultures. IRoot knot on indicator plants. Scale 0 to 5. Average of 5| plants. Elfects of soil fumigants on root-knot- nemat-odes, nutgrass, and sclerotia of the fungi causing southern blight and cotton root rot. Two tests were conducted in the summer and fall of 1941, with methods similar to those previously described. As the boiling point of methyl bromide is very low, it was introduced into treatment drums in a low temperature room at - the ice house in one case; in the other, it was poured quickly into cold soil from a bottle that had been kept in the freezing compartment of a refrigerator. This soil was then placed in the center of the drum, which was quickly filled and sealed. In the first test the nematode population was less than anticipated, and tomatoeslas indicator plants in the non-_ treated soil gave readings of only 5, 8, 7, 95 and 15 galls respectively, in 5 separate pots. Sclerotia of S. rolfsii also appeared to have been killed throughout, except in the control drum. The other test was conducted in October and November, 1941, using soil taken from a nematode-infested snapdragon bed. The soil was excessively moist when moved, and in drying to a point suitable for fumigation, the nematode population appears to have been greatly reduced. Sclerotia of the cottonroot rot fungus, Phymatotrichum omnivorum, from flask cultures on sorghum seed and soil, and corms of nutgrass were introduced into all cans, in small paper bags. Also, some large nematode galls (578 inch in diam- eter) in paper bags were inserted into the drums treated with methyl bromide, with the highest rate of chloropicrin, and in the control. Readings on, survival of the fungus were taken by plate culture methods, after a period of ventilation to rid the material of absorbed fumigants. Readings on nematode survival were delayed for two months because of an extended period of cold weather that was obviously suffi- cient to retard nematode activity. Results are shown in Table 3. For complete root-knot control, the necessary concentration of ethylene l4 BULLETIN NO. 628, TEXAS AGRICULTURAL EXPERIMENT STATION Table 3. Efiects of soil fumigation in sealed containers on cotton root rot sclerotia, nutgrass, root knot, and growth of tomato indicator plants. (Average of 5 plants in each case.) Rate 0i application Survival Height of plants Chemical ‘ after l | P. om- _ ' 1 6O days, Ml. per f Lbs. pert n1vo- 1\ut- H. nia-l inches cu. it. A. ft. 1 rum grass rioni {._._ l 1 a None (check) ______________________________ __I 0 f 0 y’ 5 l 5 1.6 9.0 l ' ; Ethylene , I dichloride- ______________________________ "J 8.3 l 1000 5 1 3 tr. 11.2‘ l ” - ............................... 10 ‘ 1200 l 0 0 tr. 10.6 . I \ i ” _ ______________________________ -- 11.0 1400 1 0 1 0 tr. 10.3 " - ................................ .- 13.3’ 1000 f 0 0 0 111* ‘ l Ohloropicrin- _____________________________ “'1 3.75 600 0 0 0 11.1‘ 4' l ‘ ” _ _____________________________ -- 5.0 r s00 l 0 I 0 0 1 11.s* 5 1 Methyl 1 l l 1 a bromide ................................. -- 3.75} see 1 0 l 0 1 0 1 111* *All significantly higher than the control plants. dichloride in this experiment would appear to be between 1400 and 1600 pounds per acre foot. Both concentrations of chloropicrin and the methyl bromide completely killed the nutgrass, the nematodes a11d sclerotia of the cotton root rot fungus. The difference between readings 1 and 2 in nematode infestation amounted to 2.5 galls per pot. As the root systems of all plants were vigorous, penetrating the soil completely in 7-inch flower pots, even thereadings of 2 appeared hardly detectable. It scarcely seems possible that so 10w an infestation could measurably affect the growth of the plants. For this reason, the fumigation apparently gave benefit in addition to nematode control. The results from inoculation of soil with large galled roots that had been exposed to certain of the treatments are not shown in the table. The Table 4. Efiects of soil fumigation of a flower bed with chloropicrin and ethylene dichloride on nematode infestation 0f tOmatO indicator plants. Chemical Lb. per 1 Cover Nematode ' I A. ft.* . survival - l 1 None (check) ----------------------------------------------- u} 0 ‘None 5-0 Ohloropicrin- _____________________________________________ --i 500 ‘lifiid-Iglflfivdt bllril 1 4 I 3p figlllfl 911a; . " _ __________ __-_ ________________________________ __ 500 Sisalkraft paper i 0 Ethylene 5 dichloride»- _________________________ _: ___________________ -_A 1560 Bag material 2-6 ” _ _______________________________________________ __3 1500 lsisalkraft paper 2.2 *An acre-foot designates the top foot 0t soil over an entire acre. \ ‘SOIL FUMIGATION FOR PLANT‘ DISEASE CONTROL 15 special gall material was removed, broken into small bits, and mixed into pots of sterilized soil for separate readings on survival. The» methyl bro- mide treatment killed all stages of the nematodes, permitting the indi cator plant to grow without a trace of root infestation; the strongest chloropicrin treatme-nt showed a trace of infestation and the comparable nontreated root galls produceda heavy infestation. Methyl bromide showed great power of penetration, with lethal effects on nematode, larvae and eggs. In a further test with methyl bromide, conducted in June, 1942, soils with 10, 121/2‘, 15, 17%, and 2O percent moisture content on the air-dry basis were fumigated in drums of two cubic feet capacity at the rate 0f 2% milliliters per cubic foot. The chemical was introduced into the previously sealed drums (Figure 8) by means of a special applicator whereby a measured amount is forced out by its ownvapor pressure through a 14-inch copper tube inserted through the paper. The tube was then rre-moved and the hole quickly sealed. Various disease organisms had been introduced into the soil, in paper bags buried 4 inches deep and 12 inches in a direct line from the point of- introduction of the gas. The introduced organisms were: moist pure-culture sclerotia of the- cotton root rot fungus "and of the southern blight fungus in cotton-stoppered test tubes; root-knot nematodes in old decayed root galls mixed with soil; root-knot nematodes in freshly collected tomato root galls 1%. inch in diameter; and ten recently dug nutgrass corms. After three days’ ex- posure to the gas in the sealed drums, the organisms were removed and tested for viability. .As tested by the~usual methods (nutrient agar plates for the fungi, and indicator plants for the nematodes with readings at 30 days) the sclersotia of the fungi and all nematodes in both lots were killed in all the fumigated soils. The nutgrass corms were discolored in_ ternally, and failed to grow after planting in pots of soil. In contrast, Figure 4. Marigolds (above) and stocks . (below) growing in soils which . were initially heavily infested, with root-knot nematodes. Left. in each case, soil fumigated with chloropicrin at about 5 milliliters per cubic foot; right, nontreated soil. Note the improved growth of the plants in the fumigated soil. 16 BULLETIN NO‘. 628, -T‘EXAS AGRICULTURAL EXPERIMENT STATION all organisms in comparable nontreated lots of soil survived. This test constitutes further proof of the high penetrating power and high fungi- cidal and nematocidal value of methyl bromide, even in soils of relatively high moisture content, when the container is adequately sealed. Funfigation of nematode-infested greenhouse soil. Figure 4 illustrates marigolds (Tagetes sp.) and stocks (Mathtola tncana) about six weeks after planting in soil fumigated with chloropicrin, as compared with nontreated, nematode-infested soils. Striking differences in growth are evident, due to control of the root-knot disease. Fumigation of outdoor plots In these tests the chemical was injected 6 inches deep into the soil with a Vermorel Injector in measured dosages in holes 12 inches apart, followed by closing- of the hole by pressure of the foot. As the operator proceeded along the plant bed, a gas-confining paper cover was placed over the bed in such a manner as to leave a 6-inch border on either side, extending down into a trench at the border (Figure 6). Soil was thrown onto the edge of the cover to hold it. in place. After the bed was com- pletely treated with the cover in place, with edges uniformly buried, the trench surrounding the bed was thoroughly wetted, thus making a wet- ’ soil barrier against the escape of gas, extending both laterally and down- ward below the edge of the cover for several inches. The efficiency of such a barrier was repeatedly demonstrated by the strong odor of the chemicals when the covers were removed four days later. Effects of chloropicrin fumigation of nematode-infested soil on subse- quent growth of sweetpeas. Attention was called to failure of sweetpeas in a nursery in 1938-39, and examination disclosed heavy nematode infestation of all of the plants. In August, 1939, one of the beds was fumigated with chloropicrin at the rate of 4 milliliters per square foot of surface. A board frame surrounding the bed was sealed at the corners and at all junctions with adhesive tape. This was the base to which was glued a sheet of glue-coated, gas-impervious duplex Kraft paper. A 6-foot sec- tion in one end of the bed was left without treatment. The ground sur- rounding the treated bed was kept thoroughly moist. After a