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CAMPUS .

A95-239-12M-L180

TEXAS AGRICULIURAL EXPERIMENT STATION

A. B. CONNER, DIRECTOR
COLLEGE STATION, BRAZOS COUNTY, TEXAS

BULLETIN NO. 573 MARCH 1939

DIVISION OF AGRONOMY

The Control of Cotton Root Rot in the
%Blacl<land Region of Texas

 

AGRICULTURAL AND MECHANICAL COLLEGE OF TEXAS .
T. O. WALTON, President

71 i   L I a n A R m
i Azricultural a Mechanical College of Texas

COIIBQB Statiant Texas

[Blank Page in Original Bulletin]

Marked and consistent reduction in the severity of cotton root rot
in fields with established infestations has resulted from rotations
with non-susceptible crops in which cotton was grown only one-
fourth of the time. Outstanding increases in cotton yields also re-
sulted from these rotations. A non-susceptible crop period of at least
two years is required before the severity of the disease is reduced;
simple alternations of susceptible and non-susceptible crops are of
little value in reducing root rot. The control of susceptible weeds
is a prerequisite to the reduction of root rot severity by crop
rotation.

Previously uninfested parts of a field may be protected from
further invasion of root rot by barriers of closely planted sorghum,
a vigorous-growing, non-susceptible crop.

The severity of cotton root rot in established locations has also
been reduced under certain conditions by the use of organic
manures, clean fallow, deep tillage, and the application of soil
amendments.

The severe losses to cotton stands that occur during the
summer throughout the Blackland region of Texas, are caused
by the fungus Phymatotrichum omnivorum (Shear) Duggar.
This fungus also attacks and causes the decay of the roots of a
great many other plants, both cultivated crops and weeds. It
‘does not, however, attack any of the cultivated or wild grasses.

Three stages in the life history of the fungus are known: (1) the
actively growing strand or vegetative stage, (2) the resting or
sclerotial stage, and (3) the spore stage. The strand growth of
root rot is the destructive stage and it serves to spread and per-
petuate the fungus. Sclerotia are formed in the strands of the
fungus and serve to perpetuate the disease over long periods, even
in the absence of susceptible plants. So far as is known, root-rot
spores neither spread nor perpetuate the fungus.

CONTENTS

Page

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Methods of Spread and Perpetuation of the Cotton Root-Rot Fungus. . . 6
Stages in the Life History of the Fungus . . . . . . . . . . . . . . . . . . . . . . . . 6

The Behavior and Functions of Vegetative Strands . . . . . . . . . . . . . . . 7

The Functions of Sclerotia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Preventing Further Spread of Cotton Root Rot . . . . . . . . . . . . . . . . . . . . . . .. 9

Rotation as a Method of Controlling Cotton Root Rot . . . . . . . . . . . . . . . . . 11

Early Observations of Rotations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 11

Early Rotation Experiments in Root-Rot Infested Areas . . . . . . . . . . . 12

Rotation Studies at Temple . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Results Secured with Cotton.= . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Root-rot Losses to cotton stands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Effect of frequency of cotton in rotation on the yield of cotton. . 16
Effect of preceding crop practices on the yield of cotton . . . . . . 17

Results Secured with Non-susceptible Crops . . . . . . . . . . . . . . . . . . . . . . 19

Effect of cropping systems on the yield of corn . . . . . . . . . . . . . . . 19

Effect of cropping systems on the yield of sorghum . . . . . . . . . . . 20

Effect of cropping systems on the yield of oats . . . . . . . . . . . . . 20
Effect of the preceding crop practices on the yield of oats. . . . 20
Conclusions from Current Rotations at Temple . . . . . . . . . . . . . . . . .. 21

Practical Uses of Rotations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Eradication 'of Susceptible Weeds . . . . . . . . . . . . . . . . . . . . . . . . . .. 22

Flexible Uses of Non-susceptible Crops . . . . . . . . . . . . . . . . . . . . . . 22

Use of a Single Non~susceptible Crop. . .- . . . . . . . . . . . . . . . . . . . . 24

Use of Two Non-susceptible Crops . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Rotation of Cotton with Three or More Non-susceptible Crops. . 25
Possibilities of Increased Utilization of Non-susceptible Crops. . 26

Methods Other Than Crop Rotation for Controlling Root Rot . . . . . . . . .. 2'7

The Use of Organic Manure Under Root-rot Conditions . . . . . . . . . .. 2'7

Root-rot Control by Clean Fallow . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 28

Control of Root Rot by Deep Tillage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Soil Amendments for Root-rot Control . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Increasing Yields of Susceptible Crops from the Use of Fertilizers. . 31
Plant Improvement and Root-rot Control . . . . . . . . . . . . . . . . . . . . . . . . 32
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

Literature cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

BULLETIN NO. 5'73 MARCH 1939

THE CONTROL OF COTTON ROOT ROT IN THE BLACKLAND
REGION OF TEXAS

By H. E. Rea, Agronomist,
Division of Agronomy

The cotton root-rot disease caused by the fungus Phymatotrichztm om-
nivorum (Shear) Duggar attacks the roots of a large number of both wild
and cultivated plants and is very destructive to cotton in some years (47,
52, 61, 66, 67, 69). Ezekiel and Taubenhaus estimated that in 1928, a typical
season, the Texas cotton crop was reduced 440,000 bales by this disease
(12). The large acreage of cotton concentrated on the fertile, highly cal-
careous soils of the Blackland region of Texas furnishes very favorable
conditions for the development and spread of root rot, and frequently
the annual loss in this region is more than half that of the entire State (1,
12, 16). On the Blackland farms where this disease has become estab-
lished and on which cotton is frequently grown on the same land year
after year, root rot is a constant hazard to cotton production.

Information concerning the control of root rot is of particular im-

portance to the farmers of the Blackland region, and it is the purpose of ~

this publication to discuss the available information that may be useful
in the practical control of root rot in this region. Much of this informa-
tion will also be useful to farmers in other parts of Texas where this
disease is prevalent.

In selecting the information that may be used, it is desired to consider
the investigations conducted in other sections and other states as well as
those conducted in the Blacklands. Cotton root rot is also serious in cer-
tain parts of Arizona, New Mexico, and Oklahoma, and is known to
occur in Arkansas, California, Utah, Nevada, and Mexico (9, 12, 19, 24, 26,
59, 63, 65, 69, '72). Extensive studies of the root-rot disease have been
made, particularly in Texas and Arizona. In Texas the current investiga-
tions are being conducted primarily by the Texas Agricultural Experiment
Station, the University of Texas, and the United States Department of
Agriculture. The Work in Arizona is carried on principally by the United
States Department of Agriculture and the University of Arizona.

Studies in recent years have greatly extended the knowledge of the
life history of the root-rot fungus; its means of spread and perpetuation
in the field, and its behavior under a wide variety of environmental con-

.ditions. Based on the available information of the disease, numerous

methods of control have been tried. Preparatory to discussing these
methods of control as applied to the Blacklands, certain fundamental in-
formation about the disease and its method of spread and perpetuation is
pointed out.

6 BULLETIN NO. 573, TEXAS AGRICULTURAL EXPERIMENT STATION

METHODS OF SPREAD AND PERPETUATION OF THE COTTON
~ ROOT-ROT FUNGUS

To appreciate fully the nature of the root-rot hazard and to understand
the principles of some of the more practical means of controlling this
disease, one must know the methods of spread and perpetuation of the
root-rot fungus.

Some classes of plants are attacked by the root-rot fungus, while others
are not. Those plants on which root rot flourishes and from which it
obtains nourishment are referred to as susceptible plants. Those on
which the disease will not grow are called non-susceptible plants. Several
previous publications have classified a large number of plants, both wild
and cultivated, some as to their immunity and others as to their degree
of susceptibility to the disease (2, 3, 4, 41, 61, 66, 67, 69). As has already
been indicated, cotton is highly susceptible to the disease and is the most
generally grown susceptible plant in the Blacklands. In these cotton
fields, a large number of fleshy tap-rooted weeds often persist that are
susceptible to the disease. Many of the legumes are also susceptible.
Grass plants, both wild and cultivated, are immune to the disease. Some
of the non-susceptible crops commonly grown in the Blacklands are corn,
sorghum, oats, and wheat. These crops do not furnish nourishment for the
root-rot fungus and under proper management may be used to control

' the disease, while the planting of susceptible crops encourages its spread

and perpetuation.

Stages in the Life History of the Fungus

In the usual cotton field in the Blacklands, large numbers of cotton
plants may be seen dying from root rot during the summer months. By
selecting a freshly wilted cotton plant and pulling it up, one may find
the root-rot fungus on the tap root. The fungus may be seen with the
naked eye as a loosely attached, whitish to buff colored, string-like growth
clinging to the root (14, 36, 39, 54, 60, 63, 65, 72). This is the actively
growing and destructive stage of the fungus. It is referred to as the
strand growth or vegetative stage of the fungus, as distinguished from
the other stages of the disease. As the freshly wilted cotton plant is
pulled up, the major portion of the root system of the plant, together with
the bulk of the strand growth of the fungus, is left in the ground. In
addition, a second stage of the fungus, known as the sclerotial stage, is
also left in the ground (8, 22, 23, 29, 3:1, 32, 35, 37, 38, 39, 68).

The sclerotial stage is the resting or dormant stage and is not destruc-
tive until it germinates and produces new fungus strands. The individual
units of the sclerotial stage of the fungus are the sclerotia, which are
specialized sections of the fungus strands usually formed as swellings.
These swellings vary from approximately the size of a mustard seed to
that of a wheat kernel and are of various shapes. After the death of
the cotton roots and the vegetative strands of the fungus, these sclerotia

CONTROL OF COTTON ROOT ROT IN BLACKLAND REGION 7

remain in the soil in a dormant but viable condition for long periods.
These root-rot sclerotia are such a small part of the soil mass and are
so near the color of other organic material in the soil that the ordinary
observer is not likely to find them. They are usually available for in-
spection only after they are located by an experienced observer.

