X Diseases and Production Problems of Cotton in Arizona · The last comprehensive discussion of cotton diseases in Arizona appeared in “Diseases of Field Crops in Arizona,” a

Cooperative Extension

Diseases and ProductionProblems of Cotton in Arizona

AZ1245July 2001

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ANT DISEASES

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2 The University of Arizona Cooperative Extension

Contents

INTRODUCTION ................................................................................................................................... 4PATHOLOGICAL COTTON PROBLEMS ......................................................................................... 5

SEEDLING DISEASE AND STAND PROBLEMS....................................................................................... 5RHIZOCTONIA SOLANI ...................................................................................................................... 5BLACK ROOT ROT OF COTTON .................................................................................................... 6

SEEDLING DISEASE CAUSED BY PYTHIUM SPP. .................................................................................. 6PHYMATOTRICHUM ROOT ROT............................................................................................................. 6VERTICILLIUM WILT....................................................................................................................................... 8LEAF DISEASES .................................................................................................................................................. 9

SOUTHEWESTERN RUST...................................................................................................................... 9ALTERNARIA LEAF SPOT .................................................................................................................... 11BACTERIAL BLIGHT .............................................................................................................................. 12

BOLL ROTS ...................................................................................................................................................... 12AFLATOXINS .................................................................................................................................................. 12

BOLL AND SEED INVASION BY A. FLAVUS ................................................................................. 12CLIMATE AND AFLATOXIN PRODUCTION IN COTTON SEED........................................ 13INSECTS AND AFLATOXIN PRODUCTION................................................................................ 13

COTTON LEAF CRUMPLE VIRUS ............................................................................................................ 14ROOT– KNOT NEMATODE ..................................................................................................................... 14

NONPATHOLOGICAL PROBLEMS ................................................................................................ 15NUTRIENT DEFICIENCY SYMPTONS .................................................................................................... 15

NITROGEN............................................................................................................................................... 15PHOSPHORUS ......................................................................................................................................... 15POTASSIUM .............................................................................................................................................. 15ZINC ........................................................................................................................................................... 16

SALINE/SODIC SOIL CONDITIONS........................................................................................................ 16SALINE SOILS ........................................................................................................................................... 16SODIC SOILS ............................................................................................................................................ 16

OTHER POSSIBLE NUTRITIONAL AND PHYSIOLOGICAL PROBLEMS............................. 17EXCESSIVE NITROGEN ............................................................................................................................... 17AMMONIA TOXICITY ................................................................................................................................. 17REDDENED LEAVES ...................................................................................................................................... 18

This information has been reveiwed by university faculty.

The University of Arizona Cooperative Extension 3

Diseases and ProductionProblems of Cotton in Arizona

MARY OLSEN

Plant Pathology Specialist

JEFFREY C. SILVERTOOTH

Agronomist

Based on material originally written by RICHARD HINE, Plant Pathologist (retired).Photographs by RICHARD HINE AND MARY OLSEN.

This information has been reviewed by university faculty.

ag.arizona.edu/pubs/diseases/az1245.pdf

AZ1245July 2001

Cooperative ExtensionCollege of Agriculture and Life Sciences

The University of ArizonaTucson, Arizona 85721


INTRODUCTION

HistoryCotton seed from Egypt was first introduced into

Arizona in 1901. Plantings for commercial observationswere made in Yuma and at a Territorial experimentalfarm affiliated with The University of Arizona in theSalt River Valley near Phoenix. In 1902 the Bureau ofPlant Industry intensified experimentation with extra-long staple Egyptian cotton in Yuma. A variety calledYuma was released in 1908. The first commercial cropof Yuma, 375 bales, was produced in 1912. In 1907,land for cotton research was acquired on the GilaIndian Reservation in Sacaton. Research therecontinued until 1957 when the Cotton Research Centerin Phoenix was constructed. Cooperative researchwith federal, state, and U A personnel took place at thisfacility until 1985 when cotton research was moved tothe Maricopa Agricultural Center in Maricopa, Arizona.The last comprehensive discussion of cotton diseasesin Arizona appeared in “Diseases of Field Crops inArizona,” a U A publication authored by plantpathologists J.G. Brown and R. B. Streets and publishedin 1934. The cotton diseases discussed in the publicationwere: Angular leaf spot, Boll rots, Soreshin,Southwestern Rust, Phymatotrichum root rot,Alternaria leaf spot, Root–knot nematode, and aphysiological disease, named Crazy-Top.

Production AreasMany different cultivars of cotton are grown in

various ecological areas in Arizona. Production areasvary greatly in elevation and annual rainfall. At oneextreme is the Yuma Valley in the southwestern cornerof Arizona along the lower Colorado River wherecotton is planted in the latter part of February.Elevations here are approximately 75-100 feet. In thisarea, average annual precipitation is less than 4 inches.In the southeastern part of Arizona in Graham, Cochise,and Greenlee counties, cotton is grown at elevationsabove 3,000 feet and plantings take place during themiddle of April. In Cochise County, for example,cotton is grown in Pearce (4,375 feet), Cochise (4,212feet), Bowie (3,765 feet) and San Simon (3,601 feet). Theproduction areas along the Gila River in Graham andGreenlee counties are approximately 3,000 feet and3,500 feet, respectively. At elevations of 4,000 feet,temperatures are approximately 12o F lower (maximumand minimum daily temperatures) than temperaturesat sea level. Also, there is a correlation in Arizonabetween elevation and rainfall. Low elevationproduction sites in the western part of Arizona averageless than 4 inches of rain annually, whereas rainfall atPearce (4,375 feet) is approximately 13-15 inchesannually. Cotton is exposed to the so-called “monsoon”season during July, August, and September. Thisperiod is characterized by heavy, irregularly distributedrainfall and high humidity. Rainfall during this periodat the higher elevations can range as high as 10-12inches during certain summers. These temperatureand rainfall differences play important roles in thedistribution and severity of cotton diseases in Arizona.