week of aeration following the 4-day treatment period, sweetpeas were planted. The precaution of Rhizobium inoculation was taken because of the known lethal effects of chloropicrin on this nitrogen-fixing-bacterium (23). The 24-foot section of the bed which was treated produced vigorous plants that attained a height of 7 feet, and bore flowers profusely from Septem- ber to April. The nontreated end of the bed produced plants only 3 feet high, and these began to die early, with much less flower production for the season. Nematode infestation was heavy in the nontreated portion but lacking or very light in the treated end. This fumigation of the soil with chloropicrin gave practical control of root knot. Control of the root-lesion nematode, Pratylenchus pratecnsis, in an egg- plant seed bed and in a chrysanthemum bed. Following repeated failures to establish eggplants in a seed bed at Substation No. 19, Winter Haven, a SO1L FUMIGJYFION FOR PLAA-NY1‘ID‘ISEAA_%E CONTROL l7 heavy infestation of the soil by the root-lesion nematode was found in May, 1937. Areas were fumigated with chloropicrin by standard methods, with adequate covers and wet borders. Plants set later in the treated beds grew rapidly from the start, in marked contrast to plantings in non- treated soil. This indicated this nematode was readily controlled by chloropicrin. Chrysanthemums in a 30-foot bed at a commercialnursery near Wes- laco had failed completely, with little or no commercial flower produc- tion. Examination disclosed the root-lesion nematode to be very abundant in the roots. Just prior to the 1939 planting, water was withheld from one plot, and it was deeply spaded twice to permit drying 0f the deeper layers. The soil was then fumigated with chloropicrin, using procedures" as in the sweetpea experiment. On the second day after treatment, a heavy rain loosened the paper cover at several points". These were again sealed and the cover left on four days longer. After aeration, 20 varieties of chrysanthemums from pots of sterilized soil were planted in rows across the 4-foot bed and the rows continued across a nearby bed of non- treated soil. Thirty days after planting, the plants in the treated bed had an average height of 14.6 inches as compared with an average height of 10.4 inches Figure 5. Chrysanthemums growing in soil that had been infested with the root- lesion nematode, Pratylenchus pratensis. Right: Soil fumi-gated with chloropicrin before planting. Left: nontreated soil. The plants in the left background were greatly dwarfed, and those in the left fore- ground largely killed by the infestation. 18 BULLETIN NO‘. 628‘, TEXAS AGRICULTURAL EXPERIMENT STATION in the nontreated plot (Figure 5). Increase in height did not adequately depict the difference in growth, as all of the branches were likewise longer and the plants consequently wider in expanse of growth. Also, in all cases there were more branches and flowers on the plants in treated soil. More striking than the difference in growth was the difference in longev- ity of_the plants, which was evident only 3 months after planting. At that time 96 percent of the plants were alive in the treated bed and only 20 percent were alive in the nontreated soil. Examination of the roots of sickly plants in the nontreated bed showed hcaxy infestation with the root-lesion nematode. There was very light infestation or none on plants in the treated bed. (Tontrol of root-knot-nematode in snapdragons. In August, 1941, snap- dragons died prematurelyand failed to produce good flowers at a florist’s nursery near Weslaco. Examination of the plants disclosed an extremely heavy infestation of root-knot nematodes. The bed was spaded twice in order to dry out excess moisture and then bedded with a 6-inch furrow on both sides. 'After three Weeks, chloropicrin at 500 pounds per acre, and ethy- lene dichloride at 1500 pounds per acre, were injected into different por- tions, of the bed, and each portion separately was covered with covers of gas-impervious materials. At time of treatment the soil moisture at 8-inch depth was about 8.5 percent on an air dry basis. It was much wetter at lower depths. When the covers were removed four days after treatment, the odors" of both gases were strong. After allowing several days for the gas to evaporate, pots of the treated soil were planted with tomato plants grown in nematode-free soil. At 30 days, readings on these plants were taken as shown in Table 4. In this test to show the relative efficiency of an acid-proof cloth bag material, as compared-With Sisal- kraft, the latter proved to be the better material for this purpose. Also, chloropicrin at 500 pounds per acre was definitely superior to ethylene dichloride at 1500 pounds in controlling root-knot nematodes under the conditions of this test. Sweet peas planted in September in the treated beds flowered profusely throughout the winter, indicating practical nema- tode control. Fumigation of‘ home flower beds with chloropicrin to eliminate nutgrass, Bermuda grass, and annual weeds. In a home flower garden at WVeslaco, nutgrass (Uyperus rotunclus), Bermuda grass (Oapriola (lactylorzi), and other weeds had become increasingly bad from year to year, requiring much hand labor to keep the beds in presentable condition. In October and November, 1941, several beds, approximately 4x30 feet, were fumigated with chloropicrin at the rate of 500 and 600 pounds per acre, using standard procedure. The covers used were 5-foot strips of Sisalkraft, a strong paper heavily reinforced with sisal fiber impregnated with asphalt- like material between two sheets of Kraft paper. At the end of eight weeks following treatment, the freedom of the treated beds from trouble- some weeds was strikingly evident. Only occasional nutgrass plants de- veloped and these always came from deep corms. In the upper layers of soil many dead corms were found. Bermuda grass, likewise, was almost SOIL FUMIGATIOL FOR PLANT DISEASE CONTROL l9 Table 5. Effect of chloropicrin fumigation on prevalence of Weeds 111 two separate flower beds. l 1 Nunibirs of plants per Lb. per ‘ sq. yd. Annual weeds Chemical A. it. i——~————~~————-~~~—-* gAv- of , Nut- Bermuda a plots) l grass grass Two months after treatment No. 1 i ~ None (check) ________ ........... -1 0 . 14.2 i 25.2 93-2 Chloropicrin ______________________ -_ 600 1 1 . 8 0-6 2 ~ 4 Four months after treatment No. 2 i 1 1 i None (check) _____ __' ______________ n] 0' g 0* 30.0 p 91 Ohloropicrin _______________________ -1‘ 500 0 ~ 1.0 § 6.1‘ *T‘his bed contained no nutgrass plants. completely eradicated. Counts of weeds in comparable treated and non- treated areas are given in Table 5. In one case the paper cover, upon re- moval, was dragged onto the lawn where it laid over night and the next day. Much top burning of the grass resulted. The release of the chloro- picrin from the asphalt material in the paper in which the gas had been absorbed was sufficient to cause the injury. After the first mowing, the injury was no longc" evident. The cost of chemical used for 120, square feet of bed, at the retail price for small quantities, was $1.60 to $1.80, depending upon rate of application; the cost of the cover, which was used repeatedly in different plots, was $1.50; the time required for treatment was less than an rour for one man. The few remaining weeds were readily removed. An ap- preciable saving in hours of labor compared with that required by hand weeding through the season was indicated. Furthermore, the injury to -growing plants brought about by repeated weedings was eliminated. For weed control alone, plant-bed fumigation apparently was‘ justified in this case. In addition, however, superior plant growth resulted even though the presence of nematodes or other plant pests in the ground may not have been recognized. During the preceding year, it had been difficult to control nutgrass, Bermuda grass, and annual weeds in this bed. Four of the above beds had been planted the year before (1941, a wet season) with Dutch bulbous iris, and when the bulbs were dug in June, a loss of approximately 50 percent from southern blight (S. rolfsii) had occurred. The same beds, fumigated with chloropicrin at 500 pounds per acre by standard methods were planted to‘ Wedgewood iris in Novem- ber and December, 1941. When dug in May, 1942, only 5 out of 780 plants showed S. rolfsii infection, and these were all growing at the margins of the beds where the soil had been wetted for gas confinement at the time of fumigation. In a comparable non-fumigated bed, 42 out of 127 iris' plants (33 percent) were killed by southern blight. In this test, satisfactory control of the disease in out-door beds was obtained by chloropicrin fumigation. \ 20 BULLETIN NO. 628, 'l“EXAS AGRICULTURAL EXPERIMENT STATION EXPERIMENTS AT JACKSONVILLE Fumigation of Field Plots Th'ese experiments were conducted on Norfolk fine sand and Ruston fine sandy loam soils in a single field in which most of the tomato plants had wilt, caused by the fungus, Fusarium lycopersici Sacc., and root knot. The soil .in the experimental plots (40, 50, or 80 square feet) was loosened t0 a depth of 10 inches in preparation for fumigation. Most of the large roots and lumps of soil were removed from these plots and the chemicals were injected at an 8-inch depth. Seed were planted in the soil one or two weeks after treatme-nt and the test plants were dug and examined 2 to 31/2 months after soil fumigation. Table 6 gives the standard con- ditions covering the results presented in Tables 7 to 9. The soil was dry or=slightly moist at the time of fumigation with a moisture content ranging from 2.8 to 4.4 (air dry basis) in Series 9, 11, and 13. Fumigation was started in soil that was 650 F. or warmer. How- ever, colder weather occurred within the 5-day period of fumigation in Series 5, 9 and 13, which helped some organisms to survive [the fumiga- Table 6. Standard data on series of soil treatments at Jacksonville. Series Date of Soil T., l Plot I - Sealing _ No. treat- deg. F‘. l border Soil cover of Applicators ment I covers 1 8-6-36 90 to 100 Boards lGlue-coated paper Banked Tube-peg boardi‘ __ H 77 77 H 1 77 g ti)??? 