Introduction
History
Cotton seed from Egypt was first introduced into Arizona in 1901.
Plantings for commercial observations were made in Yuma and at
a Territorial experimental farm affiliated with The University
of Arizona in the Salt River Valley near Phoenix. In 1902 the
Bureau of Plant Industry intensified experimentation with extra-long
staple Egyptian cotton in Yuma. A variety called Yuma was released
in 1908. The first commercial crop of Yuma, 375 bales, was produced
in 1912. In 1907, land for cotton research was acquired on the
Gila Indian Reservation in Sacaton. Research there continued until
1957 when the Cotton Research Center in Phoenix was constructed.
Cooperative research with federal, state, and U A personnel took
place at this facility until 1985 when cotton research was moved
to the Maricopa Agricultural Center in Maricopa, Arizona. The
last comprehensive discussion of cotton diseases in Arizona appeared
in “Diseases of Field Crops in Arizona,” a U A publication
authored by plant pathologists J.G. Brown and R. B. Streets and
published in 1934. The cotton diseases discussed in the publication
were: Angular leaf spot, Boll rots, Soreshin, Southwestern Rust,
Phymatotrichum root rot, Alternaria leaf spot, Root–knot
nematode, and a physiological disease, named Crazy-Top.
Production Areas
Many different cultivars of cotton are grown in various ecological
areas in Arizona. Production areas vary greatly in elevation and
annual rainfall. At one extreme is the Yuma Valley in the southwestern
corner of Arizona along the lower Colorado River where cotton
is planted in the latter part of February. Elevations here are
approximately 75-100 feet. In this area, average annual precipitation
is less than 4 inches. In the southeastern part of Arizona in
Graham, Cochise, and Greenlee counties, cotton is grown at elevations
above 3,000 feet and plantings take place during the middle of
April. In Cochise County, for example, cotton is grown in Pearce
(4,375 feet), Cochise (4,212 feet), Bowie (3,765 feet) and San
Simon (3,601 feet). The production areas along the Gila River
in Graham and Greenlee counties are approximately 3,000 feet and
3,500 feet, respectively. At elevations of 4,000 feet, temperatures
are approximately 12o F lower (maximum and minimum daily temperatures)
than temperatures at sea level. Also, there is a correlation in
Arizona between elevation and rainfall. Low elevation production
sites in the western part of Arizona average less than 4 inches
of rain annually, whereas rainfall at Pearce (4,375 feet) is approximately
13-15 inches annually. Cotton is exposed to the so-called “monsoon”
season during July, August, and September. This period is characterized
by heavy, irregularly distributed rainfall and high humidity.
Rainfall during this period at the higher elevations can range
as high as 10-12 inches during certain summers. These temperature
and rainfall differences play important roles in the distribution
and severity of cotton diseases in Arizona.
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Pathological
Cotton Problems
Seedling disease
and stand problems
Although many pathogenic organisms are involved
in cotton seedling disease throughout the Cotton Belt, only two
pathogenic fungi, Rhizoctonia solani and Thielaviopsis basicola,
cause disease problems in Arizona. These soil-borne fungi are
widespread in Arizona soils and cause seedling and root diseases
in many plants other than cotton.
 |
| A field picture showing serious stand
problems in cotton. Picture taken in Coolidge, Arizona. |
Rhizoctonia solani
Symptoms - The most
common symptom caused by R. solani is postemergence damping-off.
The fungus grows into the lower stem and the upper portion of
the root just below the soil line. Lesions on the roots are sunken
and reddish-brown in color. The infected seedling dries up and
dies. With a hand lens it is possible to see the “fuzzy”
brownish mycelium of the fungus in the girdled tissue.
Epidemiology and control - Rhizoctonia solani, a soil fungus
with a highly active saprophyte stage, survives in the soil in
the absence of cotton for indefinite periods of time. The isolates
that cause disease in cotton are, for all practical purposes,
restricted to that host. Pima and upland cottons are equally susceptible
to this disease. Typical weather favoring seedling disease occurs
when planting and emergence takes place when night temperatures
are below 50o F. Any cultural practice or weather that delays
seed germination or seedling growth increases the probability
of seedling disease. Such factors include phytotoxicity due to
weed control chemicals, irrigation during cool weather, excessive
planting depth, excessively cold or compacted soils, poor quality
seed, and rainfall prior to cap removal, which may cause crusting
problems. Temperature is frequently the most important factor
influencing the rate of seed germination. The optimum soil temperature
for cotton seed germination and emergence is between 86o and 95o
F, with a minimal temperature of about 55o F. Unfortunately, soil
temperatures are extremely variable and may be marginal for growth
and seedling emergence during the planting period.
Seedling disease caused by R. solani can be controlled by:
- Use of high quality seed.
- Fallowing or rotation of fields to crops other than cotton.
 |
| Typical symptoms on cotton seedling
caused by the damping-off pathogen, Rhizoctonia solani. |
- Planting when soil temperature at the 2-inch depth is 65o
or above at 8 a.m.,preferably for two or three consecutive days.
- Fungicide treatment. A number of highly effective fungicides
are registered for seed treatment, incorporation into the planter
box, or application into the furrow during planting. These chemicals
include PCNB, chloroneb, captan, and carboxin.
- Seedbed preparation. It is important to obtain a weed-free,
well-pulverized bed. Any preplant herbicide should be properly
applied. Improper incorporation and excessive rates of certain
herbicides, such as trifluralin, may increase seedling disease
because of reduced plant vigor.