PATHOLOGICAL COTTON PROBLEMS

SEEDLING DISEASES AND STAND PROBLEMS

Although many pathogenic organisms are involvedin cotton seedling disease throughout the Cotton Belt,only two pathogenic fungi, Rhizoctonia solani andThielaviopsis basicola, cause disease problems in Arizona.These soil-borne fungi are widespread in Arizona soilsand cause seedling and root diseases in many plantsother than cotton.

Rhizoctonia solaniSymptoms - The most common symptom caused by R.

solani is postemergence damping-off. The fungusgrows into the lower stem and the upper portion ofthe root just below the soil line. Lesions on theroots are sunken and reddish-brown in color. Theinfected seedling dries up and dies. With a handlens it is possible to see the “fuzzy” brownishmycelium of the fungus in the girdled tissue.

Epidemiology and control - Rhizoctonia solani, a soilfungus with a highly active saprophyte stage,survives in the soil in the absence of cotton forindefinite periods of time. The isolates that causedisease in cotton are, for all practical purposes,restricted to that host. Pima and upland cottonsare equally susceptible to this disease. Typicalweather favoring seedling disease occurs whenplanting and emergence takes place when nighttemperatures are below 50o F. Any cultural practice

or weather that delays seed germination or seedlinggrowth increases the probability of seedlingdisease. Such factors include phytotoxicity due toweed control chemicals, irrigation during coolweather, excessive planting depth, excessively coldor compacted soils, poor quality seed, and rainfallprior to cap removal, which may cause crustingproblems. Temperature is frequently the mostimportant factor influencing the rate of seedgermination. The optimum soil temperature forcotton seed germination and emergence is between86o and 95o F, with a minimal temperature of about55o F. Unfortunately, soil temperatures areextremely variable and may be marginal for growthand seedling emergence during the planting period.

Seedling disease caused by R. solani can becontrolled by:

1). Use of high quality seed.2). Fallowing or rotation of fields to crops other than

cotton.3). Planting when soil temperature at the 2-inch depth

is 65o or above at 8 a.m., preferably for two or threeconsecutive days.

4). Fungicide treatment. A number of highly effectivefungicides are registered for seed treatment,incorporation into the planter box, or applicationinto the furrow during planting. These chemicalsinclude PCNB, chloroneb, captan, and carboxin.

5). Seedbed preparation. It is important to obtaina weed-free, well-pulverized bed. Anypreplant herbicide should be properly applied.Improper incorporation and excessive ratesof certain herbicides, such as trifluralin, mayincrease seedling disease because of reducedplant vigor.

A field picture showing serious stand problems in cotton.Picture taken in Coolidge, A rizona.

T ypical symptoms on cotton seedling caused by thedamping-off pathogen, Rhizoctonia solani.


Black root rot of cottonBlack root rot of cotton, caused by the soilborne

fungus Thielaviopsis basicola, was first reported in NorthAmerica in Sacaton, Arizona, in 1922. The disease wasdescribed as an internal collar rot of mature American-Egyptian cotton, Gossypium barbadense L. Later, it wasfound to cause a seedling root rot of both G. barbadenseand G. hirsutum L. (upland cotton). Historically, thisdisease has been a problem in Arizona at elevationsabove 3,500 feet where soil temperatures are cooler atplanting than at lower elevations. However, recentlythe incidence of the disease at lower elevations hasincreased corresponding with an increase in acreage ofPima cotton. Pima cotton requires a longer growingseason than upland cotton and is planted earlier whencolder soil temperatures favor disease.

Symptoms. - Thielaviopsis basicola causes a black corticaldecay of the tap root, delays seedling developmentand may cause seedling death. However, infectedplants can recover because of cortical regenerationand secondary root growth. On the contrary,plants infected with R. solani usually die. Rootsymptoms disappear when rapid plant growthoccurs during warm weather.

Epidemiology and control - All varieties of uplandcotton are equally susceptible to black root rot.Pima cotton also is equally susceptible. Infectedroots are black in appearance because of the largenumber of black spores that are produced oninfected tissue. These spores survive in the soiland germinate and initiate root infection duringearly root elongation. Seedling disease is moresevere and widespread when fungal inoculumincreases because of repeated planting of cotton inthe same field. Conversely, fallowing or smallgrain rotation reduces fungal inoculum andsubsequent disease incidence and severity. Thisdisease is similar to Rhizoctonia damping-off inthe relationship of soil temperature to diseaseincidence and severity. Planting into cold soil isthe primary cause of disease. There are nocommercial seed treatments that are effective forcontrol.

Seedling Disease Caused by Pythium spp.Several species of Pythium have been described as

causing seedling disease in cotton. These soilborneorganisms cause seed decay, pre-emergence andpostemergence damping-off and necrotic lesions onboth tap and secondary roots. Damage occurs primarilywhen soils are cold and saturated with water. In Arizonathey do not cause any significant loss in cotton. Pythiumultimum, for example, which is commonly cited as animportant pathogen throughout the Cotton Belt, hasnot been identified as a pathogen of cotton seedlings inArizona. The reasons for absence of this disease are notunderstood. The most likely explanation is that cottonis planted into preirrigated beds, thus, eliminatingsaturated soil conditions during early seedling growth.The normal irrigation cycle is a preplant irrigation twoto three weeks prior to seeding followed by a secondirrigation approximately one month after seeding. Toavoid excess moisture, do not irrigate during coolweather, and allow preirrigated beds to drain beforeplanting.

Phymatotrichum Root RotPhymatotrichum root rot (Texas root rot), occurs only

in the low-organic matter soils of the SouthwesternUnited States and Central and Northern Mexico. Thefungus that causes the disease, Phymatotrichumomnivorum, has the largest host range of any knownplant pathogen. It causes a root rot in over 2,300species of dicotyledonous plants, including not onlycotton, but also other important Arizona crops such asalfalfa, stone fruits, grapes, sugar beets, and manyornamental trees and shrubs.

T ypical root symptoms in cotton caused by the black rootrot pathogen. T hielaviopsis basicola.