3g t8 g3 I ” I ” ” wet soil ” Carbona Prod‘ 4 7-26-37 90 to 100 l " Glue-tar paper ” ” " ” " 5 3-14—38 55 to 83 ‘ ” Duplex glue paper ” ” " " " 6 ! I! 7! 7! Y! , 97 i! Y1 7! 91 7 68 to so ‘ s! I s! n 11 n n 1v ‘vennorel 8 8O l! l! 77 9! i) 71 71 97 Y7 9 3-2840 58 to 83 iSheelt metals ” 1 ” ” ” ” Nailed " " 10 5-11-40 65 to 86» rDitc ‘ pecia cloth ” ” ” 11 8-26-40 72 to 90 Sheet metaliDuplex-glue paper ” Iseo Larvajectort 12 7-26-41 88 to 103 ” ” l ” ” ” Larvajector R. 1a 3-24-42 4s to es i ” " = ” ” ” " " 14_ 4-6-42 50 to 70 Ditch lWet soil, only ____ Mack’s Antiwecd t} I Gun *F'or applications with the tube-peg board and Carbona Prod, the mixture of 15‘ percent chloropicrin with 85 percent white gasoline was used {For the Vermorel Pal and the first model of the Larvajector, the fumigants were mixed with 4 percent refined cottonseed oil for lubrication of the applicaator. tion. In Series 5, the soil was too cool and moist during fumigation, and a total of 6.56 inches of precipitation from four rains within two weeks probably washed soil from unsterilized land into the treated plots. Re- sults from Series 5 are included to show the degree of root-knot control when conditions were unfavorable (Table 9). Sheet metal borders, 36 inches wide were set 30 inches deep in the soil to prevent the entrance of gophers, moles, worms‘, and nematodes from the outside through the soil or along the roots of the test plants. The tops of the metal sheets were nailed to Wooden borders. Immediately after fumigation, most of the plots‘ were covered with glue-coated paper SOIL FUMIGATION FOR PLANT DISEASE CONTROL 21 Figure 6. Above, soil plots that were fumigated with chloropicrin and carbon bisulphide, and immediately covered w i t h glue-coated paper. Photo- grflllhed at Jacksonville, March 18, 1938. Below: a Single plot at Weslaco pre- pared for fumigation. A. r011 of gas-impervious paper has been placed in position and the furrow around the p_1ot is; deep enough for covering the edges of the paper. to delay the escape of the fumigants. Because of the cost of this paper, the cheaper, wet soil (water seal) method Was tested for comparison and used as follows: . The top half inch of the soil was wetted before the fumi- gants were injected, and then the soil was soaked to depths, of two or three inches for about three days. This confined chloropicrin and carbon disul- phide fairly well, as both are insoluble in ‘water. OnLly rough lumber was used in the experiments at Jacksonville, so it was impractical to seal the edges of the cover papers onto the boards with glue. In some cases, wood strips were nailed over the edges‘ of the paper; in other cases, the paper was extended over the boards‘ and the edges were banked with moist soil. In no case was it practical to use the almost perfect seal as recommended in our description of most desirable methods. Imperfect sealing of the edges of the paper cover permitted leaking of fumigants which decreased their concentrations and efficiency. The tables show the concentrations of chemicals’ necessary for good re- sults under these conditions. Paper covers were left on the soil for five days to delay the escape of the fumigants. Thereafter, the covers were removed and the soil was stirred to accelerate the evaporation of the fumigants from the soil. The odors of the fumigant-s usually were notice- l 22 BULLETIN NO. 628, TEXAS AGRICULTURAL EXPERIMENT STATIOB Q able at this time. Other information on materials a11d methods for soil fumigation has been published previously, (Young, 72). ln these treatments, the following volatile liquids (which change to gases after injection) were tested: chloropicrin, carbon disulphide, ethylene dichloride, pentachlorethane, tetrachlorethane, and xylene. Other chemicals tested were: formaldehyde (37 percent solution) applied at the rate of 1000 pounds per acre; sodium hydroxide (2 percent solution) was sprinkled on the soil; powdered Aero Cyanamid was thoroughly mixed with the upper 10 inches of soil (in amounts that made it unfavorable for plant growth); and a powder named IN2391-A2 from E. I. du Pont de Nemours Co., was mixed thoroughly with the upper 6 inches of soil and the soil was abundantly irrigated. In a few tests, hydrocyanic acid was generated in the soil by applying'a solution of sodium cyanide, watering the soil abundantly, then watering the soil with a solution of ammonium sulphate, and finally soaking the soil with water (68). Dixie Queen watermelon, Greenpod okra, and Whippoorwill cowpeas were used as indicator plants to test the effectiveness of fumigation in controlling nematodes. The roots of the test plants were dug and washed for examination. Percentages of plants with severe root knot, as shown in Tables 7, 8, and 9, indicate the serious damage from root Table 7. Efiects of soil fumigation with different concentrations of chloropicrin and carbon disulphide on watermelon root knot and. weeds. Senes 3, l Number of Percentage of -Pounds Kind of l plants with Chemical per cover l Water-Johnson root knot _ * acre Plots melon ’ grass Other ~‘ -———j— l l l plants , stems weeds 1 Total l Severe l l l l None (check)- ....... -_ 0 lAir 10 l <12? l 055 l 212s 12.5 35.5 1' l l Ohloropicrin _________ __ 400 Glue-paper 1 2 83 2 92 0 0 " ......... -_ s00 ” l 4 1 22s 2 l 220 3.4 0.2 ” _________ _- 200 " l 2 l 11s l 0 l 100 12.4 0.0 " _________ __ 100 l ” 1 1 1 01 l 72 l 1s 52.5 l 27.9 - ‘ l ‘ | l Carbon disulphide---" 2000 ” l 1 f s0 i s l 226 1.2 l 0 ” -__-_ 2000 Water l 2 1 10s l 17 2200* 0.0 l 0 ” 1500 Glue-paper‘ a 15o 2 15s? 0.0 l 0 " _____ 1500 Water l 2_ l 93 17 l 2200* 2.2 l 0 ” _____ 1000 iGluepaperl 3 l 156 3s 139s 1.3 l 0 ” -____ 1000 lWater ‘ 2 l 104 12 2200* 0 l 0 " --__- 500 lGlue-paper l 3 3 175 12 1783 77.7 l 42.9 u *Est1'n1ated. knot and the efficiency of the control methods. Root knot was classified as severe when the roots showed many knots, or knots 1A, to 1 inch in diameter. The disease was classified as a trace when a plant showed only one or a few knots 1/32 to 1,0 inch in diameter. Control of root knot with chloropicrin. Soil fumigation with chloropicrin at rates of 300 to 600 pounds per acre, with effective covers, usually con- trolled root knot satisfactorily, as indicated by the small amount of severe root knot on the test plants in treated plots. These results are given in Tables 7 and 8, which contain data only from plots in which ... “llama l <1 W, n’, l SOIL FUMIGATION FOR PLANT‘ DISEASE CONTROL 23 conditions favored effective fumigation. Table 9 includes tests in which certain conditions were unfavorable to the best action of some chemicals. The different concentrations were grouped in this table on the basis of similar effectiveness. The nematodes were perfectly controlled in many of the test plots. Applications of 100 to 200 poundsof chloropicrin per acre usually did not give good control. The gasoline-chloropicrin mixture contained nearly six times as much gasoline as chloropicrin in Series 1 to 6, and by using 300 pounds of chloropicrin per acre, the treatment also included 1700 pounds per acre of gasoline. Root knot was well con- trolled in many plots fumigated with this mixture, but the low concen- tration of chloropicrin generally was less effective in later plots in which the chloropicrin was used alone. This indicated that they gasoline aided the chloropicrin in killing root-knot nematodes. In the later series, con- centrations of 400 to 600 pounds of chloropicrin per acre were nece-ssary for best results (Table 8). Soil with old nondecomposed roots or cold damp soil were unfavorable for efficient fumigation with chloropicrin (Table 9). Root knotlwas extremely severe in the comparable untreated plots in all of these tests. Control of root knot with carbon disulphide. Fumigation of soil with 1000 to 3000 pounds of carbon disulphide per acre controlled all or nearly all of the root-knot nematodes in plots with satisfactory covers (Tables 7, 8, and 9) while 500 to 800 pounds of carbon disulphide per acre usually failed to give satisfactory control. Carbon disulphide was ineffective against weeds (Table 7). ‘ Table 8. Effects of different chemicals and varying concentrations 1n controlling root knot. Period of years’ summary. l l Number of Percent of plants Pounds ‘ Serie Plots Plants With- With root knot Chemicals per acre No. out -—-————— l root Trace Severe - l knot l , None .......................... 0 All l 130 1188s 0 20 so chloropicrin--- _____________ -. im-iso l 2,2 2 131 I a2 f 1s 20 " __ .............. _. 