There is also a third stage of the root-rot fungus known as the spore
stage (5, 20, 23, 29, 34, 39, 63, 65, 72). This stage is found on exposed
surfaces of the soil. In the summer it may be seen occasionally during
damp, cloudy weather as whitish to buff colored mats on the soil surface.
While it has been demonstrated that these spore mats are developed by
the extension of the root-rot fungus strands, this stage apparently is
non-functional and the part it plays in the spread or perpetuation of the
root-rot disease is not known.

The vegetative and the sclerotial stages of the root-rot fungus are
functional. ‘he vegetative stage has the ability to grow and elongate and
is responsible for the spread of the disease except in rare instances. This
stage may also perpetuate the disease. The sclerotial stage functions only
to perpetuate the disease, except when moved artificially by man or by
abnormal natural agents.

The Behavior and Functions of the Vegetative Strands

The vegetative and sclerotial stages of the root-rot fungus are closely
interrelated and, as is indicated by the foregoing description, one is pro-
duced by the other. However, the behavior and functions of these stages
of the fungus can be more easily understood by considering them sep-
arately. The vegetative stage will be considered first.

When cotton is planted on land previously infested with root-rot, the
fungus is furnished an extremely favorable distribution of susceptible
plants upon which to thrive. In the cotton fields with full stands, the
fungus is provided a new food source in the form of root systems of indi-
vidual cotton plants every eight to twelve inches in every row. These
roots are not only distributed to considerable depth in the soil but are
thoroughly interspersed with the roots of their neighboring plants.
There is, then, contact or very close proximity of the roots of neighboring
cotton plants.“ As the vegetative strands attack and spread over the root
system of one plant, they soon grow on the healthy roots of the neighbor-
ing plant. By the time the disease has killed one plant in an area, or
soon thereafter, the neighboring plants become infected. The new strand
growth is the actively feeding portion of the fungus, and the older parts
of the strands, left back on the diseased and dying portion of the cotton
roots, gradually exhaust the available food and die with the root to
which they are attached. This stage of the fungus is maintained by
its ability to spread and invade the live roots of the remaining susceptible
plants. By the continued spread of the fungus from the live roots of one
cotton plant to those of another, the fungus can continue to live as long
as there are healthy, susceptible, and neighboring plants.

8 BULLETIN NO. 573, TEXAS AGRICULTURAL EXPERIMENT STATION

This spread of the vegetative strands does not cease with the coming
of winter nor is it halted by the destruction of the root crowns of the
cotton plants by the usual fall plowing. Numerous excavations during the
winter and spring months have demonstrated the presence of live cotton
roots in the soil from the previous year’s crop (47, 64). Where cotton is
grown year after year the root-rot fungus may perpetuate itself indefinitely
simply by the spread of the vegetative stage on the live roots of cotton.

If cotton, however, were the only susceptible plant growing in the field,
the omission of cotton growing for a single year would, except in rare
instances, eliminate the vegetative stage as a means of perpetuating the
disease, for cotton roots have never been found to remain alive as long
as twelve months without stalk growth. But, as previously mentioned,
root rot thrives on many fleshy tap-rooted weeds that persist in cotton
fields. The perennial weeds of this type especially aid in the maintenance
of the disease by the spread of vegetative strands. The common tie
vine, Ipomea trifida, may be used to illustrate the behavior of the fungus
on this type of weed.

Tie vines are somewhat resistant to the disease and have the ability
to put out new roots readily. By these means the infected tie vines often
remain alive for many years. These persistent perennial weeds harbor
the active strands of the fungus year after year and may be a constant
source of re-infection to cultivated crops. although cotton growing may
be interrupted for several years. In any field where the stand of sus-
ceptible perennial weeds is close, they must be controlled or eradicated
before root rot is likely to be controlled.

In rare instances, the vegetative stage of the fungus has also been
reported to survive for extended periods on dead and partially decomposed
plant tissue, even in the absence of any live roots of susceptible plants
(29, 35, 43, 44). This behavior, however, does not appear to occur fre-
quently enough to affect control practices materially.

The Function of Sclerotia

Although the presence of live roots of susceptible plants aids in the
survival of the root-rot fungus, perpetuation of this disease is not de-
pendent upon the continuity of live roots of susceptible plants. As the
fungus grows over the roots of a cotton plant or any other susceptible
plant, it produces resting bodies known as sclerotia. These sclerotia
remain in the soil, temporarily dormant, after the fungus strands that
produced them and the roots of the susceptible plants are dead. In later
seasons, when conditions are favorable, these sclerotia may germinate
and start new infections if the live roots of a susceptible plant happen
to be near. New fungus strands are developed and the vegetative stage
of the fungus again starts its spread. By means of sclerotia, root rot
may survive many years of non-susceptible crops, even in fields where no
weeds exist. These sclerotia have been found in the soil as deep as the
roots of susceptible plants grow. In cotton fields in the Blackland

CONTROL OF COTTON ROOT ROT IN BLACKLAND REGION 9

region, sclerotia have been found abundant from 3 to 36 inches deep
in the soil and most abundant from 12 to 24 inches (2, 35, 53). At such
depths it is usually impractical to affect them by tillage or chemical
treatments. With each crop of cotton or other susceptible crop, a new
lot of sclerotia may be produced, so that growing cotton year after year
permits the accumulation of an enormous number of viable sclerotia.
The presence of a close stand of perennial weeds on the land further
complicates the root-rot problem, since the weeds often survive in non-
susceptible crops and insure the perpetuation of the disease both by
actively growing vegetative strands and also by the production of repeated
lots of sclerotia. Complete information on the various factors affecting
the length of time sclerotia remain viable has not yet been developed,
but a working knowledge upon which to project control recommendations
is available.

While individual colonies of sclerotia have been known to remain viable
for rather long periods, most of the sclerotia under field conditions appar-
ently are dead by the end of the third year of a non-susceptible plant
period (53). On the other hand, it is known that the number of viable
sclerotia that can survive one year of non-susceptible plants is very great
and that there is a high carry-over of root rot in the following cotton
crop.

This information of the spread and the perpetuation of the root-rot
disease is the basis of effective means of preventing the further spread
-and of controlling the disease. »

PREVENTING THE FURTHER SPREAD OF COTTON ROOT ROT

Although root rot is generally distributed over most of the farms of the
Blackland region, there are still a number of farms in the region on
which the disease is restricted to certain portions of the field. ~On some
of these farms, the further spread of the disease may be inhibited by
unfavorable natural conditions. However, on many other farms, root rot
is still in the process of invading previously disease-free areas. Simple
and effective means of preventing the further spread of cotton root rot
into previously uninfested portions of a field may be used to great practical
value on the farms now being invaded.

In one of the early root-rot experiments at Greenville, Texas, McNamara
and Hooton used barriers to prevent the spread of root rot from infested
parts of a field into previously uninfested parts (34). The Greenville
experiment provided a double walled plank barrier with a 4-inch air space
between the walls placed at sufficient depth to intercept all the roots of
cotton plants between the infested and uninfested parts of the field
and thereby prevented the further spread of the disease (63, 65, 72). Since
that time, Taubenhaus and Ezekiel have accomplished the same results
with strips of sorghum, and they and other workers have tested numer-
ous types of barriers designed to prevent the spread of root rot (2, 9, 10,
21. 28, 29, 31). Effective barriers provide a positive gap between the

10 BULLETIN NO. 573, TEXAS AGRICULTURAL EXPERIMENT STATION

roots of the susceptible plants in the two parts of the field. They take
advantage of the information cited earlier that the root-rot disease spreads
from one location t0 another along the live roots of susceptible plants.

Barriers of closely-planted, vigorously-growing, non-susceptible crops
such as the sorghums are the most practical of the effective barriers. The
sorghum barriers prevent the contact or close proximity of the live roots of
susceptible plants in the infested and the previously uninfested portions
of a field and are effective as long as they are placed well in advance of
the spreading infestation. are sufficiently wide, are planted to a close
stand, and are free of susceptible weeds. Sorghum barriers are applicable
to any farm on which the root rot is at present restricted to a definite
portion of the farm and is in the process of spreading to the rest of the
farm. Root rot has never been known to cross a properly placed and
properly maintained sorghum barrier under dry land conditions and the
present knowledge of this disease indicates that root rot is not likely
to cross such a barrier except under very unusual conditions.

The vigorous-growing, non-susceptible crops such as the sorghums
make effective root-rot barriers because they successfully compete with
cotton for the exclusive possession of the soil below the non-susceptible
crop during the time cotton in the adjacent area is making its growth.
Only the invasion of roots of susceptible shrubs and trees which may go
below the sorghum roots is likely to endanger the effectiveness of a sorghum
barrier. Such crops as corn and oats, although non-susceptible, permit
the invasion of cotton roots from the adjacent cotton plants and can not
be expected to make effective root-rot barriers (47).

If the farmer wishes to restrict the root-rot disease to the minimum
area, the sorghum barrier should be planted parallel to the boundary of
the root-rot infestation. The location of the sorghum barrier should be
determined and marked in the fall of the year prior to planting the sorghum
in the spring. If the location of the barrier is delayed until spring, there
will be no visible evidence of the boundary between the root-rot infested
and the disease-free parts of the field. It is also advisable to mark the
barrier at least 12 feet in advance of the observed boundary to allow for
any additional spread that the disease may have made that is not evident
above ground. Also some additional spread frequently takes place dur-
ing the winter months on the live cotton roots below the plow depth.
While a barrier of one or two rows of sorghum may be eflective if prop-
erly placed and maintained, under practical field conditions the sorghum
barrier should be at least 30 feet wide. Either broadcast or row sorghum
may be used. A simple procedure for small root rot spots, would be to
plant the infested area to sorghum and extend the planting fully 30 feet
beyond the limits of the infestation. Once the sorghum barrier is estab-
lished, its ‘location from year to year may be easily identified by some
simple method such as back furrows.