Distribution - Heavily infested areas of Texas root rotare found in the flood plains and certain tributariesof the Gila River (Safford, Duncan, Solomon,Thatcher, Fort Thomas, Pima, Eden, Florence,Sacaton, Buckeye, Gila Bend, Agua Caliente,Growler, Roll, Mohawk, and Dome Valley); theSanta Cruz River (Sahuarita, Tucson, Cortaro, AvraValley, Rillito, Marana, Red Rock, and Eloy); theSan Pedro River (Hereford, St. David, Benson,

T ypical kill patterns caused by Phymatotrichum omnivorum(T exas Root Rot) in fields in Marana, A rizona. T hese aerialphotographs were taken at approximately 3,000 feetelevation during late summer. Note the varying sizes of thepatterns.

A photomicrograph showing the typical morphologicalstructure of a strand produced by Phymatotrichumomnivorum on the tap root of an infected cotton plants.

A microscopic view of the unique mycelial characteristic ofP. omnivorum.


Pomerene, Redington, Mammoth, andWinkelman); Colorado River (Parker, Poston,Ehlenberg, Yuma, Somerton, and Gadsden) andcertain locations and tributaries of the Salt Riverand Queen Creek in areas around Scottsdale, Mesa,Higley, Magma, and Queen Creek. Other infestedareas include Chandler, Aguila, San Simon, Bowie,McNeal, Douglas, and Duncan.

The “Mesa” (land at elevations above theinfluence of the Colorado River) in Yuma Countyseems to be free of the disease, whereas many“valley” production sites are infested. AlthoughPhymatotrichum root rot occurs at elevations ashigh as 4,700 feet in the Elgin and Sonoita areas ofSanta Cruz County, the disease has never beendetected in the higher elevation farming areas thatstretch south from Bonita through Willcox to KansasSettlement. In the Sulphur Springs Valley, thedisease is only found near Elfrida, McNeal, andDouglas areas.

Symptoms - All varieties of upland (Gossypium hirsutum)and Pima cotton (Gossypium barbadense) aresusceptible to Texas root rot. However, Pimacotton is usually more severely affected because ithas a longer growing season and exposure.Symptoms generally appear first during floweringand boll set. Usually, symptoms consist of rapidwilt and death of infected plants, while dead ordying leaves remain attached to the plant. The taproot is destroyed, and the fungus formscharacteristic “strands” on the surface of the rotted,cortical outer tissue. Positive identification of thedisease requires microscopic examination of the“strands” that are unique to the pathogen.

Control - Texas root rot is restricted to localized areasin individual fields; it is not spread by irrigation ortillage. Apparently this is due to survival of thefungus below plow depth. When fields arecontinuously cropped with cotton, these infestedareas appear in the same location every year.To reduce the disease, spot treatment of infestedareas with up to 20 tons/acre of manure has beenused successfully in some locations in Arizona.Summer rotations with immune grasses, such asSudan grass also, have reduced disease incidence.Double cropping systems (barley, wheat followedby late cotton) also have markedly reduced thedisease in several locations. Injection of methylbromide/chloropicrin into preirrigated cotton bedsat 18-inch depths has given good control of the

disease in Marana and Safford. The difficulty withthis treatment is high cost and erratic carry-overeffects in subsequent plantings. There are nochemical treatments that are recommended at thistime.

VERTICILLIUM WILT

Verticillium wilt is caused by the soilborne fungusVerticillium dahliae. It is a common and serious diseaseof upland cotton, Gossypium hirsutum, but not Pimacotton. Upland cotton varieties will vary in terms oftheir susceptibility to Verticillium wilt. Verticilliumwilt occurs from South Carolina across the Cotton Beltto California, and throughout most other cottonproducing areas of the world.

The fungus that causes Verticillium wilt is one of themost widespread and destructive plant pathogens. InArizona, V. dahliae, causes disease not only in cottonbut in many weeds, ornamentals, vegetables, fruitcrops and field crops, including olive, pistachio, tomato,and safflower. The fungus is able to survive indefinitelyin soil because of the production of small, seedlikestructures called microsclerotia. These microsclerotiaare produced in large numbers in the roots, stems, andleaves of infected cotton plants during late summerand early fall. They are returned to the soil duringharvesting, stalk shredding, and tillage operations.

A field picture of mature cotton plants showing typical leafsymptoms in Upland cotton of Verticillum wilt caused byvascular pathogen, Verticillum dahliae .


Symptoms - Symptoms in cotton are dependent onfour interrelated factors: the cultivar grown; soilinoculum level; the strain of V. dahliae present; andsoil and air temperature.

Verticillium wilt does not occur in Pima cotton(Gossypium barbadense) because this species isresistant to the strains of V. dahlia that occur inArizona. The cultivars of upland cotton grown inArizona, however, are susceptible to the disease.Since the fungus is most active at temperaturesfrom 70o to 80o F, symptoms of wilt normally areseen in late summer and early fall as airtemperatures decline. Midsummer temperaturesat the lower elevations in Arizona are usually toohigh for the fungus, and symptom expression ismasked. The disease is most common and seriousat elevations above 3,500 feet in Cochise, Graham,and Greenlee counties where air temperatures aremore favorable for symptom expression.

Although seedlings occasionally show effects ofthe disease during unusually cool spring weatherin Arizona, the most common symptoms occur inmature plants in late summer. Irregular, light-green areas appear first on the lower leaves betweenthe veins and along the leaf margins. These areaslater become dull-yellow and then brown in color.Defoliation and boll shedding may occur duringcool weather. Bolls of infected plants may openprematurely and have reduced fiber quality. Thefungus grows internally in the stem and causesstreaks of light to dark discoloration of the vascularsystem. In external appearance, roots remainhealthy, in contrast to Phymatotrichum root rotwhere the entire tap root is destroyed.