200-250 3,4,7»; l 11 1845 , s4 v 12 4 " __________________ _- 3004,20 l 232,040 a 317 66123 l s1 l 0s s " .................. -. 3511-750 1,4,0=-11 33 7320 l 00 3 1 Carbon disulphide- __________ -_ s00 c l s 175 22 25> 4a " ___________ __ 150 4 * 2 l 130 92 s 0 " ___________ __ 1000-2200 1.s,4,0 15 l 120s 9s 2' 0 " ........... -_; 75012000 2,4,6 l 11 1130 97 a s 0 . 1 l _ Ethylene dichloride __________ --; 1500 ll l 4 l 1.004 22 31 47 Sodium cyanide ______________ s00 4 s l 167 l so 1 s s " .............. -- 1200 4 s l 10s.; 71 l 2o s s l Cyanamid ____________________ 1000 4 l 1 09 l 45 , so 19 " ____________________ 1500 l 4 l 1 11 ; 4s 54 0 Formaldehyde ................ 1000 , 6 s 724 l as 44 2a Sodium hydroxide_ __________ -.l 1470 ~ 5 - 2 172 l 0 l 1s s7 DuPont IN2391-A2 ........... _- 500 13 l 1 l 115 l 91 l 0 a 24 BULLETIN NO‘. 628‘, TEXAS AGRICULTURAL EXPERIMENT STATION Control of root knot with other chemicals. Tetrachlorethane, pentachlo- rethane, and xylene at concentrations of 1000 and 2000 pounds per acre controlled the nematodes satisfactorily in most cases. Xylene evaporated from the soil so promptly that seed would germinate in the soil only five days after it was ventilated and planted, but tetrachlorethane remained toxic and noticeable in the soil for 10 days or longer, and pentachlore- thane was toxic 18 days after the covers were removed. The slow evapo- ration of these latter chemicals" is correlated with their low vapor pres- sures (Table 12) and seed =planted too soon in-soil treated with these materials were killed. 'Hydrocyanic acid gas released from sodium cyan- ide by ammonium sulphate reduced severe root knot to 3 percentof the plants, but left the soil very hard with black and white patches. This treatment added a ton of soluble salts per acre to the soil, and even after several inches of rain with supplementary irrigation, the toxic chemicals were not sufficiently removed to permit good growth of plants. Cyanamid failed to control root knot, and prevented good growth of watermelons. Sodium hydroxide (soda lye) apparently did not decrease the abundance of the nematodes in the soil. Although in laboratory tests by other workers, ethylene dichloride and formaldehyde controlled soil parasites, neither of these chemicals controlled nematodes satisfactorily in the field plots at Jacksonville. DuPont IN2391-A2 in a metal-bordered plot gave good control of root knot (Table 8), but was unsatisfactory in the plots with board or ditch borders (Table 9). Confining fumigants in the soil. Different kinds of covers were tested for confining chloropicrin and carbon disulphide in the soil. No important difference in thoroughness of disinfection of field plots were found to be due to differences between hoof-and-horn glue, casein glue, or vegetable paste on the cover papers, so the summary data were calculated together for the different groups of plots covered with these materials. Papers with these three kinds of gas-proofing materials were most satisfactory in aiding the fumigants to control root knot. Asphalt-covered paper gave fairly good results. In Series 10, a. special cloth cover was only partly impervious to chloropicrin and with this cover the chemicals con- trolled only 86 percent of the root knot. Tests were conducted to determine whether the water seal (wet-soil cover) would effectively confine gases in the soil for root-knot control. The wet-soil seal gave good results in most of the test plots. This pre- viously published conclusion was confirmed in Series 9. The wet-soil seal is practical for soil fumigation where the expense is to be minimized and perfect control of root knot is not required. As the comparable tests with glue-coated paper covers gave results only a little better than the wet-soil seal, data. from both kinds of covers were calculated together in Tables 7 to 9. Odors of fumigated soils. Most plots fumigated with chloropicrin, tetra- chlorethane, and pentachlorethane emitted strong oders of these chem- icals when they were uncovered within four days after fumigation was started. Soil fumigated with either chloropicrin or carbon disulphide SOIL FUMIGATION FOR PLANT DISEASE CONTROL ' 25 Table 9. Control of root-knot nematodes by ‘soil fumigation under unfavorable conditions. . ‘Percentage of Pounds Sipecial conditions during Series Number of plants with Chemical per acre and following treatment No. --- ——-f root knot Plots Plants f Trace] Severe i None ________________ __ 0 Cold soil, followed by rain 5 25' 1978 14 ' 74 Ohloropicrin ________ _- 300 ” 5 9 1135 25 | 20 . Carbon disulphide_-_- 3000 ” 5 3 387 8 1 ” ___- 1000 ” 5 9' 1318 I 34 14 None ________________ -_ 0 Soil with undecayed root I knots 12 10- 864 ; 4 91 Pentachlorethane.--" 21000 ” 12 3 495 } 1 0 Tetrachlorethane- __-- 2000 ” 12 3 533 \ 3 1 Xylene _______________ __ 2000 ” 12 2 156 J 7 3 (Jhloropicrin ........ -_ 480 ” 12 2 288 12 , 47 None“; _____________ -_ 0 Rain 2 days after treat- _ rnent 1a 11" 554 \ 3 f 94 Chloropicrin ________ __ 4.80 ” 13 2 453' , 0.2 1 Tetrachlorethane- _ , 1000 ” 13 2 222 1 0 Pentachlorethane 1000 ” 13 2 164 ‘ 21 0 Xylene _______________ __ 1000 ” 13 2 111 4 2 1 DuPont IN2391—A2_--- 500 Wooden border 13 1 89 7 9 None ________________ __ 0 Ditch borders; heavy rains 14 3 196 2 9'7 Carbon disulphide__-_ 1000 ” 14 4 398 13 14 DuPont IN2391—A2_--- 500 " 14 2 _ 207 23 69 retained a characteristic odor unlike these chemicals or unfumigated soil, after the true odor of the chemicals was no longer present. ‘ Spacing of injection holes. In Series 1 to 6, injection holes were 15 inches apart, and were 12 inches apart in Series 8 to 14. Chloropicrin was in- jected into holes 12 inches apart for comparison with injection holes 15 inches apart in Series 7. (Table 6.) There was no apparent difference in effectiveness of the chemical due to this difference in the spacing of the injection holes. The 10-inch spacing recommended in other states is not necessary in the sandy soil of East Texas. Weed control. With glue coated paper covers, chloropicrin at rates of 300 to 600 pounds per acre usually controlled most of the weeds, especially Johnson grass, crab grass, and species of Cenchrus and Amaranthus, as was shown mainly in Series 3 and 5 (Table 7). At the rate of 100 pounds per acre, chloropicrin did not control weeds, and none of the concentra- tions of carbon disulphide controlled weeds. Because dry weed seeds are resistant to chemicals, poor weed control was obtained by fumigating dry soil. Thewet-soil seal was ineffe-ctive in facilitating weed control. Fumigation of plant beds. Using glue-coated paper covers, seven hot beds and fourteen cold frames were fumigated with chloropicrin at rates of 400 to 600 pounds per acre each year from 1937 to 1942, with satisfactory con- trol of tomato diseases. The greenhouse ground bed at Substation No. 11 was fumigated with chloropicrin at 400 pounds per acre in 1939 and 1940, and with carbon disulphide in 1942 with satisfactory results. 26 BULLETIN NO. 628, TEXAS AGRICULTURAL EXPERIMENT STATION Control of (damping-off parasites Many experiments were conducted from 1936 to 1942 on the control of damping-off of tomato and cabbage seedlings by soil fumigation and seed treatment. Fertile soil abundantly infested with damping-off fungi, especially species of Pythium and Rhizoctonia, were used in these tests each year, Metal trays each holding 25 pounds of this soil were used as containers. Conditions favoring post-emergence damping-off prevailed in _ the experiments in 1937 and 1938, as the trays were kept in the labora- tory building where the seedlings received insufficient light (50-300 foot candles) which resulted in spindly seedlings. In 1939, the trays were kept in an out-door hot bed with adequate light and ventilation. Tomato seed for every tray (except those treated with formaldehyde) was dusted with red Cuprocide at the rate of 21/2 percent (by weight of seed) before planting. The different treatments were: '(1) Dry soil in an iron barrel with glue-coated paper cover was fumigated with 10 milliliters of chloropicrin per cubic foot of soil for 4 days. An excess of fumigant was used because the cover could not be glued to the top of the barrel. (2) Dry soil was also fumigated with 1 milliliter of carbon disulphide per pound of soil in the iron barrel covered with glue-coated paper. (3) Moist soil was mixed with 100 milliliters of 6 percent formaldehyde solution per tray, seeds were planted‘in the tray 1 day later, and the soil was watered abuntantly. (4) Semesan suspension (0.25 percent) was sprinkled at the rate of 142 milliliters per square foot on the soil immediately after the seed was planted, and this treatment was repeated seven days later. (5) Red Cuprocide suspension (0.25 percent) at the rate of 568 milli- liters per square foot was sprinkled on the soil immediately after the seed was planted, and this treatment was repeated seven days later. The population of seedlings changed from day to day, being increased by delayed germination of seed, and decreased by post-emergence damp- ing-off,_after an initial stand of seedlings was obtained. The seedlings) were counted every seven to ten days. As used here, the term “emerged” means the largest number of seedlings found in a given tray at any count. The “percentage of emerged seedlings” was based on the number of seeds