Sorghum barriers require a minimum cost to establish, but they are
effective for only one season at a time and have to be planted on the
same location year after year as long as protection is desired. Sorghum

CONTROL OF COTTON ROOT ROT IN BLACKLAND REGION 11

yields may decrease slightly from the continuous planting of this crop,
but the early destruction of the sorghum stubble and the occasional
sacrifice of a crop for green manuring will help to maintain the produc-
tiveness of the sorghum.

Root rot neither follows field lines nor terrace lines and to only a
minor extent row direction, and the use of a sorghum barrier may require
an artificial subdivision of a field, interfering somewhat with cultural
practices. If a farmer objects to this artificial subdivision of a field
by the irregular shape of root-rot infested areas, there is always an
opportunity to square up the root-rot infested field by placing the sorghum
barrier immediately ahead of the most advanced point of infestation and
parallel to property or terrace lines. Such a procedure, while it permits
the uninfested parts of the field behind the barrier to become infested,
will establish a definite line beyond which root rot can not spread.

ROTATION AS A METHOD OF CONTROLLING COTTON ROOT ROT

Numerous methods of controlling cotton root rot in established locations
have been tested by the several agencies studying this disease. While
none of the methods developed thus far have eradicated the disease,
several of them have materially reduced its severity. The most prac-
tical method of controlling cotton root rot on most of the farms in the
Blackland region of Texas, at present, is by the proper rotation of non-
susceptible crops with cotton. In this publication, the phrase “control of
cotton root rot” as applied to established infestations means the reduction
of stand losses of the susceptible crop caused by root rot on the treated
areas compared with the stand losses occurring on the untreated areas.
It does not mean the elimination of stand losses of the susceptible crop
caused by this disease.

Early Observations of Rotations

The value of certain types of crop rotations as a practical means of
controlling cotton root rot has been known for a long time and has been
demonstrated both by formal experiments and by the experience of many
farmers. Dr. L. H. Pammel, the first plant disease specialist of the
Texas Agricultural Experiment Station, was one of the first scientists to
point out the value and the requirements of crop rotations in controlling
this disease (41, 42). Since Pammel’s time, crop rotation experiments
under root-rot conditions have been conducted at Sar Antonio, Texas,
by the United States Department of Agriculture and on two separate
locations at Temple, Texas, by the Texas Agricultural Experiment Sta-
tion (45, 46, 50, 55, 56, 57). The accumulated information has indicated
certain rather definite requirements of a rotation to control cotton root rot.

As Pammel began his studies on cotton root rot in 1888, he found that
many of the cotton planters had already gained considerable experience
with this disease and that several of the more observant farmers already

12 BULLETIN NO. 573, TEXAS AGRICULTURAL EXPERIMENT STATICN

knew enough about root rot to control it. Pammel states in Texas Station
Bulletin No. 4, written in 1888 (41), that

“In north Texas, where small grains are a certain crop, rotation is often resorted
to to prevent the ‘dying of cotton’. As an illustration: Four years ago considerable
cotton was killed by Root Rot in a field of Mr. Ormsby Scott, near Melissa. Since
that time the field has been planted in corn, oats and wheat, and the present year
in cotton. On the first of September very little dead cotton was found in that
field. Similar cases have been found at Plano, San Marcos and Independence.

“Mr. H. R. von Bieberstein recommends for South Texas a three-year rotation,
using oats, corn and millet.

“That a simple alternation of crops is not sufficient is well illustrated in the fol-
lowing cases: At Independence, Captain Tom Clay pointed out a number of fields
to me which had been in corn the year before, yet cotton was dying quite severely.

“I think from a practical point of view a proper rotation of crops is the best
way to destroy the fungus.

“In rotation care must be taken not to follow cotton by plants which are subject
to Root Rot . . . So far as is known the fibrous rooted plants, such as corn,
oats and wheat, are exempt from the disease, and where practicable should follow
cotton. * '

“Certain weeds are also subject to the disease, hence fields ought to be kept
rigidly clean . . . . It will be of little use to rotate, if weeds which naroor tne
fungus are allowed to grow in the field.”

Thus almost fifty years ago, Pammel pointed out the most effective
factors in the successful utilization of crop rotations to control cotton
root rot. And crop rotations as recommended by Pammel have been
employed to good advantage throughout the years by many of the careful
farmers who have known the value of desirable rotations in controlling

this disease.

Early Rotation Experiments in Root-Rot Infested Areas

In summarizing the results of the crop rotations conducted at the United
States Field Station at San Antonio, Ratliife reported in 1934 that “In
these experiments 2-year rotations of cotton with non-susceptible crops
have proved of very little value in the control of root rot, 3-year rota-
tions appear to have been slightly more effective, While in 4-year rota-
tions, the disease has been effectively checked though not eliminated”
{45). The yields of crops in the San Antonio test were reported by
Ratliffe and Atkins (46) in 1931, and they conclude that “Cotton yields
were generally higher in rotation than under continuous cropping.”

The results of rotations at Temple conducted on the old Experiment
Station farm during the period 1914-24 were reported by Reynolds and
Killough (50). They stated: “It will be observed that the rotated cotton in
every case did not have as much root rot as did the cotton grown con-
tinuously on the same land. In fact, rotation of crops practically con-
trolled, although it did not eradicate the disease.” Reynolds and Killough
also reported much higher yields of cotton on the rotated than on the
non-rotated areas.

The yields of the non-susceptible crops used in the San Antonio and
Temple tests were affected in varying degrees by the rotation used (46,
50). Ratliffe and Atkins reported that in the San Antonio test the highest
yields of corn were from rotations in which early seed bed preparation for
corn was permitted. Milo yields were highest in 4-year rotations, although
the yields in 3-year rotations were nearly as high. Yields of broadcast
sorghums were increased by rotation. Oat production was seriously handi-

 

(‘IONTROL OF COTTON ROOT ROT IN BLACKLAND REGION 13

capped by the lack of varieties resistant to leaf rust and stem rust;
however, yields of oats were practically as high under continuous cropping
as when grown in rotations. Reynolds and Killough reporting on the
rotation test at Temple prior to 1928 indicate that the yields of corn were
increased considerably by rotations and were highest when corn was
preceded by a leguminous crop. The yield of wheat was increased by
rotations but oats benefited very little by rotation. In this early Temple
test no comparisons of rotated and non-rotated sorghums were available.

ROTATION STUDIES AT TEMPLE

The rotations begun at Temple in 1914 were abandoned in the winter
of 1927 when Substation No. 5 was relocated. This change in location
was made primarily to provide additional areas of severely infested land
for the purpose of studying and developing methods of controlling the
root-rot disease. In 1928 experiments were instituted at the new station
to study further the effects of cropping systems on the yield of crops
and the control of cotton root rot. These systems, with minor exceptions,
have been followed continuously.

Rogers has recently reported the results obtained on a part of the
areas included in these cropping systems. He states that “Four-year
rotations of cotton with non-susceptible crops—c0rn, sorghum, and small
grain (oats and wheat)—showed consistent reduction in root rot” (53).

In the present publication, a more complete report of the cropping sys-
tems studied at Temple from 1928 through 1936 is given. The root rot data
used in this analysis were collected by B. F. Dana and Dr. C. H. Rogers.
The yield data were collected by H. E. Dunlavy. These experiments are
being continued and further progress reports on the results secured are
to be made in the future.

The cropping systems used on the current experiments at Temple
include continuously cropped areas of cotton, corn, sorghum, and oats.

In contrast to this continuous cropping are the rotation studies involving
the use of cotton twice in three years, once in two years, once in three
years, and once in four years. In the years that the rotated areas are
not planted to cotton, the land is planted to crops immune or highly
resistant to root rot, except in a few cases where the land is fallowed.

All systems of cropping are conducted in duplicate and areas repre-
senting the various cropping treatments are planted to cotton each year.
The fields used for these studies are relatively free of perennial weeds and
the previous history of the root rot on the areas studied indicates that
the initial infestations were severe and generally distributed. The indi-
vidual cropping systems in which cotton is planted and for which records
are complete are given in Table 1.

In conducting these experiments on cropping systems, the cultural
methods—including time and method of plowing the land; time, method,
and rate of seeding; time and method of cultivation; the variety of crops
planted; and insect control practices—have been alike on both rotated and
non-rotated areas so far as seasonal conditions permitted. However, the

14 BULLETIN NO. 573, TEXAS AGRICULTURAL EXPERIMENT STATION

 

 
 

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CONTROL OF COTTON ROOT ROT IN BLACKLAND REGION 15

various soil areas used for different crop treatments in this study varied
as to productiveness and as to the prevalence of root rot.

At the beginning of these experiments two continuously cropped areas
were provided, one on Houston clay soil and the other on deep Houston
black clay soil. These areas were located on opposite sides of the farm
and varied widely in productiveness. This difference in productiveness
was associated with differences in soil type and land slope. Most of the
rotated areas were located on soils with varying degrees of similarity to
the check areas and the yields of the individual areas under rotation have
tended to parallel most closely the yields of the continuously cropped
areas to which they were most similar in soil type and land slope. The
yields observed in this experiment were influenced by (a) original differ-
ences in soil fertility and the prevalence of root rot and (b) the cropping
systems used. In this report the results secured on the more productive
continuously cropped area located on deep Houston black clay have been
used as the basis of comparing the results secured on other areas. To
eliminate as far as possible the effect of original variations, the observed
yields on each rotated area were adjusted by ratios obtained by dividing
the average yields in the pre-rotation period, 1928 to 1930, by the cor-
responding figure for the continuously cropped area. The averages for
the rotation period 1931 to 1936 "were divided by their respective ratios to
obtain the “adjusted averages,” which represent approximately the effect
of the cropping system itself on the results. Similar adjustments were
also made for root rot percentages in cases where comparisons were made
between areas that originally varied widely as to the prevalence of root
rot. ,

The results secured from the various cropping systems are presented
under the two classes of crops grown: (a) cotton, which is susceptible
toroot rot, and (b) the feed crops—corn, sorghum, and oats—which are
immune to the disease. In discussing the information secured on cotton,
attention is given first to the losses of cotton stands caused by root rot
in the various cropping systems. The relationship of the frequency with
which cotton was planted in each cropping system to the cotton yields
secured and the root-rot losses sustained are then discussed. Attention is
also given to the influence on the yield of cotton of the crop practices im-
mediately preceding cotton in the rotation. The discussion on the non-
susceptible crops gives consideration to the effect of the frequency with
which each crop is grown in the rotation on the yield of that crop. Some
information is also presented on the effect of the preceding crop practice
on the yield of oats.