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Black root rot of cotton
Black root rot of cotton, caused by the soilborne fungus Thielaviopsis
basicola, was first reported in North America in Sacaton, Arizona,
in 1922. The disease was described as an internal collar rot of
mature American-Egyptian cotton, Gossypium barbadense L. Later,
it was found to cause a seedling root rot of both G. barbadense
and G. hirsutum L. (upland cotton). Historically, this disease
has been a problem in Arizona at elevations above 3,500 feet where
soil temperatures are cooler at planting than at lower elevations.
However, recently the incidence of the disease at lower elevations
has increased corresponding with an increase in acreage of Pima
cotton. Pima cotton requires a longer growing season than upland
cotton and is planted earlier when colder soil temperatures favor
disease.
 |
| Typical root symptoms in cotton caused
by the black root rot pathogen. Thielaviopsis basicola. |
Symptoms. - Thielaviopsis
basicola causes a black cortical decay of the tap root, delays
seedling development and may cause seedling death. However, infected
plants can recover because of cortical regeneration and secondary
root growth. On the contrary, plants infected with R. solani usually
die. Root symptoms disappear when rapid plant growth occurs during
warm weather.
Epidemiology and control
- All varieties of upland cotton are equally susceptible to black
root rot. Pima cotton also is equally susceptible. Infected roots
are black in appearance because of the large number of black spores
that are produced on infected tissue. These spores survive in
the soil and germinate and initiate root infection during early
root elongation. Seedling disease is more severe and widespread
when fungal inoculum increases because of repeated planting of
cotton in the same field. Conversely, fallowing or small grain
rotation reduces fungal inoculum and subsequent disease incidence
and severity. This disease is similar to Rhizoctonia damping-off
in the relationship of soil temperature to disease incidence and
severity. Planting into cold soil is the primary cause of disease.
There are no commercial seed treatments that are effective for
control.
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Seedling disease caused by
Pythium spp.
Several species of Pythium have been described as causing seedling
disease in cotton. These soilborne organisms cause seed decay,
pre-emergence and postemergence damping-off and necrotic lesions
on both tap and secondary roots. Damage occurs primarily when
soils are cold and saturated with water. In Arizona they do not
cause any significant loss in cotton. Pythium ultimum, for example,
which is commonly cited as an important pathogen throughout the
Cotton Belt, has not been identified as a pathogen of cotton seedlings
in Arizona. The reasons for absence of this disease are not understood.
The most likely explanation is that cotton is planted into preirrigated
beds, thus, eliminating saturated soil conditions during early
seedling growth. The normal irrigation cycle is a preplant irrigation
two to three weeks prior to seeding followed by a second irrigation
approximately one month after seeding. To avoid excess moisture,
do not irrigate during cool weather, and allow preirrigated beds
to drain before planting.
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Phymatotrichum root rot
Phymatotrichum root rot (Texas root rot), occurs only
in the low-organic matter soils of the Southwestern United States
and Central and Northern Mexico. The fungus that causes the disease,
Phymatotrichum omnivorum, has the largest host range of any known
plant pathogen. It causes a root rot in over 2,300 species of
dicotyledonous plants, including not only cotton, but also other
important Arizona crops such as alfalfa, stone fruits, grapes,
sugar beets, and many ornamental trees and shrubs.
 |
 |
| Typical kill patterns caused
by Phymatotrichum omnivorum (Texas Root Rot) in fields in
Marana, Arizona. These aerial photographs were taken at approximately
3,000 feet elevation during late summer. Note the varying
sizes of the patterns. |
Distribution - Heavily infested areas of Texas root rot
are found in the flood plains and certain tributaries of the Gila
River (Safford, Duncan, Solomon, Thatcher, Fort Thomas, Pima,
Eden, Florence, Sacaton, Buckeye, Gila Bend, Agua Caliente, Growler,
Roll, Mohawk, and Dome Valley); the Santa Cruz River (Sahuarita,
Tucson, Cortaro, Avra Valley, Rillito, Marana, Red Rock, and Eloy);
the San Pedro River (Hereford, St. David, Benson, Pomerene, Redington,
Mammoth, and Winkelman); Colorado River (Parker, Poston, Ehlenberg,
Yuma, Somerton, and Gadsden) and certain locations and tributaries
of the Salt River and Queen Creek in areas around Scottsdale,
Mesa, Higley, Magma, and Queen Creek. Other infested areas include
Chandler, Aguila, San Simon, Bowie, McNeal, Douglas, and Duncan.
The “Mesa” (land at elevations above the influence
of the Colorado River) in Yuma County seems to be free of the
disease, whereas many “valley” production sites are
infested. Although Phymatotrichum root rot occurs at elevations
as high as 4,700 feet in the Elgin and Sonoita areas of Santa
Cruz County, the disease has never been detected in the higher
elevation farming areas that stretch south from Bonita through
Willcox to Kansas Settlement. In the Sulphur Springs Valley, the
disease is only found near Elfrida, McNeal, and Douglas areas.
 |
| A photomicrograph showing the typical
morphological structure of a strand produced by Phymatotrichum
omnivorum on the tap root of an infected cotton plants. |
Symptoms - All varieties
of upland (Gossypium hirsutum) and Pima cotton (Gossypium barbadense)
are susceptible to Texas root rot. However, Pima cotton is usually
more severely affected because it has a longer growing season
and exposure. Symptoms generally appear first during flowering
and boll set. Usually, symptoms consist of rapid wilt and death
of infected plants, while dead or dying leaves remain attached
to the plant. The tap root is destroyed, and the fungus forms
characteristic “strands” on the surface of the rotted,
cortical outer tissue. Positive identification of the disease
requires microscopic examination of the “strands” that
are unique to the pathogen.
Control - Texas root rot is restricted to localized areas
in individual fields; it is not spread by irrigation or tillage.
Apparently this is due to survival of the fungus below plow depth.