planted per tray, and the control of pre-emergence damping-off was sum- marized from these data. A different basis was necessary for calculating percentages of seedlings with post-emergence damping-off, as the counts came from emerged seedlings that were visibly damped-off. Hence, the total number of seedlings with post-emergence damping-off divided by the largest number of seedlings found in the tray at any one time gave the percentage of seedlings with post-emergence damping-off. Because of the different basis of calculation, there was no relation between-the per- centages with pre-emergence and post-emergence damping-off. Data on the damping-off experiments are summarized in Table 10, in which each percentage is an average of the data from 2 to 4 trays each planted with 500 tomato seeds. Chloropicrin was the most effective of l SOIL FUMIGATION FOR PLANT DISEASE CONTROL 27 Table 10. Chemical treatment of soil to control damping-oft‘ of tomato seedlings. Percentage of secd- Percentage of seedlings lings emerged lost by postemergenee Chemical damping-off 1937 1938 1939' 1937 l 1938 1939 ’ i 1 I None- ___________________________________ ...... __ 19 3e 25 4o § 7s 1 , . Chloropicrin- ______ __‘ ___________________________ __ 49 l 70 64 2 18 O Carbon disulphide .............................. __ __ 51 __ _- 50 _- \ Semesan _________________________________________ __ 32 '1 58 54 4 l 25 1 ' l Cuprocide _______________________________________ -_ 32 62 __ ? l 10 __ Formaldehyde- __________________________________ __ __ 73 31 __ 20 I 0 the chemicals used in the soil to control damping-off of seedlings. Carbon disulphide was unsatisfactory for this purpose. Formaldehyde gave ex- cellent results in 1938, but killed most of the seeds in 1939. Thus, for- maldehyde was erratic and sometimes very injurious in its effects. Seme- san and Cuprocide were both valuable in decreasing post-emergence damp- ing-off of seedlings. Fumigation box. At‘ Jacksonville, soil for pots, flats and greenhouse benches was conveniently fumigated in a gas-tight box 3x3x3 feet built of center-match lumber glued together a.nd well painted. The soil was fumigated on January 27, 1942 at the rate of 7 milliliters per cubic foot. The Llarvajector was used to inject the chloropicrin into holes one foot apart horizontally, at the 6, 18, and 30-inch vertical levels. The tightly fitting lid was placed on the box and sealed onto the sides of the box with glue-coated paper sold as French tape. Due to the cold weather, 60 feet of electric heating cable was arranged in the central part of the soil in the box and maintained the soil at 76° to 102° F. during fumigation. After a week of fumigation, the soil was ventilated and transferred to the laboratory hot bed that had been disinfected with creosote. The tomatoes grew well in the fumigated soil, and they were healthy when transplanted into a cold frame in March. FACTORS INFLUENCING THE EFFECTIVENESS OF SOIL FUMIGATION Soil type. A loose permeable soil such as sandy loam is most easily fumigated and is most likely to show good results from fumigation. Heavy clay soils are apt to show erratic results unless soil preparation has been such as to leave the soil in a uniformly loose condition to the depth to which sterilization is desired. A plow-sole may limit penetra- tion of fumigating gases, so that organisms below that layer may remain alive and cause infections in subsequent -crop plants. In any type of soil, fumigating should give effective res-ults if moisture, temperature and tilth are correct and if gas confinement is efficient. 28 BULLETIN NO. 628, TEXAS AGRICULTURAL EXPERIMENT STATION Soil moisture. Fumigants insoluble in water cannot effectively penetrate wet soil. Also, these fumigants often fail to kill large weed seeds and the sclerotia of certain fungi in powdery, dry soil because seeds and sclerotia are resistant when very dry. For best fumigation, therefore, sandy loam soil should have sufficient moisture to partially hold its shape when a sample of it is squeezed in the hand. Usually, a moisture con- tent of 5 to 15 percent on the air-dry basis is satisfactory Heavy clay soil should be nearly dry, as its colloidal properties make it nearly im- pervious to gases when more than slightly wet. Effective pest control in layers of wet soil cannot be obtained with fumigants that are- insoluble in water. Soil temperature. For best‘ results with chloropicrin, the temperature of the soil at a depth of six inches should be warmer than 65° F‘. Rapid- ity of pe-netration and killing increase with rises in temperature. Potting soils“ can be fumigated in warm weather and stored for winter use. Most of .the fumigants are inefficient below 450 F. Carbon disulphide, be- cause of its much higher vapor pressure, penetrates better than chloropi- crin at the lower temperatures. Methyl bromide has the lowest boiling point of all the fumigants tested and therefore gives uniformly good re- sults" at lower temperature. At 50° F. it is effective in killing insects, and probably would be lethal to nematodes and fungi also. Soil preparation. Any fertilizers and conditioners such as sand, peat, or manure should be added to the soil before fumigation, and the soil should be loosened by plowing or spading to the depth to which effective pest con- Figure '7. At left, three kinds of injectors of soil fumigants. Vermorel Pal in- jector (left); Isco I-arvajector, (center); and Mack Anti-Weed Gun (right). At right, the Isco I-arvajector in use. Immediately after the chemical is injected the soil is. covered with glue-coated paper the edges of which are covered with soil and wetted down. SOIL FUMIGATION FOR PLANT DISEASE CONTROL I 29 trol is desired. Large lumps should be broken or removed. Following the removal of the previous crop at least two Weeks should elapse before fumigation to permit sufficient decay of roots infested with parasites. Efiective nematode control usually cannot be obtained if undecayed roots of a previously infested crop remain in the soil. ' Injection of chemicals. For potting soils in drums, barrels, or tight boxes, or for small plots of ground, the chemicals can be poured directly into holes, which should be immediately closed. A small commercial ap- plicator called “Larvajector Jr.“ screwed onto a bottle, makes a con- venient apparatus for injecting measured dosages into small volumes of soil. Large applicators can be purchased that are conve-nient for large scale applications. The Larvajectorl, Vermorel Pali’, the Carbona Prod“, Mack’s Anti-Weed Gun", (Figure 7) are designed to insert measured- dosages as‘ deeply into the soil as required, leaving only a small opening on the surface, which can quickly be closed by a thrust of the foot. These applicators are suitable for chloropicrin, carbon disulphide, tetrachlore- thane, pentachlorethane, xylene, and ethlyene dichloride. Methyl bro- mide, with a boiling point about 40° F., requires special injection methods Figure 8. A Dow Chemical COHIPMIY methyl-bromide-app11cator in operation. The methyl bro- mide liquid is forced by its own pressure from the Orig? inal container into a closed, calibrated cylinder in quantl- ties as desired. By Opening‘ the second tap, the liquid 1S forced through a. copper tube into the soil at any desired depth. (Figure 8). Injection of this chemical as a gas directly under a gas- impervious cover is an efficient means. of application. In some experi- ments, cold methyl bromide from a container kept in a refrigerator was applied first to nearly frozen soil in a small container and then quickly 1Sold by: Innis Speiden Company, 117 Liberty St., New York; Southern Con- struction and Mill Supply Company, Houston, Texas. “Sold by P. E. Lirio, Vineland, N. J. “Sold by Wheeler, Reynolds, and Stauffer, San Francisco, California. ' is below 65° F. at the time of treatment; 30 BULLETIN NO. 628, TEXAS AGRICULTURAL EXPERIMENT STATION transferred to the large container of soil which was immediately sealed. This method gave excellent results. Rates 0f application. In soils heavily infested with nematodes, weeds, or disease-producing fungi or bacteria, effective treatment cannot be ex- pected With less than 400 pounds of chloropicrin or 1000 pounds of car- bon disulphide unless gas confinement is unusually tight, and other factors are nearly optimum. Dosages at least 50 percent larger than these rates should beapplied when: (1) it is impracticable to seal the gases com- pletely in the soil during the period of exposure; (2) the temperature (3) the moisture content of the soil is above the optimum; (4) the soil is of a heavy clay type; and (5) highest efficiency in pest control is of greater importance than economy of materials. With methyl bromide, the same general principles govern rates of application. However, preliminary results that have been reported indicate that this chemical is effective at smaller dosages than the other chemicals. One milliliter of methyl bromide per cubic foot of soil, equivalent to 166 pounds per acre "foot, is very effective. Methods of gas confinement. Two methods of gas confinement have been found to be practicable; a gas-impervious cover and a water barrier. For most thorough pest control, the surface of the soil should be promptly and completely covered with glue-coated paper or some other gas-imper- vious material. This must be