Results Secured ‘with Cotton

Root-rot losses to cotton stands. The percentages of loss to cotton
stands caused by root rot in the various cropping systems are presented
in Table 1. In this table the information on the percentages of root rot
are grouped under five types of cropping system, as follows: (a) con-
tinuous cotton, or cotton every year, (b) cotton twice in three years, or

16 BULLETIN NO. 573, TEXAS AGRICULTURAL EXPERIMENT STATION

2/ 3 of the time, (c) cotton once in two years, 0r 1/2 of the time, (d) cotton
once in three years, or 1/ 3 of the time, and (e) cotton once in four years,
‘or 1/4 of the time. As may be learned from the column headed “Cropping
Systems” in Table 1, corn, sorghum, oats, guar, or fallow was used in the
years cotton was not grown. Corn, sorghum, and oats are immune to
the root-rot disease, and guar is highly resistant. There was never a
very wide variety or a close stand of root-rot susceptible weeds on the
area used in the experiment. Also the cultural practices used were such
as to control weeds on the areas used in the rotations, and during the
year fallow was practiced, the land was plowed at regular intervals to
destroy all plant growth.

By 1931 all of the rotations had completed their first cycle and were all
fully effective during the same year for the first time. By the end of
1936, these rotations had been effective. for six years, so that the average
root-rot percentages for the period, 1931 through 1936, show the effect
of each rotation on the control of the cotton ‘root-rot disease. During this
period an average of 67 per cent of the cotton plants on the continuous
cotton area were killed by root rot, as shown in Table 1, while on the areas
planted to cotton 2/3, 1/2, 1/ 3, and 1/4 of the time, the average root-rot
percentages were 69, 63, 46, 25, respectively. These percentages show
that the severity of root rot was not reduced until cotton was withheld
from the land for at least two years; and after that the reduction was
greater the less frequently cotton was grown. In the 11-year rota-
tion of cotton, corn, sorghum, and oats where cotton was planted only 1/4
of the time, marked control of the disease was secured. Also an inspection
of the annual averagenpercentages of root rot on the continuous cotton
and the 4-year rotations given in Table 1 shows that the control obtained
by planting cotton 1/4 of the time was consistent from year to year. From
this table it may be seen that the stand losses from root rot on the con-
tinuous cotton area during 1931, 1932, 1933, 1934, 1935, and 1936, re-
spectively, were 17, 25, 35, 45, 64, and 67 percent higher than on the 4-year
rotation.

Elfect of frequency of cotton in rotation on the yield of cotton. The
most frequent use of cotton was on the continuous cotton area, where
cotton was grown every year. The next most frequent use of cotton was
in the 3-year rotations in which cotton was planted 2/ 3 of the time. In
other types of rotations, cotton was grown 1/2, 1/3, and 1/4 of the time.
The cotton yields for each of the cropping systems and the average yields
for each type of cropping system are given in Table 2 for the effective
rotation periods. From this table it may be seen that cotton yielded at
the average rate of 237, 258, 272, 300, and 324 pounds of lint per acre,
respectively, in the cropping systems in which cotton was planted all,
2/3, 1/2, 1/ 3 and 1/4 of the time. From these yields it is seen that, when
compared with continuous cotton, the rotations used in this experiment
progressively increased the yield of cotton the less frequently this crop
was grown.~

CONTROL OF COTTON ROOT ROT IN BLACKLAND REGION 17

It is also evident that the most outstanding increases in the yield of
cotton were secured in the rotations in which cotton was used only 1/ 3 and
1/4 of the time and in which the severity of root rot was reduced. Both
the greatest increase in yield and the greatest reduction in severity of
root rot were secured in the rotation using cotton only 1/4 of the time.

Table Zr-Average yield in pounds of lint cotton per acre for cropping systems at Temple, 193136

_ Adjusted
Prevlous average
Cropping systems crop 1931 1932 1933 1934 1935 1936 Average yield
1931-36
Continuous cotton . . . . . . . . . . . . . . . .. cotton 293 320 249 264 150 148 237 237
Cotton twice in 3 year's:
Cotton, cotton, oats . . . . . . . . . . . . cotton 283 168 223 182 35 102 166 210

Cotton, cotton, oats . . . . . . . . . . .. oats 310 263 259 193 140 279 241 298

Cotton, cotton, fallow . . . . . . . . .. cotton 337 245 230 245 104 279 240 267

Cotton, cotton, fallow . . . . . . . . .. fallow 341 157 352 211 175 210 241 256

Average . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222 258
Cotton once in 2 years:
Cotton, corn . . . . . . . . . . . . . . . . . . . corn 327 219 335 297 160 350 281 302

Cotton, oats . . . . . . . . . . . . . . . . . . . oats 189 262 286 222 132 220 218 260

Cotton, sorghum . . . . . . . . . . . . . .. sorghum 133 179 302 256 108 211 198 230

Cotton, guar . . . . . . . . . . . . . . . . . . . guar 292 269 327 288 203 333 285 294

Average . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246 272

Cotton once in 3 years:
Cotton, corn, oats . . . . . . . . . . . . . oats 288 200 294 188 256 240 244 294
Cotton, sorghum, oats . . . . . . . . .. oats 368 275 358 251 216 216 281 305
Average . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262 300

Cotton once in 4 years:
Cotton, corn, sorghum, oats. . . . oats 313 260 370 240 222 306 285 324

However, small increases in yield were also secured in the rotation in
which cotton was used as frequently as 2/3 and 1/2 of the time. This
suggests that factors other than the control of root rot also contributed
to the increased yields obtained on the rotated areas. The influences of
the crop practices immediately preceding cotton were factors affecting
the yield.

Eifect of preceding crop practices on the yield of cotton. The effects
of the crop practices immediately preceding cotton are indicated by the
results secured from the 2-year rotations in which cotton followed corn,
oats, sorghum and guar in different rotations. The average adjusted yields
of cotton following these crops compared with the average of continuous
cotton are taken from Table 2 and reproduced in Table 3 together with
the corresponding adjusted root-rot percentages. Adjusted averages and
root-rot percentages are used so as to eliminate as far as possible the
original differences in soil fertility and the prevalence of root rot between
these areas and thereby clarify the influence of preceding crop practices.

As previously pointed out, in the rotations in which cotton was planted
as frequently as once every two years, root rot was not consistently re-

18 BULLETIN NO. 573, TEXAS AGRICULTURAL EXPERIMENT STATION

duced. However, the yield of cotton in these rotations was affected by the
preceding crop practices. The yield of cotton following corn during the
1931-36 period was higher than the yield of continuous cotton. Cotton
following corn yielded 302 pounds of lint per acre compared with 237 for
continuous cotton. The increased yield of cotton following corn compared
with continuous cotton in these experiments was probably caused by several
factors. For one thing, the corn plant is of determinate growth and
probably takes little or no plant nutrient from the soil after it matures late
in the summer, while cotton often continues to make some growth until
frost. Also, as the corn matured, weeds and grass frequently made con-
siderable growth which, in these experiments, was turned under as green
manure. Another factor that tended to increase cotton yields in this 2-year
rotation was the early preparation of the land for cotton. Where cotton
followed corn, fall plowing was accomplished from one to two months
earlier than was practical where cotton followed cotton. These conditions
all tended to increase the plant nutrients and soil moisture available for
starting the cotton crop following corn.

Table 3.——Adjusted average cotton yields and root-rot percentages for continuous cotton and
cotton in 2-year rotations preceded by corn, oats, sorghum, and guar, respectively, 1931-36

_ Adjusted Adjusted
Preceding crop pounds of Average
lint per acre root-rot
percentages
Continuous cotton . . . . . . . . . . . . . . . . . 237 67

Corn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302 81

Oats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260 65

Sorghum . . . . . . . . . . . . . . . . . . . . . . . . . . 230 67

uar . . . . . . . . . . . . . . . . . . . . . . . . , . . . . 294 63

Similar opportunities to increase the yield of the following cotton crop
existed on the oat stubble land. Apparently these opportunities were
only‘partially realized in these experiments, since cotton following oats
gave a yield intermediate between the yield of cotton following corn and
continuous cotton. The yield of cotton following oats was 260 pounds
of lint per acre as compared with 302 following corn and 237 for con-
tinuous cotton. After the oats were harvested, clean fallow was main-
tained throughout the summer with the expectation of eradicating weeds,
some of which are carriers of root rot. This emphasis on clean fallow
precluded the production of a natural green manure crop and probably
accounts for the relatively lower yields of cotton following oats compared
with- cotton following corn.

Cotton yields following sorghums in the Temple experiments were more
satisfactory than are ordinarily obtained by Blackland farmers. In this
experiment, cotton following sorghum yielded 230 pounds of lint per acre
as compared with 237 pounds for continuous cotton. These yields are not
significantly different and it may be said that cotton following sorghum
yielded as well as continuous cotton. The difference in handling the

CONTROL OF COTTON ROOT ROT IN BLACKLAND REGION 19»

sorghum stubble in these experiments compared with the usual procedure
followed by the farmers may account in part for the results secured. In
these tests the sorghum stubble was destroyed by plowing as soon after
harvesting the first crop as was practical. This was ordinarily accom-
plished prior to September 1. This early preparation of the sorghum
stubble land provided a rather long period for the decomposition of the
stubble material prior to planting the following cotton crop. Ordinarily
farmers are late in plowing their stubble land and quite frequently they
harvest a second cutting of sorghum. As a result of this practice, the
sorghum stubble often stands until frost without being turned under.
Cotton planted on sorghum land handled in this manner usually yields-
less than continuous cotton, no doubt partially due to the depletion of avail-
able soil moisture and nitrogen at cotton planting time.