Control - Continuous planting of cotton increasespresence of the wilt organism in soil. Fallowingor rotating with non-host summer crops such assorghum, Sudan grass, or corn or winter rotationwith small grains appears to reduce diseaseincidence in certain locations. Cultural practicesthat promote early maturity or the use of earlymaturing or short-season cultivars may reducedisease incidence. Excessive use of nitrogen andirrigation should be avoided. The fungus can bespread in soil, by equipment, and in gin trash.There are no known practical methods ofchemical control.

LEAF DISEASES

Of the several leaf diseases of cotton knownthroughout the world, only three, Southwestern rust(Puccinia cacabata); Alternaria leaf spot (Alternariamacrospora) and Bacterial blight (Xanthomonasmalvacearum) are known to occur in Arizona.

Southwestern rust In Arizona, Southwestern rust is the most important

of the three foliage diseases. Rust was first describedfrom Baja California, Mexico, in 1893 and from Arizonain 1922. The disease occurs only in certain parts ofSouthern Arizona, New Mexico, West Texas, andNorthern Mexico. The fungus that causes the diseaselives on cotton and grama grass (Bouteloua spp.). Speciesof Bouteloua occurring in Arizona that are known hostsof rust include two annual species; Needle grama (B.aristidoides), Six-weeks grama, (B. barbata), and perennial

T ypical vascular discoloration of stems caused by V. dahliae.

Ground photograph at the margin of a kill pattern ofPhymatotrichum root rot in a Pima cotton field in Marana,A rizona.


Rothrock grama (B. rothrockii). The Bouteloua spp.,which are infected with P. cacabata, are the source ofinfection for cotton.

During summer rains, the spore stage on gramagrass germinates to produce airborne spores which arecarried up to eight miles and cause initial infections incotton. There is no repeating spore stage on cotton. Allnew infections on cotton are dependent upon sporeshowers from grama grass; the spores produced oncotton can only infect grama grass.

Disease incidence is usually erratic in SouthernArizona 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, SantaCruz, Graham, Greenlee, and Pima counties.

Symptoms - The most common symptom in cotton isthe appearance of bright yellow to orange spots onthe upper and lower leaf surfaces. The spotsbecome brown with age. Spots may appear on anyof the above ground parts including bracts andbolls. Severe infections may cause defoliation anddwarfing of bolls. Spores from these lesions do notinfect cotton. If the weather is favorable, severalspore showers from grama grass may occurthroughout the summer “rainy season.” The firstsymptoms on grama grass are elongate brownishspots on the leaves (uredial lesions). The sporesproduced in these lesions are spread by air andmay reinfect grama grass. A black spore stage ongrama grass (telial lesions) appears later. Thespores produced from this stage during summerrains infect cotton to complete the cycle.

Epidemiology and Control - Typically cotton rustappears in Southeastern and Southern Arizonaduring the so-called “monsoon season” in July andAugust. Leaf wetness periods in excess of 16 hourscoupled with high humidities and moderatetemperatures are factors necessary for epidemicsto occur. Rust is a very erratic disease because ofthe dependance on extended wet and high rainfallperiods. Severe outbreaks can cause yieldreductions of over 50 percent. The disease isoccasionally found as far north as Pima County butessentially it is restricted to the higher elevations,

T ypical leaf symptoms of cotton rust.

A close-up view of the leaf lesions caused by the cotton rustfungus.

T ypical stem lesions caused by the cotton rust fungus ongrama grass.


higher rainfall and cooler producing areas in thesouthern and southwestern parts of Arizona. Rustmay be controlled by three to four aerialapplications during July and August of mancozeb(Penncozeb, Dithane M-45 or Manzate 200) plus asticker in 10 gals/water/acre. Follow labelrecommendations. Applications should be madeprior to the first “spore showers” because thesefungicides are only protective.

Alternaria leaf spotThe first report of Alternaria leaf spot of cotton in

Arizona was in 1922. Upland cottons (Gossypiumhirsutum) in general are considered to be highly resistantto the pathogen (Alternaria macrospora) whereas varietiesof American Pima cotton (G. barbadense) are consideredto be susceptible. However, recent studies in Arizonaindicate that some cultivars of upland cotton aresusceptible to the disease. For example, the increasedacreage of Deltapine 90 since its introduction in 1982,the reduced acreage of Deltapine 61, and increasedplantings of Pima S-6 explain the increased reports ofAlternaria leaf spot in the late 1980’s and early 1990’s.Deltapine 90 and Pima S-6 are susceptible whereasDeltapine 61 was resistant. The disease occurs in thecentral and southwestern parts of Arizona but not inthe hotter and drier western areas. Often times, asvarieties change so do the frequency of occurrences ofAlternaria leaf. Often times, as varieties change so dothe frequency of occurences of Alternaria leaf spot.

Symptoms - On Pima cotton, Alternaria produces leafspots that start as tiny, dull-brown, circular lesions,

which may enlarge to spots 1/4 to 1/2 inch indiameter. Because of higher humidity, the lowerleaves are more commonly affected. Spots alsomay occur on bolls. Under favorable wet conditionsduring July and August, leaf defoliation may occur.

Epidemiology and control - The most important factorin the development of this disease is the length oftime that leaves remain wet. In most years there isinsufficient rainfall to enable leaf infection to occur.The fungus survives in dried infected leaf tissue.Spores are produced during the summer rainyperiod and are carried to plant tissue by wind.

T ypical lesions of A lternaria leaf spot on leave of PimaCotton.

A photomicrograph of A lternaria macrospora, the causalfungus of A lternaria leaf spot.

Symptoms on bolls caused by Xanthomonas malvacearum(A ngular Leaf Spot). Note the water-soaked, rounded lesions.


They grow only when a film of water is on the leafsurface. Studies indicate that maximum infectiontakes place when leaves are wet for more than 12hours. No chemical control methods are currentlyrecommended.