adequately sealed at the edges and ends 0f the treated plot, either by sealing it to a previously prepared board border or by burying the edges of the paper to a depth of 5 inches and saturating the soil around the entire border with water. Several paper companies‘ manufacture either a gjue-coated single sheet or a duplex Kraft paper with glue as an adhesive for holding the two sheets together. Both are efficient for gas confinement, but neither will withstand re- peated use. A water-resisting paper of the Kraft duplex type, suitable for repeated use, would be highly desirable. Another material, in com- mon use for other purposes in greenhouses is Sisalkraft’, a very strong duplex paper fortified by sisal fibers impregnated with an asphalt com- pound. It is not completely desirable because of the absorption of chloropicrin by the asphalt. However, its strength and water resisting properties, and the fact that it can be used repeatedly, make it useful for confining the gases". Practical tests in field plots have shown that this paper confines the gas adequately for four days. Other materials that have been used are rubberized materials (not sufficiently impervious tmehloropicrin) and cellophane covered fabrics or cellulose acetate im- pregnated materials, which are not sufficiently durable. to the commercial use of the gas impervious coverl is its cost and the ex- pense of application. It is to be recommended primarily for seed and nursery beds and for special nursery and home gardens in which practi- cally complete control of pests is highly desirable. lThe Western Waxed Paper Co., North Portland, Oregon; Chase Bag Co., 1111 Ifian§ar St., S. Dallas, Texas; Arkell Safety Bag Co., 10 E. 40th St., New York. ‘m. Sisalkraft 00., 205 W. Wacker Drive, Chicago, Ill. An objection- 4A SOIL FUMIGATION FOR PLANT DISEASE CONTROL- 31 With the water-seal method, the top half-in-ch of soil is" wetted before fumigation, the fumigant is injected, andthe soil then is soaked to a depth of one inch and kept wet for the entire period of fumigation. A modification of this is to cover the soil with newspaper, burlap, peat moss, or manure, which is kept thoroughly wet during the period of fumigation. A Weakness" of the method is the danger of permitting the covering layer to become dry on a sunny day or during a period of low humidity before the fumigation is complete. Because the gases quickly escape from the soil through such materials when dry, thus lowering the efficiency of fumigation, repeated sprinklings are required. Another ob- jection is that nematodes, weed seeds, and other pests within the layer of wet soil are protected from the fumigating gas. However, the water seal (wet soil) method has been found to give 95 percent or better con- trol of nematodes, which is adequate for most annual crops. .It is not satisfactory for‘ weed control. The method is less costly than one requir- ing an expensive covering material. A reliable criterion as to the effectiveness of gas-confining measures is the presence or absence of the characteristic odor of the gas‘ in the soil at the close of the fumigation period. Effective pest control can be ob- tained only if, within two days after application, the odor of the gas is strongly present in the soil. If the odor is still present at the end of four days, very good results may be expected. Gas elimination. Gas-confi_ning covers should be removed after four or more days. If the wet-top-soil method has been used, drying of this layer can be hastened by cultivation. With most of the fumigants, complete elimi- nation of the gas can be expected in two to four days, provided correct con- ditions of moisture and temperature were maintained at time of application and later. Soils that were too wet, too cool, or sticky when treated, or that were wet by rains following the treatment, require one of three weeks be fore planting. Sometimes reworking the soil may be necessary. It is not safe to plant seed or set plants in fumigated soils when the characteristic odor of the gas used can still be detected in a handful of soil taken from a depth of eight inches. A Effects of fumigation on the soil. Chloropicrin, carbon disulphide, ethy- lene dichloride, xylene, and methyl bromide all evaporate almost completely, and leave no toxic residues in the soil. They do not change the physical structure of the soil. Plants grow well in soils fumigated with these chemicals probably because of the reduction or elimination of plant pests. Prevention of recontamination. Care should be taken to prevent recon- tamination of fumigated soils-. Recontamination commonly occurs by ((1) direct mixing of nontreated soil with the treated soil; (2) use of contaminated tools that were used recently in infested soils; (3) move- ment of parasites in soil by water flowing from infested areas onto the treated plots; (4) invasion by organisms from nontreated adjacent areas; (5) storage of treated soil in contaminated containers and (6) movement of gophers and moles through the soil from infested areas. 32 BULLETIN NO. 628, TEXAS AGRICULTURAL EXPERIMENT STATION Pots, flats, and tools can be fumigated readily in a fumigation box or bin, or they can be dipped in any strong disinfectant such as‘ 5 percent formaldehyde solution before being used with fumigated soil. Greenhouse benches can be cleaned and disinfected by forcing live steam intoall cracks and corners, by hot water, or by any strong disinfectant. Only plants from treated soils should be set in the fumigated soil. Border contamination can be avoided by inserting a barrier, even a strip of tar paper, vertically into a trench 2 or 3 feet deep around the edges of the treatedsoil. Soil fumigation may be effective through several plantings until the soil becomes recontaminated, or until a residue of infestation increases to injurious proportions. COST OF SOIL FUMIGATION All treatments that control soil-borne plant pests are expensive. How- ever, the co'sts of soil fumigation compare favorably with those of other methods that are equally effective. In choosing a method, factors be- sides cost should be considered. Effects on the soil and date of safe planting following treatment are important. If the fumigation method is selected, the choice of the fumigant should be based on the nature of the predominating pest to be combated as well as on the -cost of material. If only one type of organism is to be fought, the cheapest available ma- Table 11. Comparison of fumigants as to effectiveness and the relative cost of treatment. ; . l 1 Lb. er Effectiveness against l: Cost Chemical 1000 ; ; sq. ft. j Nema- - Per. lb. Per 1000 l 1 todes Fungi Insects Weeds bulk i sq. it. l 1 Ghloropicrin ______________ "l 9 +++* +++ +++ +++ $ 0.80 s 7.20 Carbon disulphide ........ 21 ++ + +++ - .15 3.15 Ethylene dichloride ______ h! s4 1 ++ r 1 +++ A - m 2.38 Methyl bromide __________ _- 3.6 +++ ? +++ ? .65 2.34 l i Tetrachlorethane _________ --j 46 g +++ ? 1 ? +++ .19 ’ 8.74 > Pentachlorethane _________ _ -1 46 I + +.+ ? ‘B ? .20 9.20 *+*+t+' represents high killing power, ++ moderate, and + low effectiveness. terial that is effective will probably be selected. If more than one organism is to be controlled, the-n a fumigant that will be general in ef- fectiveness would be desirable. Other factors to be considered are: ready availability of the fumigant and applicators, ease of application Without danger to the operator, amount of risk involved such as the explosion hazard and danger to nearby growing plants. ' E Table 11 lists the fumigants considered in this bulletin, their range of practical effectiveness in pest control (indicated by + signs), and the approximate cost per thousand square feet of soil surface. a ‘SOIL FUMIGATION FOR PLANT DISEASE CONTROL 33 PRACTICAL USES FOR SOIL FUMIGATION Seed beds, hot beds and cold frames. Disease-free seedlings for fieltl planting are very important. Fumigation is probably the most efficient and economical method for adequate sterilization of seed-bed soils. Nursery plant beds. Profit from nursery plants often depends upon free- dom from nematodes and other pests. Soil fumigation can be used ad- vantageously in the nursery plant bed, the cutting bench, and the nur- sery row. I Home flower and vegetable gardens. Because home flower and vegetable gardens usually are limited in size and location, it is often important to eliminate nematodes and other pests from the soil. The cost of treatment is often a minor factor. Soil fumigation is probably the most practical means of eliminating soil-borne injurious organisms in home gardens. Commercial flower and vegetable gardens. Considerable work in vege- table gardens in the eastern states has shown substantial profits from soil fumigation with chloropicrin. Orchards. Efficient spot treatments, which enable shrubs or trees to get a good start, may make the difference between fair plants or none in new plantings of peaches, figs, papayas, or other nematode-susceptible trees in root-knot areas. A Greenhouse benches. The cost of soil renewal and replacement, involving the movement by hand of tons of soil is one of the main cost items in the greenhouse culture of plants. Proper fumigation of greenhouse soils should eliminate ‘completely the need for the removal of the soil, leaving only the maintenance of adequate organic material, acidity and