From the results presented in_ Table 3, it may be seen that cotton fol-
lowing guar yielded 294 pounds of lint per acre as compared with 302.
pounds following corn and 237 pounds from continuous cotton. While the
yield of cotton following guar was much higher than the yield of con-
tinuous cotton, it is surprising that the yield was not also higher than
for cotton following corn. It is normally expected that the yield of cotton
following a legume, such as guar, will exceed that of cotton following a
non-leguminous crop, such as corn. However, repeated examination dur-
ing the summers of 1935 and 1936 showed the roots of guar to be prac-
tically devoid of nodules, so that these crops most likely obtained very
little nitrogen from the air and therefore added little or no nitrogen to
the soil from this source. Also in most years the, guar was harvested and
removed from the land and only a limited amount of the crop residues
plowed under. Under different crop management where inoculation of
the legume is provided and the entire crop turned under as green manure,
higher yields of cotton following a legume might be expected.

Results Secured with Non-Susceptible Crops

The records from these tests furnish information for corn, sorghum,
and oats as to the effect of frequency with which each crop was included
in the rotation on the yield of the individual crop. In the case of oats,
records are also available on the effect of the preceding crop practices.
Yield records on guar are incomplete and for this reason were not used
in this analysis. The 1931-36 average yields of corn, sorghum, and oats
adjusted to eliminate original variation are given in Table 4 for the
various cropping systems. '

Effect of cropping systems on the yield of corn. From the records
presented in Table 4, it may be seen that corn grown in rotation with
other crops during the period 1931-36 yielded an average of 29 bushels
of shelled corn per acre, while corn grown continuously on the same land
produced only 22.7 bushels. This is a difference of 6.3 bushels, or 28
per cent, in favor of rotation. It is evident that the frequency with which
corn was grown in the rotation had little or no effect on the yield of corn.

20 BULLETIN NO. 573, TEXAS AGRICULTURAL EXPERIMENT STATION

Corn grown every other year yielded justias much as corn grown only
every fourth year. i

Table 4.—Effect of frequency of use in the rotation on the yield per acre of corn, sorghum,
and oats, 1931-36

Corn Sorghum
Time in each crop shelled, Oats,
bushels Grain, bu. Forage, tons bushels
Continuous . . . . . . . . . . . . . . . . . . . . . . . 22.7 24.2 2.82 32.1
One-half . . . . . . . . . . . . . . . . . . . . . . . . . . 29.1 31.2 3.95 32.5
One-third . . . . . . . . . . . . . . . . . . . . . . . . . 28.9 26.7 3.93 31.1
One-fourth . . . . . . . . . . . . . . . . . . . . . . . . 29.1 33.6 4.58 incomplete
I’ . record
Rotation average . . . . . . . . . . . . . . . . . . 29.0 30.5 4.15 31.8

Effect of cropping systems on the yield of sorghum. Sorghum under
rotation gave somewhat higher yields than continuous sorghum. Darso
was the sorghum variety grown in these tests from 1931 through 1936.
As shown in Table 4 darso grown in rotation with other crops produced
6.3 bushels of grain and 1.3 tons of cured forage more per acre than when
grown continuously on the same land. Part of these differences in the
yield of sorghum were probably due to the sorghum disease, Phthium

orrhenomanes. This disease was prevalent on the continuously cropped

and rotated areas in 1937, and the comparatively low yields recorded for
the continuously cropped area in 1935 and 1936 may have been due
in part to this disease. There appeared to be only a slight tendency
for the grain yields to increase the less frequently darso was grown.
There was, however, an increase in the yield of forage from darso
grown only once every four years compared with darso grown every
other year. Darso grown only once every four years produced 4.58 tons
of forage per acre as compared with 3.95 tons when grown every other
year.

Effect of cropping systems on the yield of oats. The average yield of
oats in rotation, as shown in Table 4, was 31.8 bushels per acre as com‘-
pared with 32.1 for continuous oats. From these records, it is apparent
that there was no significantdifference in the yield of oats on the rotated
and non-rotated areas and that the frequency with which oats was used
did not affect the yields.

Elfect of the preceding crop practices on the yield of oats. The pre-v

ceding crop practices, however, did affect the yield of oats, as shown in
Table 5.

The influence of the previous crop on the yield of oats in these tests
was closely associated with the condition of the seed bed at the time
of sowing oats, except in the case where sorghum was the previous crop.

The highest yields of oats were obtained by sowing on cotton stalk land

CONTROL OF COTTON ROOT ROT IN BLACKLAND REGION 2L

which furnished a .clean, firm seed bed. Oats following oats were sown
on a moderately loose seed bed, while oats following corn were sown on
a very loose seed bed. To prepare corn land for fall seeding of oats, it
was necessary to plow the land at least 3 1/2 to 4 inches deep to turn
under the vegetative litter present on the corn land. Both the plowing
operations and also the vegetation plowed under resulted in a loose seed
bed. The decomposition of the vegetative litter probably also deprived
the growing oat crop of considerable nitrogen during the early growth of
the crop. Sorghum probably leaves the soil temporarily depleted in both
soil moisture and available nitrogen. The interval between the sorghum
and oat crop was usually too short to permit the normal activities of soil
organisms and weather to correct this condition. The detrimental effects
of the sorghum on the following oat crop were most evident in seasons
when seed bed preparation for oats was either delayed or omitted. The
difliculty of following oats after sorghum was largely responsible for the
failure to secure complete records for oats in the 4-year rotation in
Table 4.

, Table 5.—Efl'ect of preceding crop practices on the yield of oats, 1931-36

Acre yield,
Preceding crop bushels_ of
grain

Cotton . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34.2
Oats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32. 1
Corn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30.0
Sorghum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 .3

Conclusions from Current Rotations at Temple

Although cotton root rot was not eradicated, its severity was reduced
in those rotations providing at least a two-year non-susceptible crop period
between cotton crops under the conditions of a scattered stand of weeds
that carry root rot. Marked and consistent reduction in severity of the
disease was obtained in rotations using cotton only 1/4 of the time.

All the rotations used increased the yield of cotton whether they. re-
duced the severity of root rot or not. Cotton yields were progressively
increased the less frequently cotton was grown in the rotation and the
major increases in cotton yields were closely associated with the degree
to which root rot infestation was reduced. Outstanding increases in yield
of rotated cotton over that of continuous cotton were secured in rotations
in which cotton was grown only 1/4 of the time.

Factors other than the reduction of cotton root rot contributed to in-
creases in cotton yields secured by rotation. The other factors contribut-
ing to increased cotton yields in these rotations apparently were associated
with the management of the crop immediately preceding cotton. In these

22 BULLETIN NO. 573, TEXAS AGRICULTURAL EXPERIMENT STATION

experiments, cotton yields were increased most when preceded by corn,
guar, and oats, in the order named. Under the crop management used in
these experiments, sorghum did not decrease the yield of cotton when it
preceded cotton.

Rotation with other crops increased the yield of corn and sorghum
when compared with continuous growing of these crops. However, non-
rotated oats made yields just as high as did rotated oats.

The frequency with which corn and sorghum were grown in the rota-
tion had very little effect on their yields.

The preceding crop practices affected the yield of oats and the highest
yields were obtained when the preceding crop provided a clean, firm seed
bed for oats. Oats preceded by cotton produced the highest yields, while
oats preceded by sorghum made the lowest yield.

Practical Uses of Rotations

From a review of the rotation experiments conducted in the Blacklands
prior to 1928 and from an analysis of the current experiments at Temple,
it is evident that the factors essential to the control of root rot by crop
rotation originally recommended by the Experiment Station have been con-
firmed and redemonstrated. These recommendations, in brief, were: (a)
eradicate root-rot susceptible weeds, (b) follow cotton by three years of non-
suscepible crops for effective root-rot control and marked increases in
cotton yields._ These recommendations have stood the test of time and are
now, nearly fifty years after they were first made, the most practical
means of controlling root rot under field conditions in the Blacklands.

Eradication of susceptible weeds. The eradication of susceptible weeds
is a prerequisite to the control of cotton root rot by crop rotations. Cer-
tain perennial weeds may persist on the land whether the crops are rotated
-or not, and the rotation of crops may have only an indirect influence upon
their eradication by using crops such as sorghum that tend to smother
the Weeds or by making the land available for cultural operations at
unusual times of the year. Continuous cotton land is never available
for summer plowing; so certain types of weeds become prevalent, while
summer or early fall plowing to destroy these weeds may be accomplished
following the harvest of almost any of the non-susceptible crops (47).
The management of the individual kinds of crops grown, and not the
rotation used, largely determines the extent to which susceptible weeds
may be controlled. Thorough and sustained cultivation is the most ef-
fective means of eradicating weeds. Where sheep are available, they
may often be used to considerable advantage in controlling weeds.

Flexible uses of non-susceptible crops. In addition to eradicating
root-rot susceptible weeds from his field, the farmer also has to meet
the requirements of rotations designed to control root rot. The rotation
used by any individual Blackland farmer is naturally influenced by the
restricted number of non-susceptible crops he grows.

 

1
l

CONTROL OF COTTON ROOT ROT IN BLACKLAND REGION 23

Varieties of corn and sorghum adapted to almost any conditions found
in the Blackland region are available and corn particularly is exten-
sively grown. Many excellent varieties of sorghum are available but some
of them are of more recent distribution and the value of sorghum is not
fully appreciated by many farmers of the region. Grain sorghums are
grown‘ most extensively on those Blackland farms and in the sections
where corn is likely t0 suffer from droughts. The varieties of wheat and
oats now in use by Blackland farmers are susceptible to leaf and stem
rust and are extensively planted only in those sections and on those soils
of the region where serious damage from rust is less likely to occur.