When fields are continuously cropped with cotton, these infested
areas appear in the same location every year.
 |
| A microscopic view of the unique mycelial
characteristic of P. omnivorum. |
To reduce the disease, spot treatment of infested areas with
up to 20 tons/acre of manure has been used successfully in some
locations in Arizona. Summer rotations with immune grasses, such
as Sudan grass also, have reduced disease incidence. Double cropping
systems (barley, wheat followed by late cotton) also have markedly
reduced the disease in several locations. Injection of methyl
bromide/chloropicrin into preirrigated cotton beds at 18-inch
depths has given good control of the disease in Marana and Safford.
The difficulty with this treatment is high cost and erratic carry-over
effects in subsequent plantings. There are no chemical treatments
that are recommended at this time.
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Verticillium wilt
Verticillium wilt is caused by the soilborne fungus Verticillium
dahliae. It is a common and serious disease of upland cotton,
Gossypium hirsutum, but not Pima cotton. Upland cotton
varieties will vary in terms of their susceptibility to Verticillium
wilt. Verticillium wilt occurs from South Carolina across the
Cotton Belt to California, and throughout most other cotton producing
areas of the world.
 |
| A field picture of mature cotton plants
showing typical leaf symptoms in Upland cotton of Verticillum
wilt caused by vascular pathogen, Verticillum dahliae . |
The fungus that causes Verticillium wilt is one of the most widespread
and destructive plant pathogens. In Arizona, V. dahliae,
causes disease not only in cotton but in many weeds, ornamentals,
vegetables, fruit crops and field crops, including olive, pistachio,
tomato, and safflower. The fungus is able to survive indefinitely
in soil because of the production of small, seedlike structures
called microsclerotia. These microsclerotia are produced in large
numbers in the roots, stems, and leaves of infected cotton plants
during late summer and early fall. They are returned to the soil
during harvesting, stalk shredding, and tillage operations.
Symptoms - Symptoms in cotton are dependent on four interrelated
factors: the cultivar grown; soil inoculum level; the strain of
V. dahliae present; and soil and air temperature.
Verticillium wilt does not occur in Pima cotton (Gossypium
barbadense) because this species is resistant to the strains
of V. dahlia that occur in Arizona. The cultivars of upland cotton
grown in Arizona, however, are susceptible to the disease. Since
the fungus is most active at temperatures from 70o to 80o F, symptoms
of wilt normally are seen in late summer and early fall as air
temperatures decline. Midsummer temperatures at the lower elevations
in Arizona are usually too high for the fungus, and symptom expression
is masked. The disease is most common and serious at elevations
above 3,500 feet in Cochise, Graham, and Greenlee counties where
air temperatures are more favorable for symptom expression.
 |
| Typical vascular discoloration of stems
caused by V. dahliae. |
Although seedlings occasionally show effects of the disease during
unusually cool spring weather in Arizona, the most common symptoms
occur in mature plants in late summer. Irregular, light-green
areas appear first on the lower leaves between the veins and along
the leaf margins. These areas later become dull-yellow and then
brown in color. Defoliation and boll shedding may occur during
cool weather. Bolls of infected plants may open prematurely and
have reduced fiber quality. The fungus grows internally in the
stem and causes streaks of light to dark discoloration of the
vascular system. In external appearance, roots remain healthy,
in contrast to Phymatotrichum root rot where the entire tap root
is destroyed.
Control - Continuous planting of cotton increases presence
of the wilt organism in soil. Fallowing or rotating with non-host
summer crops such as sorghum, Sudan grass, or corn or winter rotation
with small grains appears to reduce disease incidence in certain
locations. Cultural practices that promote early maturity or the
use of early maturing or short-season cultivars may reduce disease
incidence. Excessive use of nitrogen and irrigation should be
avoided. The fungus can be spread in soil, by equipment, and in
gin trash. There are no known practical methods of chemical control.
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Leaf diseases
Of the several leaf diseases of cotton known throughout the world,
only three, Southwestern rust (Puccinia cacabata); Alternaria
leaf spot (Alternaria macrospora) and Bacterial blight
(Xanthomonas malvacearum) are known to occur in Arizona.
 |
| Ground photograph at the margin of a
kill pattern of Phymatotrichum root rot in a Pima cotton field
in Marana, Arizona. |
 |
| Typical leaf symptoms of cotton rust. |
Southwestern rust
In Arizona, Southwestern
rust is the most important of the three foliage diseases. Rust
was first described from Baja California, Mexico, in 1893 and
from Arizona in 1922. The disease occurs only in certain parts
of Southern Arizona, New Mexico, West Texas, and Northern Mexico.
The fungus that causes the disease lives on cotton and grama grass
(Bouteloua spp.). Species of Bouteloua occurring in Arizona
that are known hosts of rust include two annual species; Needle
grama (B. aristidoides), Six-weeks grama, (B. barbata),
and perennial Rothrock grama (B. rothrockii). The Bouteloua
spp., which are infected with P. cacabata, are the source
of infection for cotton.
During summer
rains, the spore stage on grama grass germinates to produce airborne
spores which are carried up to eight miles and cause initial infections
in cotton. There is no repeating spore stage on cotton. All new
infections on cotton are dependent upon spore showers from grama
grass; the spores produced on cotton can only infect grama grass.
Disease incidence is usually erratic in Southern Arizona and
depends on summer rains, high humidity and an infected source
of grama grass for inoculum. The disease is known to occur only
in Cochise, Santa Cruz, Graham, Greenlee, and Pima counties.
Symptoms - The most common symptom in cotton is the appearance
of bright yellow to orange spots on the upper and lower leaf surfaces.