fertility to kee-p plants in good condtion. Greenhouse potting soils. Fumigation of potting soil is believed by the writers to be one of the most widely useful forms of soil sterilization. With proper equipment it is simple and inexpensive (Figures 8 and 9). GENERAL DIRECTIONS FOR SOIL FUMIGATION Ground plots. For seed beds, nursery plant beds, nursery rows, or for home or commercial flower and vegetable gardens, use soil fumigation only when the soil is fairly dry, uniformly loose, free from lumps and undecayed roots of a previous infested crop, and preferably when the temperature is 70° F. or Warmer. Insert the chemical at recommended dosages, at points approximately twelve inches apart and quickly apply a means of sealing the gas into the soil. After four days remove covers and culti- vate if necessary to accelerate gas elimination. Plant in the soil only after the odor of gas has completely disappeared. Details of the methods have been given in the descriptions of experiments. Greenhouse benches. Use methods as for ground plots, governing dosages by the volume of soil to be treated. Methods should be adapted to the bench construction. All cracks in the bottom of the bench as well as the edges‘ of the covering material should be sealed to prevent escape of the 34 BULLETIN NO. 628, TEXAS AGRICULTURAL EXPERIMENT STATION gas. Some modification of standard bench construction may contribute to best fumigation-gas confinement. _ Greenhouse potting soils. Use only airtight containers and apply dosages in accordance with cubic-foot contents". For small operations several large garbage cans "or barrels capable of being completely sealed would be adequate equipment. In the containers illustrated, glue-coated paper was used for sealing. After the chemical is injected preferably at levels not greater than twelve inches apart, the covering material is glued tightly into place. Canner’s label-lap-end paste is an excellent, quick drying ad- hesive for this purpose. French tape may also be used on smooth surfaces. Tying covers with a wire or a string does not provide sufficient sealing. For large amounts of soil, one or more especially built boxes like the one shown in Figure 9 would be desirable. Such boxes must be airtight in construction. The box shown was built of center mat-ch flooring with a liberal application of carpenters’ glue between the boards. Specifica- tions for a two-cubic-yard box will be furnished upon request to the Lower Figure 9. A soil fumigation box of two ' cubic yards capacity, with gas- impervious lid. The lid is made to rest upon cleats within the box, so that strips of adhesive Paper tape will readily seal the outside edges. Rio Grande Valley Experiment Station, Weslaco, Texas. For operators who use from one to several cubic yards of soil a day, a numberof such boxes would be required. While one box is being used a second could be _ undergoing fumigation, and a third could be aerating. Precautions must be observed if this method is followed. For winter use a warm room ad- jacent to the head house and just back ofthe potting bench would be de- sirable. Boxes could be dumped by block and tackle to hasten gas elimi- nation following treatment. Ventilation fans may be desirable when the soil boxes are _in an enclosed room. ' i SOIL FUMIGATION FOR PLANT DISEASE CONTROL 35 PRECAUTIONS Although precautions in handling soil fumigants-‘vary with the different chemicals used, certain general precautions are advisable for all materials. General Precautions Avoid spilling the chemicals needlessly. Use a funnel for pouring liquid fumigants from one container to another, or into applicators. In outdoor work, keep containers at arms length, and work on the Wind- ward side 0f the chemical. For all indoor applications requiring more than a minute or two, it is advisable to use the gas mask prescribed by the manufacturer for the particular fumigant used. Use commercial applicators Whenever the extent of operations justifie-s the cost. Wash applicators thoroughly after use with gasoline or kero- sene to remove all traces of the chemical. Remove the Washing material by ejecting it as in regular use. _ Finally operate the machine with motor oil, leaving a portion of it in the apparatus to keep the valve gaskets soft and‘ in working order. Specific precautions Carbon disulphide is highly inflammable and explosive when mixed With air. All possible contact with sparks or open flames should be avoided. Breathing the gas momentarily is not dangerous. _ Chlloropiclin is highly toxic but is non-explosive. It has been used as a war gas but it is not dangerous when ordinary precautions are taken, as no one can willingly remain in a room with more thana small fraction of a dangerous concentration. A concentration as low as seven parts per million in the air will cause smarting and watering of the eyesl A deep breath Will produce violent coughing, but a deep breathing of pure air afterward will bring quick relief. Repeated exposure to the gas for several days in succession is to be avoided, for injurious" effects are cumu- lative and have been known to be fatal. In confined places such as greenhouses, a gas mask must be worn. Chloropicrin is injurious to liv- ing plants in very low concentrations. All plants must be removed from a greenhouse even if only a portion of the soil in the house is to be fumigated with this chemical. When the gas is being used outside, every care should be taken to avoid the possibility of leakage of the gas into a greenhouse with valuable plants. Soil fumigation with chloropicrin kills the beneficial nodule-forming bacteria (Rhizobium $12.), so if sweetpeas, lupines, or other legumes are to be planted, the seed or soil should be inoculated with the proper nitrogen-fixing bacteria. Ethylene dichloride has an odor and physiological effects like chloro- form. Prolonged exposure to the fumes should be avoided. It is non- inflammable. - Methyl bromide is toxic to people, but there is little danger if ordinary precautions are taken against breathing the fumes. For indoor use, a gas mask should be Worn. Because of its low boiling point, methyl bromide evaporates immediately upon exposure to open air at ordinary ~ 36 BULLETIN NO‘. 628, TEXAS AGRICULTURAL EXPERIMENT STATION temperatures. It is Well to become thoroughly familiar with the direc- tions provide-d by the manufacturers before fumigating with this chemical. PHYSICAL PROPER-TIES OF SOIL FUMIIGANTS The important physical properties of the leading fumigants discussed in this bulletin are recorded in Table 12. The figures for vapor pressure Table 12. Physical properties of soil fumigants. i Specific Weight i Boiling Vapor pressure I Specific gravity as per point i ———— gravity Chemical a liquid gal.-—lbs.*;——-——-i 20°C. i 25°C‘. of gas . “C. i - (air=1) i i i Carbon g 1 Y disulphide ____________________ __ 1 .2561‘ 10.48 i 46.2 i 297.5 ‘ 361.0 2.8 Ghloropicrin______~_ ____________ __ 1,671 14.00~ ‘ 112.0 i 12.2 i 24.0 5.7 i i ’ i Ethylene i i i dichloride- ................... -- 1.2571 10.53 p $3.5 60.4 i 77.1 3.5 Methyl . i ‘ i bromide ______________________ -_ 1.7321 14.47 ; 4.5 1315.0 1 1824.0 3.3 Pentachlorethane ______________ -_ 1.6811‘ 14.03 i 1601.5 i 3.0 4.0 7.0 Tetrachlorethane- _____________ __ 1.5961 13.32 i 146.3 5.1 i 6.8 5.8 *1 gallon of water weighs 8.34\p-Ol1I1dS. tAt 20° O. A are of particular interest in that they (together with molecular weights) constitute a measure of the penetrating power of the gases. Carbon disul- phide and methyl bromide, for example, have greater vapor pressures and lower molecular weights than chloropicrin and, if adequately confined, their powers of penetration are very great. The low vapor pressure and high molecular weight of pentachlorethane explain its delay in evaporat- ing from the soil. DOSAGES AND CONVERSION FIGURES For the convenience of the operator, Table 13 gives the correct figures for converting standard dosages per cubic foot into cubic yard figures (for the treatment of potting soils); and into rates per thousand square feet and per acre for ground-plot application. Where nursery rows or crop-plant soil are to be treated in the row only, leaving the middles un- treated as is done with pineapples (23, 33, 34), the acre rate is reduced in proportion t0 the area actually treated. The formula. given b‘y Young (72) can be adapted to other chemicals as well as chloropicrin and carbon disulphide. In fumigating with chloropicrin, it is well to remember that 3 milliliters of chloropicrin per square foot of soil equals 480 pounds per acre. Similar general formulae can be developed for other fumigants. For determining the rate of application of limited amounts of a chemical, the pounds of fumigant per acre multiplied by the conversion constant, 0.023, equals approximately the number of pounds of fumigant necessary for 1000 square feet of soil (Table 13). r SOIL FUMIGATION FOR PLANT DISEASE CONTROL 3“? ' Table 13. Dosages of soil fumigants. Milliliters per‘ Pounds per Chemical Thousand Cubic Cubic square Acre foot foot yard feet Chloropicrim .......................................... _- '2 a4 7.4 s20 " . . _ . . . . . _ . _ _ _ _ _ -_ 2.5 67 9.2 400 " .......................................... -_ s s1 11.04 480 " ________________ _-,. ....................... _- 4. 