Bacterial blightBacterial blight of cotton or angular leaf spot, caused

by the bacterium Xanthomonas malvaearum, was firstnoted in Arizona in 1922. Prior to the use of acid-delinting, the disease was relatively important becausethe bacterium survived and was spread on fuzzy seed.With the advent of the acid-delinting process, whicheliminates the pathogen from seed, the disease becamerare and unimportant in Arizona. However,occasionally, bacterial blight has appeared underunusually wet summer “monsoon” conditions in cottonareas above 3,000 feet in Southeastern Arizona. Thedisease has not been detected in any other cottonproducing areas in Arizona. The bacterium is able tosurvive in infected, dry plant tissue for many years.

Symptoms - In Arizona, symptoms have only beenseen on bolls and leaves. Lesions on bolls arecircular, dark-green, water-soaked and greasy inappearance. Water-soaked, angular lesions appearon the leaves.

Control - Because of the infrequency of the disease, nocontrol measures are recommended for Arizona.

BOLL ROTS

Boll rots occur primarily during the monsoon seasonwhen cotton is exposed to rainfall and high humidity.Most boll decay is found in the lower half of the plantcanopy because of high humidity. The three pathogensdescribed under Leaf Diseases, Alternaria macrospora,Puccinia cacabata and Xanthomonas malvacearum alsoinfect 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 andcause extensive rot. This disease, however, is very rarein Arizona. Another pathogen, Phytophthora capsacioccurs only in the Sulfur Springs Valley of CochiseCounty. This invades uninjured bolls under wetconditions and can causes extensive decay, but disease,is not widespread, and no chemical treatment isrecommended for control.

Although a large number of fungi have beenimplicated as causal agents in boll rots, most of theseorganisms cannot penetrate healthy boll tissue.Infection is caused by airborne spores that invade bollsafter opening. If insects such as the pink boll worm,tobacco budworm, or any other factor that causespremature opening exists, then the incidence of bolldecay is increased.

The most significant problem associated with bolland fiber development is the aflatoxin disease causedby the fungus Aspergillus flavus. Because of theimportance of this disease it will be discussed in detail.

AFLATOXINS

Aflatoxins are carcinogens produced in many nuts,grains and cottonseed by the fungus Aspergillus flavus.The fungus occurs throughout the United States, butaflatoxin problems in cotton are serious only in WesternArizona, the Southern California desert, and the lowerRio Grande Valley in Texas. In Arizona, aflatoxin issignificant only where cotton is grown below 2,000 feetelevation. When A. flavus infects lint and cottonseed,the main problem created is the production by thefungus of aflatoxins. Two factors, high temperatureand lint moisture content above 15 percent are necessaryfor the fungus to infect and produce these carcinogens.

Boll and seed invasion by A. flavusAspergillus flavus colonizes dead organic matter and

produces large numbers of spores. The entry point intothe cotton boll is often the exit hole of the larval stageof the pink bollworm. Once colonization takes place,A. flavus invades the cotton fiber. This lint invasionresults in weakened fiber and discoloration called

Leaf symptoms caused by infection by Xanthomonasmalvacearum. Both photographs were taken in Bowie,A rizona during an unusually wet monsoon season.


“yellow stain.” After initial lint invasion, the funguspenetrates the seed coat and colonizes the meat portionof the seed. Aflatoxins are produced in this portion,but not in the seed coat or lint. Seed invasion and theformation of aflatoxins will continue as long as seedmoisture is in excess of approximately 15 percent.

Climate and aflatoxin production incottonseed

Temperature and humidity are the primary factorsinfluencing boll invasion by A. flavus and the subsequentformation of aflatoxins in cottonseed. The hightemperatures and humidities found in Arizona andSouthern California cotton fields favor boll invasionby A. flavus, and moist lint of an opening boll is verysusceptible to infection.

The critical period for boll invasion and contaminationby aflatoxins in Arizona is early August to mid-September when bolls start to open. Approximately 95percent of the seed infection and formation of aflatoxinstakes place in bolls on the lower one-third to one-halfof the plant, where dense leaf canopies maintain highhumidities and prevent air movement.

The average level of aflatoxins detected in thecottonseed crop fluctuates from year to year.Temperature is the most important factor thatdetermines whether contamination by aflatoxins ofseed will take place. When nighttime minimums areconsistently below 70o F for this period, as is the case atthe higher elevations in Arizona, the San Joaquinproduction areas of California, and other productionareas of the United States, aflatoxin seed contaminationis 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 14to 21 days after flowering when they are mostsusceptible. Under dry conditions yield reductions arenot significant unless over 25 percent of the bolls areinfected. With the occurrence of high humidities andrainfall, however, during the latter half of July intoAugust and September, extensive losses can occurbecause damaged bolls are invaded by miscellaneousfungi that cause boll rots. The exit hole of the pinkbollworm becomes an entry point for infection byairborne spores of A. flavus . Bollworms (Heliothus spp.)feeding sites as well as any other insect punctures alsoenable A. flavus to enter into boll tissue. Aspergillusflavus is not able to infect and penetrate undamaged,intact boll tissue.

Control - Currently, there is no practical field control ofcottonseed invasion by Aspergillus flavus andsubsequent contamination by aflatoxin. In Arizona,approximately 80 percent of the whole seedproduced is processed for meal, oil, linters, andhulls. In the delinting, dehulling, and oil extractionprocess, almost all of the aflatoxin resides with themeal. The meal represents only about 40 percent to50 percent of the seed weight, so, in effect, theaflatoxin level doubles in this by-product of wholecottonseed processing. In reality, a seed producermust control to 10 parts per billion (ppb) in orderfor the processor successfully to produce 20 ppbmeal.

Humidity may be regulated by improving airmovement through the lower leaf canopy. Skip-row planting in a 2 x 1 or 4 x 1 mode will improveair movement around the bolls in a lower canopyand will result in less aflatoxin than is encounteredin similar solid planted fields. Unfortunately,skip-row cultivation will only maintain aflatoxinlevels below 10-20 ppb when a very light season isencountered. Currently there is no way to predictat planting time the degree of aflatoxincontamination that will occur during August andSeptember.