The spots become brown with age. Spots may appear on any of the
above ground parts including bracts and bolls. Severe
 |
| A close-up view of the leaf lesions caused
by the cotton rust fungus. |
 |
| Typical stem lesions caused by the cotton
rust fungus on grama grass. |
infections may cause defoliation and dwarfing of bolls. Spores
from these lesions do not infect cotton. If the weather is favorable,
several spore showers from grama grass may occur throughout the
summer “rainy season.” The first symptoms on grama grass
are elongate brownish spots on the leaves (uredial lesions). The
spores produced in these lesions are spread by air and may reinfect
grama grass. A black spore stage on grama grass (telial lesions)
appears later. The spores produced from this stage during summer
rains infect cotton to complete the cycle.
Epidemiology and Control - Typically cotton rust appears
in Southeastern and Southern Arizona during the so-called “monsoon
season” in July and August. Leaf wetness periods in excess
of 16 hours coupled with high humidities and moderate temperatures
are factors necessary for epidemics to occur. Rust is a very erratic
disease because of the dependance on extended wet and high rainfall
periods. Severe outbreaks can cause yield reductions of over 50
percent. The disease is occasionally found as far north as Pima
County but essentially it is restricted to the higher elevations,
higher rainfall and cooler producing areas in the southern and
southwestern parts of Arizona. Rust may be controlled by three
to four aerial applications during July and August of mancozeb
(Penncozeb, Dithane M-45 or Manzate 200) plus a sticker in 10
gals/water/acre. Follow label recommendations. Applications should
be made prior to the first “spore showers” because these
fungicides are only protective.
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| Typical lesions of Alternaria leaf spot
on leave of Pima Cotton. |
Alternaria leaf spot
The first report of Alternaria leaf spot of cotton in Arizona
was in 1922. Upland cottons (Gossypium hirsutum) in general
are considered to be highly resistant to the pathogen (Alternaria
macrospora) whereas varieties of American Pima cotton (G.
barbadense) are considered to be susceptible. However, recent
studies in Arizona indicate that some cultivars of upland cotton
are susceptible to the disease. For example, the increased acreage
of Deltapine 90 since its introduction in 1982, the reduced acreage
of Deltapine 61, and increased plantings of Pima S-6 explain the
increased reports of Alternaria leaf spot in the late 1980’s
and early
 |
| A photomicrograph of Alternaria macrospora,
the causal fungus of Alternaria leaf spot. |
 |
| Symptoms on bolls caused by Xanthomonas
malvacearum (Angular Leaf Spot). Note the water-soaked, rounded
lesions. |
1990’s. Deltapine 90 and Pima S-6 are susceptible whereas
Deltapine 61 was resistant. The disease occurs in the central
and southwestern parts of Arizona but not in the hotter and drier
western areas. Often times, as varieties change so do the frequency
of occurrences of Alternaria leaf. Often times, as varieties change
so do the frequency of occurences of Alternaria leaf spot.
Symptoms - On Pima cotton, Alternaria produces leaf spots
that start as tiny, dull brown, circular lesions, which may enlarge
to spots 1/4 to 1/2 inch in diameter. Because of higher humidity,
the lower leaves are more commonly affected. Spots also may occur
on bolls. Under favorable wet conditions during July and August,
leaf defoliation may occur.
Epidemiology and control - The most important factor in
the development of this disease is the length of time that leaves
remain wet. In most years there is insufficient rainfall to enable
leaf infection to occur. The fungus survives in dried infected
leaf tissue. Spores are produced during the summer rainy period
and are carried to plant tissue by wind. They grow only when a
film of water is on the leaf surface. Studies indicate that maximum
infection takes place when leaves are wet for more than 12 hours.
No chemical control methods are currently recommended.
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 |
| Leaf symptoms caused by infection by
Xanthomonas malvacearum. Both photographs were taken in Bowie,
Arizona during an unusually wet monsoon season. |
Bacterial blight
Bacterial blight of cotton or angular leaf spot, caused by the
bacterium Xanthomonas malvaearum, was first noted in Arizona
in 1922. Prior to the use of acid-delinting, the disease was relatively
important because the bacterium survived and was spread on fuzzy
seed. With the advent of the acid-delinting process, which eliminates
the pathogen from seed, the disease became rare and unimportant
in Arizona. However, occasionally, bacterial blight has appeared
under unusually wet summer “monsoon” conditions in cotton
areas above 3,000 feet in Southeastern Arizona. The disease has
not been detected in any other cotton producing areas in Arizona.
The bacterium is able to survive in infected, dry plant tissue
for many years.
Symptoms - In Arizona, symptoms have only been seen on
bolls and leaves. Lesions on bolls are circular, dark-green, water-soaked
and greasy in appearance. Water-soaked, angular lesions appear
on the leaves.
Control - Because of the infrequency of the disease, no
control measures are recommended for Arizona.
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Boll Rots
Boll rots occur primarily during the monsoon season when cotton
is exposed to rainfall and high humidity. Most boll decay is found
in the lower half of the plant canopy because of high humidity.
The three pathogens described under Leaf Diseases, Alternaria
macrospora, Puccinia cacabata and Xanthomonas malvacearum
also infect and cause specific symptoms on cotton bolls. These
symptoms are discussed in the previous section. O f the three,
only, X. vesicatoria, is able to invade and cause extensive
rot. This disease, however, is very rare in Arizona. Another pathogen,
Phytophthora capsaci occurs only in the Sulfur Springs
Valley of Cochise County. This invades uninjured bolls under wet
conditions and can causes extensive decay, but disease, is not
widespread, and no chemical treatment is recommended for control.
Although a large number of fungi have been implicated as causal
agents in boll rots, most of these organisms cannot penetrate
healthy boll tissue. Infection is caused by airborne spores that
invade bolls after opening. If insects such as the pink boll worm,
tobacco budworm, or any other factor that causes premature opening
exists, then the incidence of boll decay is increased.