10s _ 14.7 640 Carbon disulphide ___________________________________ __ 8.3 224 23.0 1000 Ethylene dichloride- _________________________________ -_ 11.6 313 32.0 1400 Methyl bromide ______________________________________ __ 1 m 3.8 166 Pentaehlerethane ----- --» _____________________________ -- 12.4 335 46.0 2000 Tetrachlorethane _____________________________________ __ 13 350 464) 2000 *1 milliliter (m1.) = about 20 drops; 30 ml. = 1 (c.c.). A teaspoon holds about 5 m1. liquid ounce; 1 ml. = 1 cubic centimeter t" 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 5'°9°“"“?"°‘ BULLETIN NO‘. 628, TEXAS AGRICULTURAL EXPERIMENT STATION I-ITERATUBIE CITED Bakke, A. L. The use of tetrachlorethane in the eradication of the Euro- pean bindweed. Jour. Amer. Soc. Agron. 33:759-761. 1941. Bickerton, J. M. Timely suggestions for the carnation grower. Larvacide Log 3(1):12. 1940. Buehrer, E. M. and G. Steiner. noaerréatode (Heterodera marioni). U. S‘ D 1 3 . Byars, L. P. Experiments on the control of the root-knot nematode, Hetero- dera radioicola (Greef) Mueller I. The use of hydrocyanic-acid gas in loam soil in the field. Phytopath. 9:93-103. 1919. Clayton, E. E., J. G. Gaines, G. M. Stone, and K. J. Shaw. for tobacco plant beds. v Phytopath. 31:6. 1941. Chitwood, B. G. A rapid method for determining “k" cides. Proc. Helminth. Soc. Wash. 6:66-70. 1939. Chitwood, B. G. Soil treatments with volatile liquids for control of nema- todes. Phytopath. 31:818-824. 1941. Chitwood, B. G. and A. G. Newhall. The efficacy of certain nematocides in the control of onion bloat in muck soil. Phytopath. 32:254-258. 1942. Davey, A. E. and L. D. Leach. Experiments with fungicides for use against Sclerotium rolfsii in soils. Hilgardia 13:523-547. 1941. deOng, E. Ralph. Hydrocynanic-acid gas as a soil fumigant. Res. 11:421-436. 1917. deOng, E. Ralph and Jocelyn Tyler. A list of plants attacked by the root-knot . A. P1. Dis. Rptr. 17(7.):64-96. Soil treatment values of nemato- J our. Agr. Potassium xanthate as a soil fumi- gant. Indust. and Eng. Chem. 20:912. 1928. Dimock, A. The Rhizoctonia foot-rot of annual stocks (Matthiola in- cana). Phytopath. 31:87-91. 1941. Ezekiel, W. N. Tests with pentachlorethane, tetrachlorethane, and xylol to determine thein efficiency in eradication of Phymatotrichum root rot in the field. Jour. Agr. Res. 56:579-594. 1938. Gersdorff, W. A. 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Increased ef- ficiency of chloropicrin for nematode control with better confinement of the gas. Phytopath. 241332-1346. 1934. Godfrey, G. H. and P. A. Young. Directions for the use of chloropicrin and carbon bisulphide as soil fumigants. Tex. Agr. Exp. Sta. Progress Rept. 514. (Larvacide Log. 1:71-75.) 1938. Guba, E. F. Carbon disulphide emulsion for the control of the root-knot The use of methyl bromide to Proc. Helminth. Science n. s. nematode. Mass. Agr. Exp. Sta. Bul. 292. 1932. _ Haenseler, C. M. Soil fumigation. Crop Protect. Inst. Clrc. 16(1): Feb- 23, 1942. Hamilton, C. C. Methyl bromide fumigation for control of Asiatic beetle grubs attacking Azalea plants. Jour. Econ. Ent. 33-486-490. .1940. Howard, F. L. Field tomatoes——chloropicrin soil treatment and fungiclde trials. Larvacide Log 4(2):45-47. 1941. Howard, F. L. and F. K. Crandall. Response of field grown tomatoes t0 soil fumigation with chloropicrin. Proc. Amer. Soc. Hort. Sci. 37:939- 9 1. 1939. Howard, F. L., F. L. Stark, and J. B. Smith. Chemical control of nema- todes in tomato greenhouses. Phytopath. 29:11. 1939. Hurst, Richard. On the relative distribution of cysts of Heterodera schach- tii and a chemical dressing incorporated with infected land by means of a rototiller. Jour. Helminth. 16:57-60. 1938. Jackson, K. E. Chloropicrin. Chem. Rev. 14(2):251-286. 1934. JOIIHSOII. Maxwell O. The Pineapple. Paradise of the Pacific Press. oulu. 1935. Hon- 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. SOIL FUMIGATION FOR PLANT DISEASE CONTROL 39 Johnson, Maxwell O. and G. H. Godfrey. trol. Ind. Eng. Chem. 24:311-313. 193 Kligman, A. M. Control of fungi in mushroom casing soil by sterilization with chloropicrin. Phytopath. 32:978-985. 1942. Leukel, R. W. Chloropicrin as a disinfectant for plant beds. Phytopath. 32:1034-1036. 1942. Linford, M. B. Chemical and heat treatments. U. S. D. A. Pl. Dis. Rptr. Supplement 109. 1938. McCulloch, L. Control of gladiolus yellows by soil treatment with choloro- picrin. Florists Review, May 2, and Larvacide Log 3(1):2. 1940, McLaughlin, J. Harvey, and‘ I. E. Melhus. The response of some field crops . on soils treated with chloropicrin. Phytopath. 32:13. 1942. Monteith, John and A. E. Rabbitt. Killing weeds in soil with chloropicrin (tear gas). Turf Culture 1(1):63-79. 1939 and Larvacide Log 1(4): 120-128. 1938. Morrison, H. E. and Don C. Mote. A progress report of investigations con- cerning the symphylid and its control. Ore. Agr. Exp. Sta. Circ. of Inform. No. 242. 1941. Neifert, I. E. and G. L. Garrison. zChloropicrin for nematode con- Experiments on the toxic action of cer- tain gasses on insects, seeds, and fungi. U. S. D. A. Bul. 893. 1920. Neller, J. R. and R. V. Allison. A machine for the» subsurface treatment of soils with chloropicrin and carbon bisulphide for nematode control under field conditions. Soil Sci. 40:173-178. 1935. Newhall, A. G., Charles ChuDD. and C. E. F. Guterman. Soil treatment for the control of diseases in the greenhouse and the seedbed. N. Y. (Cor- nell) Agr. Exp. Sta. Bul. 217 (rev.). 1940. Newhall, A. G. and F. L. Stark, Jr. Chloropicrin and ethylene dichloride for root-knot nematode controL Phytopath. 32:626-630. 1942, Newton, W., J. E. Boshen, and R. J. Hastings. The treatment of glass- house soils with chloropicrin for the control of Heterodera mariowi (Cornu) Goodey, and other soil pathogens. Can. Jour. Res. 15(5):182- 186. 1937. Roark, R. C. Bibliography of chloropicrin. 1848-1932. U. S. D. A. Misc- Bibliography of chloropicrin, 1932-1934. Pub. No. 176. 1934. Roark, R. C. and R. L. Busbey. ¥).35S. D A. Bur. Ent. and Plant Quar. Cir. (mimeographed) E-351. Schwardt, H. H. and C. G. Lincoln. Soil fumigation for. control of the al- falfa snout beetle. Jour. Econ. Ent. 33:460-462. 1940. Shaw, L. Results of preliminary experiments on the control of root dis- eases of the peach. Phytopath. 32:649. 1942. Smith, N. R. The partial sterilization of soil by chloropicrin. Proc. Soil Sci. Soc. Amer. 3:188. 1938. Snapp, O. I. The peach tree boner. U. S. D. A. Farmers’ Bul. 1246. 1940. Stark, Frank L. Jr. Effective sterilization of greenhouse soil achieved withz chloropicrin Flor. Rev. Aug. 29, 1940 and Larvacide Log 3(1)! 11-1. 1940. Stark, Frank L. Jr., F. L. Howard, and J. B. Smith. Effect of chloropicrin in the soil solution on plant growth. Plt. Physiol. 15:142-145. 1940. Stark, Frank L. Jr., J. B. Smith and F. L. Howard. Effect of chloropicrin fumigation on nitrification and ammonification in soil. Soil Sci. 48: 433-442. 1939. Taylor, A. L. Chemical and heat treatments. U. S. D. A. Pl. Dis. Rptr. Supplement 109135-136. 1938. Taylor, A. L. Efficient spacing of soil fumigants for field applications. Proc. Helminth. Soc. Wash. 6(2):62-66. 1939. Taylor,1 A. L. Chemical control of root-knot. Larvacide Log 4(2)63-66. 194 Taylor, A. L. Chemical control_of root-knot. Phytopath. 32:650. 1942. Taylor, A. L., H. D. Barker, and O. P. Owens. Preliminary report on cotton wilt-nematode experiment at Lumberton, 291752. 1939. Taylor, A. L. and C. W. McBeth. Preliminary tests of methyl bromide as a nematocide. Proc. Helminth. Soc. Wash. 7:94-96. 1940. Taylor, A. L. and C. W. McBeth. A practical method of using methyl lzarozngide éasla nematocide in the field. Proc. Helminth. Soc. Wash. 8: 6- . 1 4. Taylor, A. L. and C. W. McBeth. ethylene dichloride for. control of root knot. Wash. 8:53-55. 194i. Thomas, H. E. and L. O. Lawyer. The use of carbon bisulphide in the control of Armillaria root rot. Phytopath. 29:827. 1939. Thorne, Gerald. Some factors governing the success of chemical treat- nbents of soil for nematode control. Proc. Helminth. Soc. Wash. 6: 6 -62. 1939. How to prevent or lessen its ravages. N. C. Phytopath. Spot treatments with chloropicrin and Proc. Helminth. Soc. 40 66. 67. 68. 69. 70. 71. 72. 73. 74. BULLETIN NO. 628, TEXAS AGRICULTURAL EXPERIMENT STATION Thorne, G, M. W. Allen, James Hare, and M] A. Lindsay. Populations of root-knot nematode larvae in two Kenn County, California, fields. Phytopath. 32:650. 1942. Watson, J. R. Control of root knot by calcium cyanamide. Fla. Agr. Exp. Sta. Bul. 136. 1917. Watson, J. R. and C. C. Goff. Control of root~knot in Florida. Fla. Agr. Exp. Sta. Bul. 311. 1937. Welton, F. A. and J. C. Carroll. Control of lawn weeds and the renova- tion of lawns. Ohio Agr. Exp. Sta. Bul. 619. 1941. Young, P. A. Controlling damping-off of tomato seedlings with li§ht and chemicals on seed and soil. Amer. Jour. Bot. 25(10):18s. 193 . Young, P. Chemical soil treatment to control Fusarium lycopersici, Heterodera mariomi, and weeds. Phytopath 29:25. 1939. Young, P. A. Soil fumigation with chloropicrin and carbon bisulphide to control tomato root-knot and wilt Phytopath. 30:860-865. 1940. Young, P. A. Chemical treatment of soil to control the root-knot nematode. U. S. D. A. P1. Dis. Rptr. Supplement 1241150. 1940. Young, P. A. Soil fumigation to control crop pests. Tex. Agr. Exp. Sta- Ann. Rpt. 51:145. 1941. ONIGTIHH HOHVBSSH 3W0}! "IVEIREI HEADS VIIVILS