“Rank” cotton is usually more susceptible to theproblem. Careful management of water andfertilizer on land known to produce rank cottonwill add in maintaining lower levels. Reduction ofthe growing season, and limiting the number ofirrigations in August show promise for reducingaflatoxin levels.

Good midseason insect control, particularly thatof the pink bollworm complex, from mid-July toearly September, is important for the maintenanceof low levels of aflatoxin.

There is a piece of equipment, common to cottonproduction, which has the ability of segregatingseed with lower levels of aflatoxin from highlycontaminated seed. The spindle harvester wasdesigned to remove fluffed lint efficiently and isinefficient in removing unfluffed or tight loculesand/or PBW-bollworm complex damaged bolls,which contain most of the aflatoxin. Unfortunately,this advantage is lost when a ground gleanerfollows the spindle harvester. Ground gleanerswill retrieve most of the seed cotton remaining onthe plant from the ground. This seed is thenroutinely mixed with spindle-picked seed at thegin. In several studies in Arizona, aflatoxin levelswere 50 times higher in seed retrieved by ground


harvesters. Segregation on the basis of harvestertype during a light aflatoxin year would serve toprovide a large quantity of seed with acceptablelevels of aflatoxin.

Cottonseed and meal above the tolerances set bythe U.S. Food and Drug Administration (20 to 300ppb) depending on the intended use of the feedmust be treated with ammonia or mixed withuncontaminated feeds to produce a mixture withinthe limits of the tolerance.

COTTON LEAF CRUMPLE VIRUS

History - This disease was first described fromCalifornia in 1954 and from Arizona in 1960. Amajor epidemic of cotton leaf crumple occurred inArizona cotton in 1981. The epidemic wasassociated with high populations of the sweetpotato whitefly (Bemisia tabaci) and large acreagesof 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. Plantsinfected when young are stunted.

Host range - Studies in 1986 indicated that Stonevillegerm plasm is less susceptible than Deltapine germplasm. Pima cotton is also susceptible. Cheeseweed(Malva parviflora) and bean (Phaseolus vulgaris) arehosts of the virus in inoculated greenhouse plants,but its natural host range is not known.

Epidemiology and control - Cotton leaf crumple, ageminivirus, is the only virus disease of cottondescribed in the United States. It is vectored onlyby the whitefly. The virus is not mechanicallytransmitted. The disease was most important inArizona when ratooning of cotton was practiced inthe warmer western and central production areas.The ratooning of infected plants enabled the virusto establish itself systemically in young plants,causing significant yield losses. Seed cotton wasnever as seriously damaged. Currently, the diseaseis scattered throughout areas where whitefliesexist. Cotton is no longer ratooned in Arizona andthe disease is economically important only whenyoung plants are infected. Infections that occur inlate July or August cause symptom developmentbut little or no yield loss.

ROOT–KNOT NEMATODE

The Southern root–knot nematode (Meloidogyneincognita) is the only nematode affecting cotton inArizona. The nematode is restricted to sandy soilswhere it attacks both short and long staple varieties.

Symptoms - Galls and root swellings are small butvisible on roots. The galls appear as early as onemonth after planting. Galls on cotton roots aresmaller than root–knot galls on other hosts such astomato, cucurbits, and other vegetables.Rotation may be the best method for control. InArizona, cotton can safely be planted followinglettuce and most nondormant varieties of alfalfa.Summer fallowing can be an effective means ofcontrol, providing there is root destruction of theprevious crops, and susceptible weeds areprevented from growing.When crop rotation is impractical, considerationmust be given to soil treatment with a suitablenematicide. The decision is based on the cost oftreatment, the number of root knot nematodes inthe soil, the anticipated temperature during seedgermination and development of the tap root, thepercent of the field with sandy areas that exceeds60 percent or more sand and the anticipated priceof cotton. At present, soil fumigants remain themost effective and economical method ofcontrolling root knot nematodes attacking cotton.The only soil fumigants presently available for usein cotton are various formulations of 1-3, D(dichloropropene). These liquid compounds mustbe injected into the soil before planting. Soil shouldalways be in excellent tilth with temperaturesbetween 60o F and 80o F. The manufacturer’s labelcarries instructions for the correct dosage. For best

Symptoms of cotton leaf crumple on leaves of Upland cotton.


results with soil fumigants, the beds should belisted and irrigated before the material is applied.

NONPATHOLOGICAL COTTON

PROBLEMS

NUTRIENT DEFICIENCY SYMPTOMS

NitrogenNitrogen (N) is the plant nutrient required in largest

amounts and the only mineral nutrient to which cottonhas consistently been shown to respond in Arizona. InArizona, with the exception of water, N is the firstlimiting nutrient for plant growth. Practically all soilswill respond to applications of fertilizer N in terms ofcotton growth and development. Nitrogen is oneelement that growers have control over, and it provesto be very important in Arizona cotton production.

In the plant, N is a mobile nutrient in that asdeficiencies develop, N-containing components in oldertissues (lower on the plant) break down, releasing Nforms which are then translocated to younger,developing portions of the plant (near terminal areas).Therefore, when N deficiencies develop, they arecommonly seen first as yellow or red leaves on theolder, lower portions of the plant. A crop that isdeveloped under a continual deficiency of N will tendto mature early and have reduced yields.

The N nutritional status of a cotton crop can beevaluated in-season by use of petiole sampling andanalysis for NO-N. Guidelines for use of this techniquein N fertility management is outlined by Penningtonand Tucker, 1984 (The Cotton Petiole, a NitrogenFertilization Guide, the University of Arizona Collegeof Agriculture, Publication No. 8373). Preseason soilsamples for residual NO-N can also be of benefit tomanaging a N fertility program for cotton. In general,split applications of fertilizer N throughout the growingseason are highly recommended on all soils, particularlymedium- and coarse-textured soils versus larger one-time applications. Also, unfavorable soil physicalconditions such as compaction, saturation, or drynesscan reduce N uptake by cotton.

PhosphorusPhosphorus (P) is an essential plant nutrient used in

the storage and transfer of energy within the plant.Important functions of P include formation and transferof energy, and formation and utilization ofcarbohydrates.