The most significant problem associated with boll and fiber development
is the aflatoxin disease caused by the fungus Aspergillus flavus.
Because of the importance of this disease it will be discussed
in detail.
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Aflatoxins
Aflatoxins are carcinogens produced in many nuts, grains and
cottonseed by the fungus Aspergillus flavus. The fungus
occurs throughout the United States, but aflatoxin problems in
cotton are serious only in Western Arizona, the Southern California
desert, and the lower Rio Grande Valley in Texas. In Arizona,
aflatoxin is significant only where cotton is grown below 2,000
feet elevation. When A. flavus infects lint and cottonseed,
the main problem created is the production by the fungus of aflatoxins.
Two factors, high temperature and lint moisture content above
15 percent are necessary for the fungus to infect and produce
these carcinogens.
Boll and seed invasion by A. flavus
Aspergillus flavus colonizes dead organic matter and produces
large numbers of spores. The entry point into the cotton boll
is often the exit hole of the larval stage of the pink bollworm.
Once colonization takes place, A. flavus invades the cotton
fiber. This lint invasion results in weakened fiber and discoloration
called “yellow stain.” After initial lint invasion,
the fungus penetrates the seed coat and colonizes the meat portion
of the seed. Aflatoxins are produced in this portion, but not
in the seed coat or lint. Seed invasion and the formation of aflatoxins
will continue as long as seed moisture is in excess of approximately
15 percent.
Climate and aflatoxin production in cottonseed
Temperature and humidity are the primary factors influencing
boll invasion by A. flavus and the subsequent formation
of aflatoxins in cottonseed. The high temperatures and humidities
found in Arizona and Southern California cotton fields favor boll
invasion by A. flavus, and moist lint of an opening boll
is very susceptible to infection.
The critical period for boll invasion and contamination by aflatoxins
in Arizona is early August to mid-September when bolls start to
open. Approximately 95 percent of the seed infection and formation
of aflatoxins takes place in bolls on the lower one-third to one-half
of the plant, where dense leaf canopies maintain high humidities
and prevent air movement.
The average level of aflatoxins detected in the cottonseed crop
fluctuates from year to year. Temperature is the most important
factor that determines whether contamination by aflatoxins of
seed will take place. When nighttime minimums are consistently
below 70o F for this period, as is the case at the higher elevations
in Arizona, the San Joaquin production areas of California, and
other production areas of the United States, aflatoxin seed contamination
is insignificant.
Insects and aflatoxin production
The pink bollworm, Pectinophora gossypiella, is a major
factor contributing to the aflatoxin problem. Larvae of this moth
invade bolls of upland cottons 14 to 21 days after flowering when
they are most susceptible. Under dry conditions yield reductions
are not significant unless over 25 percent of the bolls are infected.
With the occurrence of high humidities and rainfall, however,
during the latter half of July into August and September, extensive
losses can occur because damaged bolls are invaded by miscellaneous
fungi that cause boll rots. The exit hole of the pink bollworm
becomes an entry point for infection by airborne spores of A.
flavus . Bollworms (Heliothus spp.) feeding sites as
well as any other insect punctures also enable A. flavus
to enter into boll tissue. Aspergillus flavus is not able
to infect and penetrate undamaged, intact boll tissue.
Control - Currently, there is no practical field control
of cottonseed invasion by Aspergillus flavus and subsequent
contamination by aflatoxin. In Arizona, approximately 80 percent
of the whole seed produced is processed for meal, oil, linters,
and hulls. In the delinting, dehulling, and oil extraction process,
almost all of the aflatoxin resides with the meal. The meal represents
only about 40 percent to 50 percent of the seed weight, so, in
effect, the aflatoxin level doubles in this by-product of whole
cottonseed processing. In reality, a seed producer must control
to 10 parts per billion (ppb) in order for the processor successfully
to produce 20 ppb meal.
Humidity may be regulated by improving air movement through the
lower leaf canopy. Skip-row planting in a 2 x 1 or 4 x 1 mode
will improve air movement around the bolls in a lower canopy and
will result in less aflatoxin than is encountered in similar solid
planted fields. Unfortunately, skip-row cultivation will only
maintain aflatoxin levels below 10-20 ppb when a very light season
is encountered. Currently there is no way to predict at planting
time the degree of aflatoxin contamination that will occur during
August and September.
“Rank” cotton is usually more susceptible to the problem.
Careful management of water and fertilizer on land known to produce
rank cotton will add in maintaining lower levels. Reduction of
the growing season, and limiting the number of irrigations in
August show promise for reducing aflatoxin levels.
Good midseason insect control, particularly that of the pink
bollworm complex, from mid-July to early September, is important
for the maintenance of low levels of aflatoxin.
There is a piece of equipment, common to cotton production, which
has the ability of segregating seed with lower levels of aflatoxin
from highly contaminated seed. The spindle harvester was designed
to remove fluffed lint efficiently and is inefficient in removing
unfluffed or tight locules and/or PBW-bollworm complex damaged
bolls, which contain most of the aflatoxin. Unfortunately, this
advantage is lost when a ground gleaner follows the spindle harvester.
Ground gleaners will retrieve most of the seed cotton remaining
on the plant from the ground. This seed is then routinely mixed
with spindle-picked seed at the gin. In several studies in Arizona,
aflatoxin levels were 50 times higher in seed retrieved by ground
harvesters. Segregation on the basis of harvester type during
a light aflatoxin year would serve to provide a large quantity
of seed with acceptable levels of aflatoxin.
Cottonseed and meal above the tolerances set by the U.S. Food
and Drug Administration (20 to 300 ppb) depending on the intended
use of the feed must be treated with ammonia or mixed with uncontaminated
feeds to produce a mixture within the limits of the tolerance.