Individual farmers throughout the Blacklands have not only eradicated
susceptible weeds on their land, but also have grown sufficient acreages
and kinds of non-susceptible crops to permit the extensive use of rotations
in which cotton is grown as infrequently as 1/4 of the time. Where such
practices have been followed over long periods, root-rot infestations are
of minor importance and cotton yields are comparatively high. But most
Blackland farmers, although they rotate their crops, do not provide in-
tervals of sufficient duration between cotton crops to effect root-rot
control. In some cases this may have been because the farmer has not
understood the principal features of a rotation designed to control root
rot or because he has not visualized the elasticity of the non-susceptible
crop schedule recommended to him. In the past the word “rotation” when
applied to cropping systems has ordinarily meant “equal acreages of dif-
ferent crops grown in a definite sequence or fixed order.” In conducting
formal crop rotation experiments this definition of the word “rotation”
has been used so as to simplify the prosecution and interpretation of the
experiments. However, in the following discussion of the application of
these results to conditions of Blackland farmers, the word “rotation” is
used to mean “the recurrent interruptions of cotton growing or other root-
rot susceptible crops on any particular piece of land by one or more years
of root-rot immune crops, independent of the kinds, acreages, or sequence
of these crops.” From the standpoint of root-rot control, the principal
feature of a rotation is the number of years between cotton or other sus-
ceptible crops on the same piece of land. If material reduction in the
prevalence of the disease is to be expected, the land in question should be
free of cotton and all other root-rot susceptible plants, both wild and
cultivated, for three years.

During the three-year non-susceptible plant period any number, kinds,
or acreages of cultivated grasses such as corn, sorghum, oats, wheat,
barley, sudan grass, and millet may be planted in any order or combina-
tion desired. Also wild grasses may be permitted to grow in these crops
without materially affecting the degree of root-rot control. The non-
susceptible crop schedule on any particular piece of land may be as
elastic as the crop adaptation of the land and the tolerance of the crop
to be planted to the crop that preceded it will permit. Within these
limitations, which are independent of root-rot control, the farmer may

24 BULLETIN NO. 573, TEXAS AGRICULTURAL EXPERIMENT STATION

select the kinds and acreages of non-susceptible crops he islto plant 0n
any portion of his farm from year to year and on the basis of the prob-
able market demands, requirements for home consumption, and his facil-
ities for producing each crop. Furthermore, a rotation designed to con-
trol root rot may be used on any part of a farm where this disease is
particularly destructive without affecting the cropping system on another
part of the farm where the disease is less severe or absent altogether.
Used in this way a rotation to control root rot on severely infested areas
need not upset the balance of cash and feed crops for the entire farm.

Use of a single non-susceptible crop. Many Blackland farmers grow
only two major crops, cotton and corn. If corn is the only root-rot immune
crop grown, only a simple alternation of crops is feasible. Such a
rotation seldom, if ever, reduces the severity of root rot, even though
weeds are absent. Very frequently this is because too many viable
sclerotia survive a single year of non-susceptible plants. A two-crop
farmer, if one of the crops is highly susceptible to root rot, usually has
very little opportunity to follow the kind of rotation that is necessary
for the control of the disease. This is particularly true if the non-
susceptitble crop is corn or sorghum, since neither of these crops can
follow itself without danger of reducing the yield materially (46, 50).
However, the farmer who grows only cotton and corn can follow cropping
practices that will increase the yield of cotton, although root rot is not
controlled. As shown by the results presented in Table 3, the yield of
cotton was comparatively high in the 2-year rotation of cotton and corn
when liberal amounts of crop residue were plowed under early in the _fall
following the harvest of corn.

‘On the two-crop farm where the crops grown are cotton and small
grain, the cropping system necessary for the control of root rot may be
followed, at least on selected portions of the farm. In a previous
section of this report, it has been shown that, although the yield of
oats at Temple was higher when oats followed cotton than when oats
followed oats, the yield of continuous oats over a nine-year period was
almost as high as rotated oats. Similar results with oats have also
been secured by other workers (46, 50). Thus a 4-year rotation of cotton
with oats in which cotton is planted only 1/4 of the time is practical,
since continuous oats may be used for the three years required. Such a
cotton-and-oats rotation may be used for selected portions of the farm or
for the entire farm, depending upon the proportional acreage of cotton
and oats normally grown. Other small grains may be used in a similar
manner. If weeds are controlled, such a rotation may be expected to
control root rot effectively and to increase materially the yields of
cotton with only a slight sacrifice in the yield of small grain.

Use of two non-susceptible crops. On farms where two or more non-
susceptible crops are normally grown, the required type of rotation to
control root rot can nearly always be used, if not for the entire farm,

CONTROL OF COTTON ROOT ROT IN BLACKLAND REGION 25

at least on selected areas. If cotton, corn, and sorghum are the crops
grown, for example, it is practical to use a 3-year rotation for as large
an area as is planted to the non-susceptible crop with the smallest acreage.
In "the Blacklands, the smallest acreage on a given farm is at present
likely to be sorghum. In this case, a 3-year rotation using cotton only
1/3 of the time can be used on an area the size of the sorghum acreage.
Such a procedure may be expected to give moderate protection from root
rot losses on the area involved if careful attention is given to eradicating
weeds. This practice cannot be expected to control root rot as regularly
or as effectively as using cotton only 1/4 of the time. It is, however,
superior to the simple alternation of the corn and sorghum with cotton
or continuous cotton.

Where small grain and at least one other non-susceptible crop are
grown, a rotation using cotton only 1/4 of the time is nearly always

‘feasible. If, for example, the crops grown are cotton, small grain, and

corn, a rotation of cotton the first year, small grain the second and
third years, and corn the fourth year followed again by cotton should
prove effective. Again the acreage involved in the rotation may depend
upon the area planted to the non-susceptible crop with the lowest acreage.
Also, as indicated previously, the effectiveness of the rotation will be
partially or entirely lost if weeds are not eradicated. If weeds are
eliminated and cotton is grown only 1/4 of the time, effective control
of root rot and marked increase in yield of cotton compared to continuous
planting of cotton may be expected.

Rotation of cotton with three or more non-susceptible crops. On farms
where a large number of non-susceptible crops, such as corn, sorghum,
oats, wheat, sudan grass, millet, and other grass crops are well adapted
and are normally planted or may be profitable planted, the matter of fol-
lowing a 4-year rotation using cotton only 1/4 of the time is simplified.
In the future many more farmers may be expected to grow high acreages
on non-susceptible crops as current results of crops breeding by State
and Federal workers specializing on certain of these crops promise to
yield even more widely adapted and higher yielding strains on non-
susceptible crops than are now available (2). This is especially apt to
be the case with small grains as high-yielding, rust-resistant strains
are distributed. As the acreage of these non-susceptible crops increases
in proportion to the acreage of cotton grown, a greater and greater
portion of the farms may be involved in the required rotation. On
those farms where the cotton acreage is relatively small and the acreage
of the non-susceptible grass crops is relatively large, the maximum
benefits of crop rotation in controlling cotton root rot may be obtained.
But on those farms where cotton is planted almost exclusively and there
is not the will nor the opportunity for the farmer to use increased kinds
and acreages of non-susceptible crops, there is very little opportunity to
control cotton root rot by crop rotations. On the majority of farms in
the Blackland region, with conditions intermediate between these two

26 BULLETIN NO. 573, TEXAS AGRICULTURAL EXPERIMENT STATION

extremes, crop rotations properly managed may be used to considerable

advantage in reducing the severity of root rot.

Possibilities of increased utilization of non-susceptible crops. The

cropping systems needed for the most effective control of cotton root rot

require the planting of relatively large proportions of the cultivated
land on Blackland farms to non-susceptible crops and only moderate
acreages to cotton. In the past the average income per acre made from
cotton even under serious root-rot infestations has usually been greater
than the average income from most non-susceptible crops. Except on

certain soils and in certain sections of the Blacklands where conditions,

are more favorable for the non-susceptible crops than for cotton, these
crops have rarely been grown for sale as cash crops but have been grown
for consumptive purposes and only the surpluses sold.

In contrast to the usual practice, individual farmers throughout the
Blacklands who control sufficient crop acreage to make it feasible to grow’
a number of crops and who have adequate supplies of stock water have
for years diversified their farming activities. These farmers have long-
grown relatively high acreages of non-susceptible crops and only moderate
acreages of cotton. By utilizing their non-susceptible feed crops in the
production of salable livestock or livestock products to supplement their
income from cotton production, these farmers have made relatively high
farm incomes. As a result of their diversified farming, these farmers
have sufiicient acreages of non-susceptible crops to provide the type of
rotations required to control root rot effectively. While the majority of
Blackland farmers have not followed this procedure, the success of those
who have indicates that many others with similar resources might well
do so.

No radical change in farm organization is recommended within a short
period. Any change that is to be made should be made gradually and
should pay its way as the change is made. Non-susceptible crops should
not be planted at the risk of materially lowering farm income, regardless
of their usefulness in controlling root rot or in improving soil fertility.
Only when such a crop promises to add materially to the net farm income
is there sufficient justification for planting it. However, farmers should
not attach too much importance to high income per acre. The cost of
producing a crop as well as the number of acres a man can tend are
also to be considered. One man, for example, can tend rather large
acreages of oats without seriously interfering with the normal size of
his cotton crop; and oats, although a low income crop, is relatively in-
expensive to produce. If sufiicient land is available, both cotton and oats
can be produced with only moderate additional expense. Often the only

' income received from oats is from the sale of grain. Yet where this

crop is planted in the fall, it may be grazed by salable livestock at least
five months of the year, without serious risk of lowering grain yield,
and the value of this pasturage may materially increase the per-acre
income from oats (58).