Although soils of Arizona are often high in total P,the amount of P available for plant uptake andutilization often has been a point of concern. Arizonasoils commonly used for cotton production generallyhave high pH conditions (pH $ 7.5) and are verycalcareous in nature. These conditions often lead to aconsiderable degree of P “fixation.” However, researchresults, show no consistent response of cotton to Pfertilization. Several experiments have been carriedout with broadcast P methods for a number of years,and experiments with banded P fertilization have beenconducted only on a limited basis thus far. Therefore,P deficiencies in Arizona cotton are very rare. This isoften attributed to the very warm conditions in thegrowing season, and possibly to cotton’s lowrequirement for P.

Symptoms of P deficiency include slow earlygrowth with darkened or red plant tissue (anthocyaninaccumulation) particularly at early stages of growth.Plants generally grow out of symptoms as the soiltemperatures warm, allowing greater root growth.Correction of suspected P deficiencies after planting isvery difficult.

PotassiumPotassium (K) is the third macronutrient (besides N

and P). Within the plant, K is not incorporated into anycompounds and tends to remain in ionic form (K+).Potassium is essential for photosynthesis, starchformation, and translocation of sugars within the plant.K is necessary for chlorophyll development, but it isnot 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 beendocumented in Arizona. Deficiency symptoms,therefore, are not likely to be encountered. If, however,one suspects K deficiency, symptoms include: bronzingand scorching of leaves, weak stems, and reduced bollsize and fiber development. For most plants, K isconsidered as a mobile nutrient in the plant leading todeficiency symptoms developing first on the lower,older leaves of the plant. However, when K deficiencieshave been diagnosed on cotton, it has proven to besomewhat of an exception. In cotton, characteristic Kdeficiency symptoms often develop first on youngerleaves. This is particularly true when symptomsdevelop late in the season as bolls are developing, andsymptoms appear on leaves adjacent to rapidly fillingbolls. This is attributed to strong sink demand by thebolls, 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 Kdeficiencies are very difficult to accomplish. If Kdeficiencies are suspected, one should have soil samplesanalyzed for exchangeable soil-K by 1-foot incrementsto a depth of 3 to 4 feet.

ZincZinc (Zn) is a micronutrient essential for plant growth

and development. Its designation as a “micronutrient”does not reflect any lesser degree of importance interms of essentiality, only that it is required in verysmall amounts relative to the macronutrients (N, P,and K) or secondary nutrients (Ca, Mg, and S). Zinc isessential as an activator (cofactor) for many plantenzymes during all stages of plant growth. Zinc isrelatively immobile in the plant and therefore, smallbut consistent levels of Zn must be taken up by cottonplants all season. In agricultural soils from Arizona,extractions performed with DTPA that result in $ 0.65parts per million (ppm) Zn are considered to besufficient for cotton production. In recent experimentsworking with soil and/or foliar applications of Zn, noyield responses were measured in upland or Pimacotton.

When Zn deficiencies have been confirmed on cottonplants, symptoms often include: shortened internodesand small leaves with interveinal chlorosis (white orbleached-out tissue between the veins of leaves). Laterin the season, very small bolls (“ping-pong” bolls)develop under Zn-deficient conditions. When Zn-deficiency symptoms develop early in the season, it isoften due to poor root development. Poor rootdevelopment could be caused by soil compaction orcold soil conditions. In the latter case, plants will oftengrow out of the Zn-deficient symptoms when soiltemperatures warm.

SALINE/SODIC SOIL CONDITIONSMany soils in the desert southwest are naturally

saline and/or sodic. These soils are usually the objectof reclamation and management techniques to controland minimize salt and sodium problems. Saline and/or sodic soil conditions often are the cause of difficultiesin getting and keeping a good stand of cotton in manyfields.

Saline SoilsSaline soils are defined as “nonsodic soils that have

sufficient soluble salt to affect adversely the growth ofmost plants.” Salt contents which result in electricalconductivity measurement of 4 mmhos per cm orgreater from a saturated extract (ECe), are generallyreferred to as saline soils. Cotton plants are considered

to be moderately salt-tolerant. This is certainly trueand cotton plants are often pushed to their limits interms of salt tolerance. When present, excessive salts(ECe values of 10 mmhos per cm or greater) causestunted, slow-growing plants. They often arecharacterized as having very short internodes, toughleathery textured leaves, and often a burn or scar markat 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 waterhigh in soluble salts. In such cases, rotation to cropssuch as small grains or alfalfa, which permit borderirrigation, is found to be helpful. Actually, it is theleaching of the salts that is of most benefit from theborder flooding. Also, alternate row irrigation can behelpful in preventing a salt accumulation at the top ofthe bed (in the row) which will occur with every-rowirrigation.

Sodic SoilsSodic soils are defined as “nonsaline soils containing

sufficient exchangeable sodium to affect adverselycrop production and soil structure.” Sodic soils haveexchangeable sodium percentages (ESP) of 15 or greater.They are characterized as having sodium adsorptionratios from the saturated extract (SARe) of 13 or greater.Soils high in sodium are found to have water penetrationproblems due to dispersing soil particles, which in turncauses a sealing of the soil. Soil sealing due to dispersionis often seen as a crusting problem and the associateddifficulties with poor seedling vigor. However,additional irrigations and cultivations will not solvethis problem, perhaps only making it worse.

In some cases, the sodic conditions within a field maybe within tolerable limits, only to be pushed into adangerous excess by an inadvertent act of management.Many growers apply fertilizer nitrogen (N) asanhydrous ammonia directed through the irrigationwater. This may cause an increase in the pH of theirrigation water due to the ammonium hydroxide thatis formed upon the addition of the anhydrous ammonia.The resultant increase in the pH of the water may causesoluble calcium and magnesium in the water toprecipitate from the solution by combining withbicarbonate, while any sodium in the water remains insolution. This may then result in irrigation water thatis high in its proportion of sodium to calcium andmagnesium. Continuous use of this practice and theresulting water, can create a sodic soil and fieldconditions that are increasingly difficult to manage.This can be a particularly tough problem at the time ofseedling establishment.