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 |
| Symptoms of cotton leaf crumple on leaves
of Upland cotton. |
Cotton leaf crumple virus
History - This
disease was first described from California in 1954 and from Arizona
in 1960. A major epidemic of cotton leaf crumple occurred in Arizona
cotton in 1981. The epidemic was associated with high populations
of the sweet potato whitefly (Bemisia tabaci) and large
acreages of perennially grown (stub) cotton.
Symptoms -
Leaves on infected plants are small in size, cupped downwards
and crinkled in appearance. Leaf veins are distorted and thickened.
Plants infected when young are stunted.
Host range - Studies in 1986 indicated that Stoneville
germ plasm is less susceptible than Deltapine germ plasm. Pima
cotton is also susceptible. Cheeseweed (Malva parviflora)
and bean (Phaseolus vulgaris) are hosts of the virus in
inoculated greenhouse plants, but its natural host range is not
known.
Epidemiology and control - Cotton leaf crumple, a geminivirus,
is the only virus disease of cotton described in the United States.
It is vectored only by the whitefly. The virus is not mechanically
transmitted. The disease was most important in Arizona when ratooning
of cotton was practiced in the warmer western and central production
areas. The ratooning of infected plants enabled the virus to establish
itself systemically in young plants, causing significant yield
losses. Seed cotton was never as seriously damaged. Currently,
the disease is scattered throughout areas where whiteflies exist.
Cotton is no longer ratooned in Arizona and the disease is economically
important only when young plants are infected. Infections that
occur in late July or August cause symptom development but little
or no yield loss.
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Root Knot Nematode
The Southern root–knot nematode (Meloidogyne incognita)
is the only nematode affecting cotton in Arizona. The nematode
is restricted to sandy soils where it attacks both short and long
staple varieties.
Symptoms - Galls and root swellings are small but visible
on roots. The galls appear as early as one month after planting.
Galls on cotton roots are smaller than root–knot galls on
other hosts such as tomato, cucurbits, and other vegetables.
Rotation may be the best method for control. In Arizona, cotton
can safely be planted following lettuce and most nondormant varieties
of alfalfa.
Summer fallowing can be an effective means of control, providing
there is root destruction of the previous crops, and susceptible
weeds are prevented from growing.
When crop rotation is impractical, consideration must be given
to soil treatment with a suitable nematicide. The decision is
based on the cost of treatment, the number of root knot nematodes
in the soil, the anticipated temperature during seed germination
and development of the tap root, the percent of the field with
sandy areas that exceeds 60 percent or more sand and the anticipated
price of cotton. At present, soil fumigants remain the most effective
and economical method of controlling root knot nematodes attacking
cotton.
The only soil fumigants presently available for use in cotton
are various formulations of 1-3, D (dichloropropene). These liquid
compounds must be injected into the soil before planting. Soil
should always be in excellent tilth with temperatures between
60o F and 80o F. The manufacturer’s label carries instructions
for the correct dosage. For best results with soil fumigants,
the beds should be listed and irrigated before the material is
applied.
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Nonpathological
Cotton Problems
Nutrient Deficiency Symptoms
Nitrogen
Nitrogen (N) is the plant nutrient required in largest amounts
and the only mineral nutrient to which cotton has consistently
been shown to respond in Arizona. In Arizona, with the exception
of water, N is the first limiting nutrient for plant growth. Practically
all soils will respond to applications of fertilizer N in terms
of cotton growth and development. Nitrogen is one element that
growers have control over, and it proves to be very important
in Arizona cotton production.
In the plant, N is a mobile nutrient in that as deficiencies
develop, N-containing components in older tissues (lower on the
plant) break down, releasing N forms which are then translocated
to younger, developing portions of the plant (near terminal areas).
Therefore, when N deficiencies develop, they are commonly seen
first as yellow or red leaves on the older, lower portions of
the plant. A crop that is developed under a continual deficiency
of N will tend to mature early and have reduced yields.
The N nutritional status of a cotton crop can be evaluated in-season
by use of petiole sampling and analysis for NO-N. Guidelines for
use of this technique in N fertility management is outlined by
Pennington and Tucker, 1984 (The Cotton Petiole, a Nitrogen Fertilization
Guide, the University of Arizona College of Agriculture, Publication
No. 8373). Preseason soil samples for residual NO-N can also be
of benefit to managing a N fertility program for cotton. In general,
split applications of fertilizer N throughout the growing season
are highly recommended on all soils, particularly medium- and
coarse-textured soils versus larger one-time applications. Also,
unfavorable soil physical conditions such as compaction, saturation,
or dryness can reduce N uptake by cotton.
Phosphorus
Phosphorus (P) is an essential plant nutrient used in the storage
and transfer of energy within the plant. Important functions of
P include formation and transfer of energy, and formation and
utilization of carbohydrates.
Although soils of Arizona are often high in total P, the amount
of P available for plant uptake and utilization often has been
a point of concern. Arizona soils commonly used for cotton production
generally have high pH conditions (pH $ 7.5) and are very calcareous
in nature. These conditions often lead to a considerable degree
of P “fixation.” However, research results, show no
consistent response of cotton to P fertilization. Several experiments
have been carried out with broadcast P methods for a number of
years, and experiments with banded P fertilization have been conducted
only on a limited basis thus far. Therefore, P deficiencies in
Arizona cotton are very rare. This is often attributed to the
very warm conditions in the growing season, and possibly to cotton’s
low requirement for P.
Symptoms of P deficiency include slow early growth with darkened
or red plant tissue (anthocyanin accumulation) particularly at
early stages of growth. Plants generally grow out of symptoms
as the soil temperatures warm, allowing greater root growth. Correction
of suspected P deficiencies after planting is very difficult.