CONTROL OF COTTON ROOT ROT IN BLACKLAND REGION 27

In addition to small grains, other non-susceptible crops such as corn,

‘sorghum, and sudan grass may be grown without prohibitive competition

with cotton for labor and equipment, and many opportunities to secure
increased incomes from these crops are available to farmers who are
willing to utilize them. The accessibility of extensive acreages of pre-
viously little-used, small-grain pasturage and the ease of providing high-
carrying-capacity summer pastures of sudan and other grasses together
with the opportunity to produce large quantities of cheap forage from
planting increased acreages of sorghum should attract many Blackland
cotton farmers to fatten beef cattle as a supplementary enterprise to
cotton production. Small dairy enterprises should appeal to others. And
in still other instances, farm flocks of sheep may be used.

The benefits of these opportunities can gradually be obtained by most
Blackland farmers, and the early steps toward a more profitable farming
system are easy ones if they are chosen intelligently. Any Blackland
farmer who is not already doing so might well start toward a better type
of farming with a family garden and a home supply of poultry, pork,
beef, and dairy products as a foundation and expand into desirable live-
stock enterprises to supplement farm income. Only a short way from the
farm pantry are other steps such as improved farm pastures and trench
silos, steps that may be taken by most farmers. ‘Working from the
farmyard to the fields, farmers may soon realize the benefits of non-
susceptible feed crops as a basis of enterprises to supplement cotton farm-
ing, and only after suflicient acreages of non-susceptible crops are grown

will it be practical to control cotton root rot by the required rotation.

METHODS OTHER THAN CROP ROTATIONS FOR CONTROLLING
COTTON ROOT ROT

In addition to the use of crop rotations, cotton root rot can be controlled
under certain conditions by other methods, most of which are of little
practical value except when used in conjunction with crop rotations and
are only occasionally of sufficient promise to be considered as independent
methods of controlling the disease. Some of the other methods that have
been studied are: (a) the use of organic manures, (b) clean fallow, (c)
deep tillage, (d) the use of chemical soil amendments including com-
mercial fertilizers, and (e) the selection of root-rot resistant strains within
susceptible crops. A discussion of these special methods should serve
to clarify the means by which progress in controlling established root-
rot infestations may be expected. Of particular interest are the ways
some of these specialized methods may be used in conjunction with crop
rotations.

The Use of Organic Manures Under Root-Rot Conditions

Under some specialized conditions root rot has been controlled by the
use of heavy applications of organic materials. In Arizona, King con-

28 BULLETIN NO. 573, TEXAS AGRICULTURAL EXPERIMENT STATION

trolled root rot under irrigated conditions by annual applications of animal
manures at rates of 12 t0 14 tons per acre (6, 20, 21, 25, 27, 28, 30, 31).
He also secured satisfactory results with equivalent amounts of green
manure. The extent to which organic manures may be used for root-rot
control under other conditions has not been fully studied but under dry land
conditions in the Blackland region of Texas the results secured have not
been encouraging. Manure treatments for the control of root rot tried
by Scofield (55), Taubenhaus and Killough (72), and Ratliffe (45) gave
negative or inconclusive results.

Few attempts have been made to control root rot in the Blackland soils
by the use of green manures and before green manures are apt to be used
for any purpose in this region more satisfactory green manuring prac-
tices are needed. In other sections of the United States where green
manuring is practiced, legumes are generally used. In the Blacklands,
however, few legumes are well adapted. Most of the winter legumes that
have been tested make only mediocre and uncertain growth and often inter-
fere with seed bed preparation of the following money crop. The vigorous-
growing spring and summer legumes available seldom benefit the land.
sufliciently to justify the use of the land for a season. Also most of these
legumes are highly susceptible to root rot and their use on root-rot in-
fested land is of doubtful value as they tend to spread root rot.

Since so many of the legumes are susceptible to root rot and are only
mediocre forage plants, consideration has been given to the value of non--
leguminous crops for green manure. In this class, sorghums offer consid-
erable possibility, since they are not only immune to root rot but are also
heavy forage yielders, even under favorable conditions of rather short
duration. Also sorghums frequently offer an opportunity to secure a
green manuring crop incidental to the production of a feed or revenue crop.
Where this is feasible considerable benefit may be obtained by turning
under second growth and catch crops of sorghum. Although such green
manuring practices may have some value in reducing root rot, their prin-
cipal value is to stimulate the yield of plants that escape the disease.
For this reason the best results may be expected when sorghums for
green manure are used in rotations designed to control root rot.

Root Rot Control by Clean Fallow

Cotton root rot has been controlled by clean fallow of sufficient duration
to kill the roots of susceptible crops, to eradicate or materially reduce
the stand of susceptible weeds, and to permit a material reduction in the
population of viable sclerotia by natural deaths. In most cases studied,
at least two years of clean fallow were necessary to secure control. Clean
fallow experiments have been conducted under a wide variety of condi-
tions and at a number of locations throughout the Blackland region of
Texas, and practically every agency studying the cotton root-rot disease,
both in Texas and in other states; has at some time conducted clean fallow
experiments designed to control the disease (27, 29, 30, 31, 33, 34, 36, 45,

CONTROL OF COTTON ROOT ROT IN BLACKLAND REGION 29

47, 53, 55, 72). Various experiments have provided clean fallow periods
during the winter months, during the summer months, for one year,
two years,.and more. One of the longest clean fallow periods studied
thus far was for eight years. Few of the clean fallow treatments main-
tained for less than two years were successful. In common with most other
methods of controlling root rot, the control resulting from clean fallow
of two or more years was fully efiective for only one cotton crop imme-
diately after the treatment. The development of root rot in the second
and later cotton crops after the fallow is often just as severe as on the
continuous cotton areas.

As an illustration of the results secured with clean culture the follow-
ing cases may be citedz‘ McNamara and Hooton of Greenville report that
“One plat that showed more than 90 per cent infection for the 3 years
period from 1919 to 1921, inclusive, showed no infection when returned
to cotton in 1924 after a 2-year clean fallow” (34).

With reference to the clean fallow experiments-at Temple, Rogers states:
“Cotton planted in 1935 on Acre E6, after five and one-half years of clean
fallow, had an end season root-rot kill of 6.7 per cent. Part of the area
planted to cotton continuously for three years after three-year fallow had
98.2 per cent of the cotton killed by root rot in the same year. Both areas
planted again to cotton in 1936 had a 92.9 per cent kill on the part having
only 6.7 per cent in 1935 and 98 per cent kill on the part having 98.2
per cent dead in 1935” (2).

Clean fallow was not as effective in root-rot control as the use of non-
susceptible crops, except in the eradication of perennial weeds which
carry the disease. The use of clean fallow for this purpose is probably
justified when the treatment does not interrupt crop production. As an
independent method of controlling root rot, clean fallows were impractical
in almost every situation they were tried, since the treatments required
a long non-crop period and gave only temporary control of the disease.
The clean fallow method of controlling the disease was very expensive
both in terms of sacrificed income from idle land and also in the extra cost
of the tillage operations. As the primary benefit of clean fallow is the
earlier eradication of perennial weed carriers of the disease, it is probable
that many of these weeds can be eradicated at a much lower cost by other
cultural practices and the wise management of livestock grazing in con-
junction with the rotation of non-susceptible crops with cotton.

Control of Root Rot by Deep Tillage

Deep plowing has long appealed to the popular imagination as a prom-
ising method of controlling soil inhabiting diseases and various types of
deep tillage have been advocated from time to time as possible methods of
controlling cotton root rot. Even before very much fundamental infor-
mation concerning the root-rot fungus was developed, Shear and Miles
recommended deep plowing in conjunction with crop rotations as a method
of controlling this disease (56). From deep plowing and subsoiling

L l B R A R Y
Agricultural 8. Mechanical College of Texas
College Station, Texas.

30 BULLETIN NO. 573, TEXAS AGRICULTURAL EXPERIMENT STATION

experiments conducted at Temple from 1917 to 1921, Taubenhaus and
Killough concluded that deep plowing was of little value in controlling
root rot (72). In 1926 King reported that deep fall plowing was not
adapted to conditions in the irrigated sections of Arizona and that deep
spring plowing had little or no effect on the severity of root rot (27).

The early deep tillage tests were for the most part concerned with
horse drawn implements and involved only moderate depths. Interest in
deep tillage as a root-rot control method has continued, and since 1928
there has ben available at several locations throughout the Blackland
region of Texas heavy tractor-drawn subsoiling equipment and various
commercial and institutional agencies have tested tillage to depths of 14
to 28 inches as a means of controlling root rot. In most of the trials
made, root rot was less severe on the subsoiled areas than on the un-
subsoiled areas (2, 8, 9, 10). A marked degree of root-rot control was
obtained in some years and in the majority of the cases the yield of cotton
was also increased. The results secured by Dunlavy (2) at Temple from
1930 to 1933 are presnted in Table 6.

Table (Sr-The efiect of subsoiling on the control of cotton root rot and the yield of cotton

 

 

October lst Average
Treatment Percentage of root rot Pounds of lint cotton per acre
1930 1931 1932 ' 1933 f Average‘ 1930 1931 l 1932 l 1933 Average
Untreated . . . . . . . . . . . . . . . . . . . . . . .. 42.6 41.9 63.8 76.0 56.1 167 206 156 320 212
Subsoiled . . . . . . . . . . . . . . . . . . . . . . . .. 34.1 25.6 52.7 48.1 40.1 166 262 192 307 232

 

In the Temple trials root rot was less severe on the subsoiled plats.
than on the untreated plats each year. During the 1930-33 period root rot
killed an average of 40.1 per cent of the cotton plants on subsoiled plats
as compared with 56.1 per cent on the untreated plats. The subsoiled
plats yielded an average of 232 pounds of lint cotton per acre as com-
pared with 212 pounds for the untreated plats. Although subsoiling
resulted in an average increase of 20 pounds of lint cotton per acre,
the value of this increased yield was not sufiicient to pay the cost of the
subsoiling operation. From a practical point of view, subsoiling to depths
of 14 to 28 inches is not a promising method of controlling cotton root rot
because of the expensiveness of the subsoiling operations. '

Soil Amendments for Root Rot Control

Many experiments involving the use of a wide variety of soil amend-
ments have been conducted but most of the treatments used have failed
to control the disease (2, 7, 8, 9, 10, 11, 13, 18, 21, 23, 25, 29,40, 41, 42,
49, 59). There are a number of chemicals that are toxic to root rot when
they are brought in contact with the fungus. However, the problem of
securing penetration and distribution throughout the soil of the material
used so as to insure contact of the chemical with the fungus is very

CONTROL OF COTTON ROOT ROT IN BLACKLAND REGION 31

diflicult. Even in some of the more porous soils, these treatments have
been only moderately successful although extreme care and elaborate
methods were used.