There are several approaches to correcting a sodicproblem after it has been properly identified. One soilamendment that can be used is gypsum. The first stepin this reclamation process is the incorporation of thegypsum at a satisfactory rate. The calcium from thegypsum will exchange for the sodium in the soil,leaving a soluble form of sodium. The second step isthen to leach the soluble sodium (often sodium sulfate)with excess (good quality) irrigation water. The use ofsulfuric acid on sodium-affected soils can also serve asa good reclamation effort when the soils containsufficient free lime (calcium carbonate). The soilreactions 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 soilprofile with sufficient (quality and quantity) irrigationwater. These efforts at reclamation should be beneficialwhen exchangeable sodium percentages (ESP)approach or exceed 10.

OTHER POSSIBLE NUTRITIONAL

AND PHYSIOLOGICAL PROBLEMS

Excessive NitrogenA key principle in managing the nitrogen (N) fertility

of a cotton crop is to supply the N to the plants inadequate, but not excessive, amounts. In cottonproduction, a critical management objective is tooptimize the vegetative/reproductive balance in thecrop so as to obtain the highest lint production aspossible. Growers in Arizona recognize theresponsiveness 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 (orwrong) time can stimulate vegetative growth in theprocess of sacrificing reproduction and yield potential.Excessive N particularly late in the growing season,can promote vegetativeness, delay maturity, and createfurther problems with crop termination and defoliation.

Symptomatically, one may look for excessive Nconditions by sampling petioles and analyzing forNO-N over a period of several weeks. These valuesshould be referenced against guidelines outlined byPennington and Tucker, in accordance to stage ofgrowth. Research results indicate that petiole NO-Nlevels can be drawn below 2,000 ppm and 1,000 ppmby late in the season (mid-September) withoutsacrificing yield potential and providing for plantdevelopment conducive to senescence and defoliation.

The best management against excessive N fertility

status of cotton is to employ diagnostic tools such aspreseason soil samples to evaluate residual soil NO3levels, irrigation water analysis to estimate NO3contributions with each irrigation, and in-seasonsampling and analysis of cotton petioles for NO-Ncontent. Historically, water stress has been amanagement tool used in some cases to counteract thevegetative tendencies brought on by elevated N fertilityin cotton. However, it is not an advisable practice toimpose intentionally a water stress on a crop,recognizing the sacrifice in yield potential that resultsfrom water stress conditions. The best advisablepractice is to manage N fertility with available toolsfrom the beginning of the season so as to provideadequate levels of N for crop production, but avoidingexcesses.

Ammonia ToxicityOne of the occasional consequences of fertilization

with anhydrous ammonia as a sidedress application, isthat of plant damage due to direct exposure to theammonia (NH3) gas. When side-dressing young cottonplants with an application of anhydrous ammonia, it isimportant to make the application a sufficient distanceaway from the crop row so as to avoid damage to thecotton roots. Also, it is important to make applicationsunder proper soil moisture conditions so that the soilseals after the passage of the shank applicator.Otherwise ammonia gas can escape causing both theloss of fertilizer N and damage to the abovegroundportion of the plants.

Anhydrous ammonia gas is very hydrophilic (water-loving) and therefore, exerts a severe and caustic burnto any plant tissue (root or shoot) that comes into directcontact with it. Direct exposure to anhydrous ammoniagas severely damages cell membranes and causes ablackening of the exposed tissue. Wilting also mayoccur upon direct root exposure shortly after applicationof the anhydrous ammonia. Damaged plants usuallywill recover despite a setback before recovery. It is alsoimportant to point out that applicators themselvesshould exercise necessary precautions when usinganhydrous ammonia due to the hazards associatedwith exposure to any skin tissue (particularly the eyes).

Reddened LeavesA condition that often draws some attention in a

cotton field is the development of reddened leaves.This phenomenon can be caused by several differentfactors. It seems that a natural response of the cottonplant (particularly upland) to a variety of stressfulsituations is to lose its chlorophyll content, which of


course diminishes the green coloration of the leaves.With the loss of the chlorophyll, another natural plantpigment becomes readily apparent which are theanthocyanins, leading to the distinct red coloration onthe leaves. The senescing of plant tissue, whethernatural or induced by some stress, also often acceleratesthe formation of anthocyanin. Therefore, a naturaldevelopment of red coloration of the leaves could beexpected to some extent late in the season as the cropis naturally aging on senescing. This is particularlytrue in conditions where much cooler weather isexperienced.

There are also some other causes of red leaves, whichone must be conscious of if the condition develops.Nitrogen deficiency symptoms often are noted asreddened leaves on the older, lower portions of theplant. This possibility could be further diagnosed byuse of tissue sampling (petioles) extent of the affectedareas in the field, age of the crop, and other possibleproblems. Another very common cause of red leavesis that of spider mite infestations. In the case of spidermites, the development of red leaves and red leaf spotswill be restricted to small, localized areas within afield, particularly at early stages of the population’sdevelopment. Often it will also be restricted to theolder, lower portions of the plant at very early stagesof development. One can pursue this possibility byinspecting the leaves (underside particularly) for thepresence of spider mite colonies. The mites and theireggs can easily be seen with the aid of a hand lens, andalso with the naked eye. Spider mite colonies usuallyhave a substantial amount of webbing developed onthe surface of affected leaves. Control of both spidermites and N deficiency is possible, but early detectionand diagnosis is very important in that regard. Anotherpossible cause of reddened leaves, particularly a distinctpattern of red leaf spots, may be due to some types ofinsecticides. One should consider recent insecticideapplications, extent of the affected areas, and anypatterns when considering this as a possible cause.


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Documents

X Diseases and Production Problems of Cotton in Arizona · The last comprehensive discussion of cotton diseases in Arizona appeared in “Diseases of Field Crops in Arizona,” a