Potassium
Potassium (K) is the third macronutrient (besides N and P). Within
the plant, K is not incorporated into any compounds and tends
to remain in ionic form (K+). Potassium is essential for photosynthesis,
starch formation, and translocation of sugars within the plant.
K is necessary for chlorophyll development, but it is not an actual
part of its molecular structure.
Arizona soils are generally high in plant-available K, and deficiencies
have not been confirmed. Accordingly, responses to K fertilization
by cotton have not been documented in Arizona. Deficiency symptoms,
therefore, are not likely to be encountered. If, however, one
suspects K deficiency, symptoms include: bronzing and scorching
of leaves, weak stems, and reduced boll size and fiber development.
For most plants, K is considered as a mobile nutrient in the plant
leading to deficiency symptoms developing first on the lower,
older leaves of the plant. However, when K deficiencies have been
diagnosed on cotton, it has proven to be somewhat of an exception.
In cotton, characteristic K deficiency symptoms often develop
first on younger leaves. This is particularly true when symptoms
develop late in the season as bolls are developing, and symptoms
appear on leaves adjacent to rapidly filling bolls. This is attributed
to strong sink demand by the bolls, causing export of K from adjacent
tissue (leaves) which are often in younger portions of the plant.
Similar to P, in-season corrections of suspected K deficiencies
are very difficult to accomplish. If K deficiencies are suspected,
one should have soil samples analyzed for exchangeable soil-K
by 1-foot increments to a depth of 3 to 4 feet.
Zinc
Zinc (Zn) is a micronutrient essential for plant growth and development.
Its designation as a “micronutrient” does not reflect
any lesser degree of importance in terms of essentiality, only
that it is required in very small amounts relative to the macronutrients
(N, P, and K) or secondary nutrients (Ca, Mg, and S). Zinc is
essential as an activator (cofactor) for many plant enzymes during
all stages of plant growth. Zinc is relatively immobile in the
plant and therefore, small but consistent levels of Zn must be
taken up by cotton plants all season. In agricultural soils from
Arizona, extractions performed with DTPA that result in $ 0.65
parts per million (ppm) Zn are considered to be sufficient for
cotton production. In recent experiments working with soil and/or
foliar applications of Zn, no yield responses were measured in
upland or Pima cotton.
When Zn deficiencies have been confirmed on cotton plants, symptoms
often include: shortened internodes and small leaves with interveinal
chlorosis (white or bleached-out tissue between the veins of leaves).
Later in the season, very small bolls (“ping-pong” bolls)
develop under Zn-deficient conditions. When Zn-deficiency symptoms
develop early in the season, it is often due to poor root development.
Poor root development could be caused by soil compaction or cold
soil conditions. In the latter case, plants will often grow out
of the Zn-deficient symptoms when soil temperatures warm.
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Saline/Sodic Soil Conditions
Many soils in the desert southwest are naturally saline and/or
sodic. These soils are usually the object of reclamation and management
techniques to control and minimize salt and sodium problems. Saline
and/or sodic soil conditions often are the cause of difficulties
in getting and keeping a good stand of cotton in many fields.
Saline Soils
Saline soils are defined as “nonsodic soils that have sufficient
soluble salt to affect adversely the growth of most plants.”
Salt contents which result in electrical conductivity measurement
of 4 mmhos per cm or greater from a saturated extract (ECe), are
generally referred to as saline soils. Cotton plants are considered
to be moderately salt-tolerant. This is certainly true and cotton
plants are often pushed to their limits in terms of salt tolerance.
When present, excessive salts (ECe values of 10 mmhos per cm or
greater) cause stunted, slow-growing plants. They often are characterized
as having very short internodes, tough leathery textured leaves,
and often a burn or scar mark at the soil surface, as well as
poorly developed roots.
Prevention is usually the best cure for salt problems. Saline
conditions are often the result of irrigation water high in soluble
salts. In such cases, rotation to crops such as small grains or
alfalfa, which permit border irrigation, is found to be helpful.
Actually, it is the leaching of the salts that is of most benefit
from the border flooding. Also, alternate row irrigation can be
helpful in preventing a salt accumulation at the top of the bed
(in the row) which will occur with every-row irrigation.
Sodic Soils
Sodic soils are defined as “nonsaline soils containing sufficient
exchangeable sodium to affect adversely crop production and soil
structure.” Sodic soils have exchangeable sodium percentages
(ESP) of 15 or greater. They are characterized as having sodium
adsorption ratios from the saturated extract (SARe) of 13 or greater.
Soils high in sodium are found to have water penetration problems
due to dispersing soil particles, which in turn causes a sealing
of the soil. Soil sealing due to dispersion is often seen as a
crusting problem and the associated difficulties with poor seedling
vigor. However, additional irrigations and cultivations will not
solve this problem, perhaps only making it worse.
In some cases, the sodic conditions within a field may be within
tolerable limits, only to be pushed into a dangerous excess by
an inadvertent act of management. Many growers apply fertilizer
nitrogen (N) as anhydrous ammonia directed through the irrigation
water. This may cause an increase in the pH of the irrigation
water due to the ammonium hydroxide that is formed upon the addition
of the anhydrous ammonia. The resultant increase in the pH of
the water may cause soluble calcium and magnesium in the water
to precipitate from the solution by combining with bicarbonate,
while any sodium in the water remains in solution. This may then
result in irrigation water that is high in its proportion of sodium
to calcium and magnesium. Continuous use of this practice and
the resulting water, can create a sodic soil and field conditions
that are increasingly difficult to manage. This can be a particularly
tough problem at the time of seedling establishment.