One group of soil amendments that has been extensively studied is the
soil acidifying chemicals. Interest in the soil acidifying agents was
developed following a laboratory demonstration that the growth of the
root-rot fungus in pure culture could be controlled by changing the
reaction of the media on which the fungus grew from alkaline to acid
(72). It has long been known that the cotton root-rot disease is most
generally distributed and most severe in neutral to basic soils and that
its distribution and severity are very much restricted in low lime content
soils. Detailed studies of the major soil types in Texas have shown
that there is a close connection between the soil reaction and the severity
of root rot (1, 2, 7, 8, 9, 10, 12, 16, 41, 42, 63, 65, 70, 71, 72).

In an effort to make practical application of this information, sulphur
was used in a number of tests as one of the more economical soil-
acidifying agents. In small experimental plats planted to cotton, the
prevalence of the root-rot disease was considerably reduced by applica-
tion of the sulphur (7, 8, 10, 13, 63, 65, 70). However, attempts to make
practical the use of sulphur on a field scale to control root rot on low
lime content soils were disappointing. The results secured indicated
that sulphur applied to such soils was not likely to give satisfactory
control of the disease, and that there was considerable danger of injury
to the cotton (8). Tests made on certain high lime content soils of the
Blackland region indicated that rates of sulphur application necessary
to affect the soil reaction materially were far too expensive for practical
use (48).

Increasing Yields of Susceptible Crops from the Use of Fertilizers

Chemical fertilizers are another class of soil amendments that have»
been given considerable attention. Since these materials are expected to»
have their influence primarily through the plant, their method of appli-
cation is not so exacting as for chemicals where contact with the fungus
is required. However, recent experiments show that reasonably deep
placement of fertilizer in the clay soils of the Blackland region permits
the fertilizers to be more effective than shallow placement (17). During
the past 10 years a large number of fertilizer experiments with cotton
have been conducted under root-rot conditions in the Blackland region
both by the Texas Agricultural Experiment Station and the Division
of Soil Fertility Investigations, United States Department of Agricul-
ture. In general the fertilizers used have increased the yield of cotton,
particularly the fertilizers high in nitrogen and phosphorus (2, 17, 49, 51).
The Division of Soil Fertility Investigations has also pointed out that
certain fertilizers delay the occurrence of root rot early in the summer"
(17). Under conditions of favorable cotton prices, the increased cotton
yields obtained from fertilizers on several of the soils of the region may

32 BULLETIN NO. 573, TEXAS AGRICULTURAL EXPERIMENT STATION

be quite profitable, especially if the fertilizer is used in conjunction
with crop rotations.

Plant Improvement and Root Rot Control

Repeated attempts have been made and are still being made to find
cotton and legume strains that are resistant t0 cotton root rot (2, 3, 4, 7,
8, 9, 10, 15, 59, 61, 62, 63, 65, 67, 69, 72). Although the discovery of a
resistant strain of cotton might be the most effective means of controlling
the disease, the results secured with cotton thus far give little indication
of resistance in this crop. Some of the legumes, such as sesbania and
guar, however, appear to be highly resistant to the disease, and in more
recent experiments, observations have been made that certain varieties
of cowpeas are less susceptible to root rot than are others (2, 69).

The development of high yielding strains of forage legumes resistant
"to root rot would contribute materially to the profitableness of Blackland
agriculture and greatly encourage the use of crop rotation. The occur-
rence of some very acceptable plant material for legume breeding and
the existence of apparent resistance to root rot in certain legumes offers
some encouragement that this objective might be attained. The improve-
ment recently made with non-susceptible grass crops such (as rust resist-
ance in wheat and oats and the development of more productive strains
-of corn and grain sorghum promise to increase the acreage planted to
these crops (2). With the planting of suificient acreages of these non-
susceptible crops, the control of root rot by crop rotations will be simplified.

SUMMARY

The fungus Phymatotrichum omnivorum (Shear) Duggar attacks and
causes the decay of the roots of cotton and many other plants, both
cultivated crops and weeds.

This disease is particularly destructive to cotton in the Blackland region
of Texas where the soil, climate, and flora are generally favorable for the
spread and perpetuation of the fungus.

Three stages in the life history of the fungus are known: (1) the active
growing strand or vegetative stage, (2) the resting or sclerotial stage,
and (3) the spore stage.

The strand growth of the root-rot fungus is the destructive stage,
and it spreads and perpetuates the fungus by continuous growth of the
strands, chiefly along the live roots of susceptible plants and from one
plant to another where continuous culture of cotton is practiced and close
stands of susceptible weeds occur. Roots of susceptible perennial weeds
may harbor and perpetuate the strand growth of the fungus for long
periods.

Root rot sclerotia are formed in the strands of the fungus and serve to
perpetuate the fungus in a dormant but viable condition over long periods,
even in the absence of susceptible plants. Sclerotia may be found in the

CONTROL OF COTTON ROOT ROT IN BLACKLAND REGION 33

soil as deep as roots of susceptible plants grow and are infected with
root rot. Sclerotia may remain viable in the soil for many years, although
the majority apparently lose their vitality by the end of three years.

The further spread of root rot into a previously uninfested portion of a
cotton field may be prevented by growing a barrier of sorghum between
the infested and the uninfested parts of the field. The sorghum must
be planted in the same location year after year as long as protection from
root rot invasion is desired. Under practical field conditions the sorghum
barrier should be at least 30 feet wide. Corn, small grain, and many‘
other immune crops will not make effective barriers, since their root
systems permit the invasion of the roots of adjacent susceptible crops.

The value and requirements of crop rotations in controlling root rot
were pointed out by the Experiment Station as early as 1888. Since that time
rotation experiments at San Antonio and Temple have confirmed and fur-
ther demonstrated the factors essential to the control of root rot by crop
rotations. Marked and consistent control of cotton root rot was secured.
in rotations of cotton with non-susceptible crops in which cotton was
grown only 1/4 of the time.’ A non-susceptible crop period of at least
two years between cotton crops was required to control the disease. Simple
alternation of susceptible and non-susceptible crops was of little value in
controlling root rot.

The control of susceptible weeds was a prerequisite to the control of root
rot by crop rotations.’ l H

Yields of cotton on rotated areas were generally higher than on non-
rotated areas. At Temple the increases in~ the yield of rotated cotton
compared with non-rotated were greater the less frequently cotton was
planted and the major increases were associated with the degree to which
root rot was controlled. Outstanding increases in yield of cotton over
that of continuous cotton were secured in rotation in which cotton was
planted only 1/ 40f the time.

The yields of corn and sorghum were increased by rotation. Rota-
tions of short durantion were as effective as long rotations in increas-
ing the yields of these crops. The yield of oats was practically as high
when grown continuously as when rotated. '

Increased acreages of non-susceptible crops are frequently needed on
the farms of the Blackland region to enable farmers to secure the max-
imum benefit from crop rotations in controlling root rot. Plant improve-
ment projects now in progress promise to make available in the near-
future new strains of the root-rot immune crops that should attract
farmers to grow these crops more extensively. Higher yielding strains
of Wheat and oats are already in the possession of state and federal
plant breeders. Higher yielding and more dependable strains of corn
and sorghum are being developed. Improved strains of pasture grasses
may also be expected.

LIncreaSQd-farm incomes from the root-rot immune crops may be secured
on most of the Blackland farms where sufiicient stock water is available
by marketing these crops through salable livestock. Grazed by live-

:34 BULLETIN NO. 573, TEXAS AGRICULTURAL EXPERIMENT STATION

stock, weeds and grass in corn fields may be converted into farm income.
'Winter grazing of small grains may materially increase the value of
these crops. Increased markets for sorghum, a well adapted, inexpensive,
and high yielding crop in the Blacklands, may be greatly extended through
the use of livestock. Sorghum for grain, bundle feed, and silage is well

suited to cattle feeding. Greatly increased acreages of sorghum may be i

grown for silage to be stored in inexpensive trench silos to insure the
abundance of cheap feed. Increasing acreages of sudan may furnish
excellent and dependable summer pasture for livestock and when used to
supplement other feed resources may be a source of considerable farm
income. The sodding of eroded areas and waste land along drainage
ways to permanent pasture grasses may easily add revenue-producing

acreage to many Blackland farms where livestock are kept. The classes,

of livestock that may be kept advantageously depends on the physical
resources available, the location of the farm with reference to markets,
and the aptitude of the individual farmer.

Methods of controlling root rot other than by crop rotations are less
applicable to Blackland conditions. Clean fallows of sufficient duration to
‘control the disease are no more effective than crop rotations and make
poor use of land and labor. Deep tillage is usually very effective but
requires the use of expensive machinery and is costly to perform. Soil
amendments are seldom effective under practical field conditions. Com-
mercial fertilizers are of most value when used in conjunction with crop
rotations. The use of heavy applications of animal manure, although
successful in irrigated sections, has failed to control root rot under dry
land conditions. Control of root rot by green manuring practices is
uncertain and the crops and land used in such a practice usually have
their highest value when used in conjunction with a crop rotation and
livestock program. While root-rot resistant varieties of cotton may be
found, the prospects are not as encouraging as are those for the further
improvement of root-rot immune crops that may be grown in rotation
"with cotton. l

LITERATURE CITED

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CONTROL OF COTTON ROOT ROT IN BLACKLAND REGION 35

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36‘

i
\

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U. S.

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