There are several approaches to correcting a sodic problem after
it has been properly identified. One soil amendment that can be
used is gypsum. The first step in this reclamation process is
the incorporation of the gypsum at a satisfactory rate. The calcium
from the gypsum will exchange for the sodium in the soil, leaving
a soluble form of sodium. The second step is then to leach the
soluble sodium (often sodium sulfate) with excess (good quality)
irrigation water. The use of sulfuric acid on sodium-affected
soils can also serve as a good reclamation effort when the soils
contain sufficient free lime (calcium carbonate). The soil reactions
involved are similar as when gypsum is used. In each case, the
calcium is exchanged for the sodium, which then can be leached
down through the soil profile with sufficient (quality and quantity)
irrigation water. These efforts at reclamation should be beneficial
when exchangeable sodium percentages (ESP) approach or exceed
10.
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Other
Possible Nutritional and Physiological Problems
Excessive Nitrogen
A key principle in managing the nitrogen (N) fertility of a cotton
crop is to supply the N to the plants in adequate, but not excessive,
amounts. In cotton production, a critical management objective
is to optimize the vegetative/reproductive balance in the crop
so as to obtain the highest lint production as possible. Growers
in Arizona recognize the responsiveness of cotton to both water
and N. However, due to the indeterminate nature of the cotton
plant, conditions such as excessive N or water at the right (or
wrong) time can stimulate vegetative growth in the process of
sacrificing reproduction and yield potential. Excessive N particularly
late in the growing season, can promote vegetativeness, delay
maturity, and create further problems with crop termination and
defoliation.
Symptomatically, one may look for excessive N conditions by sampling
petioles and analyzing for NO-N over a period of several weeks.
These values should be referenced against guidelines outlined
by Pennington and Tucker, in accordance to stage of growth. Research
results indicate that petiole NO-N levels can be drawn below 2,000
ppm and 1,000 ppm by late in the season (mid-September) without
sacrificing yield potential and providing for plant development
conducive to senescence and defoliation.
The best management against excessive N fertility status of cotton
is to employ diagnostic tools such as preseason soil samples to
evaluate residual soil NO3 levels, irrigation water analysis to
estimate NO3 contributions with each irrigation, and in-season
sampling and analysis of cotton petioles for NO-N content. Historically,
water stress has been a management tool used in some cases to
counteract the vegetative tendencies brought on by elevated N
fertility in cotton. However, it is not an advisable practice
to impose intentionally a water stress on a crop, recognizing
the sacrifice in yield potential that results from water stress
conditions. The best advisable practice is to manage N fertility
with available tools from the beginning of the season so as to
provide adequate levels of N for crop production, but avoiding
excesses.
Ammonia Toxicity
One of the occasional consequences of fertilization with anhydrous
ammonia as a sidedress application, is that of plant damage due
to direct exposure to the ammonia (NH3) gas. When side-dressing
young cotton plants with an application of anhydrous ammonia,
it is important to make the application a sufficient distance
away from the crop row so as to avoid damage to the cotton roots.
Also, it is important to make applications under proper soil moisture
conditions so that the soil seals after the passage of the shank
applicator. Otherwise ammonia gas can escape causing both the
loss of fertilizer N and damage to the aboveground portion of
the plants.
Anhydrous ammonia gas is very hydrophilic (water-loving) and
therefore, exerts a severe and caustic burn to any plant tissue
(root or shoot) that comes into direct contact with it. Direct
exposure to anhydrous ammonia gas severely damages cell membranes
and causes a blackening of the exposed tissue. Wilting also may
occur upon direct root exposure shortly after application of the
anhydrous ammonia. Damaged plants usually will recover despite
a setback before recovery. It is also important to point out that
applicators themselves should exercise necessary precautions when
using anhydrous ammonia due to the hazards associated with exposure
to any skin tissue (particularly the eyes).
Reddened Leaves
A condition that often draws some attention in a cotton field
is the development of reddened leaves. This phenomenon can be
caused by several different factors. It seems that a natural response
of the cotton plant (particularly upland) to a variety of stressful
situations is to lose its chlorophyll content, which of course
diminishes the green coloration of the leaves. With the loss of
the chlorophyll, another natural plant pigment becomes readily
apparent which are the anthocyanins, leading to the distinct red
coloration on the leaves. The senescing of plant tissue, whether
natural or induced by some stress, also often accelerates the
formation of anthocyanin. Therefore, a natural development of
red coloration of the leaves could be expected to some extent
late in the season as the crop is naturally aging on senescing.
This is particularly true in conditions where much cooler weather
is experienced.
There are also some other causes of red leaves, which one must
be conscious of if the condition develops. Nitrogen deficiency
symptoms often are noted as reddened leaves on the older, lower
portions of the plant. This possibility could be further diagnosed
by use of tissue sampling (petioles) extent of the affected areas
in the field, age of the crop, and other possible problems. Another
very common cause of red leaves is that of spider mite infestations.
In the case of spider mites, the development of red leaves and
red leaf spots will be restricted to small, localized areas within
a field, particularly at early stages of the population’s
development. Often it will also be restricted to the older, lower
portions of the plant at very early stages of development. One
can pursue this possibility by inspecting the leaves (underside
particularly) for the presence of spider mite colonies. The mites
and their eggs can easily be seen with the aid of a hand lens,
and also with the naked eye. Spider mite colonies usually have
a substantial amount of webbing developed on the surface of affected
leaves. Control of both spider mites and N deficiency is possible,
but early detection and diagnosis is very important in that regard.
Another possible cause of reddened leaves, particularly a distinct
pattern of red leaf spots, may be due to some types of insecticides.
One should consider recent insecticide applications, extent of
the affected areas, and any patterns when considering this as
a possible cause.
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are mentioned, shown, or indirectly implied in this publication
do not imply endorsement by the University of Arizona.
Document located
http://ag.arizona.edu/pubs/diseases/az1245/
Published July 2001
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