Cover Photo:AspergiI/us flavus, the fungus that produces aflatoxins.

Prepared by:Harry E. Duncan, Specialist in Charge, Extension Plant Pathology; K. J. Leonard,Professor (USDA), Plant Pathology, and Gary Payne, Assistant Professor, PlantPathology

Published byTHE NORTH CAROLINA AGRICULTURAL EXTENSION SERVICE

North Carolina State University at Raleigh, North Carolina Agricultural and Techni-cal State University at Greensboro, and the U. S. Department of Agriculture, Coop-erating. State University Station, Raleigh, N. 0., Chester D. Black, Director. Distrib-uted in furtherance of the Acts of Congress of May 8 and June 30, 1914. The NorthCarolina Agricultural Extension Service offers its programs to all eligible persons re-gardless of race, color, or national origin, and is an equal opportunity employer.7-84-5M AG-343

Major Corn Diseases

in North Carolina

Each year diseases cause some losses in corn production in NorthCarolina. A number of diseases affect corn with individual fields of-ten suffering severe losses.All parts of the plant may be attacked—the ears, leaves, stalks androots—at various stages of development. Diseases reduce the valueand quality of the grain and may increase harvesting costs when af-fected plants lodge.Corn leaf, ear and stalk diseases generally are favored by warm,wet weather. Most root rots such as Pythium root rot, are moresevere in wet than in dry soils, but charcoal rot is favored by hot, dryconditions. Seedling diseases are favored by cool soil temperaturesthat delay emergence and growth of seedlings and are more severewhen planting time is unusually wet. Symptoms of nematode damageare more pronounced in sandy soils, which can support largenematode populations, and during dry weather, when the effects ofroot damage are apparent as increased plant stress.There are no completely effective measures for controlling all corn

diseases; however, losses can be minimized by following certainrecommended practices. Growers should become acquainted with dis-ease problems and factors affecting the severity of the disease.Seed Rots and Seedling Blights (caused by species of Fusarium

Diplodia, Pythium, and other fungi). Germinating corn kernels maybe attacked by a number of soilborne or seedborne fungi that causeseed rots and seedling blights. The terms “preemergence andpostemergence damping-0f ” are often used to specify the affectedgrowth stage. These diseases are more prevalent in poorly drained,excessively compacted, or cold, wet soils. Planting old or poor qualityseed with mechanical injury to the pericarp will increase seed rot andseedling blight as will planting seed too deep in wet, heavy soils.Hybrids differ in genetic resistance to the fungi that cause seed rotand seedling blight. Seed treatment with a good fungicide is an im-portant method for control of these fungi.Southern Corn Leaf Blight (caused by the fungus Bipola’ris

maydis [Helminthospom'um mag/dish. Southern corn leaf blight oc-curs worldwide, but is particularly damaging in regions of warm,moist weather. Lesions on the leaves (Fig. 1) caused by Race 0 of thefungus are elongated between the veins, tan, up to one inch long, with

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limited parallel margins and buff to brown borders. Lesions producedby Race T (which was very prevalent in the early 1970s) are tan, IA to1/2 inch x M: to 1 inch, spindle-shaped or elliptical, with yellow-green orchlorotic halos. Later, the Race T lesions often have dark, reddish-brown borders and may occur on all parts of the plants (Fig. 2). RaceT is commonly found only when hybrids with Texas male sterilecytoplasm are grown. Both races of the fungus overwinter on corndebris in the field. Thus, rotation and destruction of residue willreduce losses due to this disease. Resistant hybrids are also available.Northern Corn Leaf Blight (caused by the fungus Exserohilum

turcicum [Helminthospom’um turcicumfi. Symptoms of this disease(Fig. 3) are long elliptical, grayish-green or tan lesions ranging from 1to 6 inches in length, developing first on lower leaves and later caus-ing severe damage to the upper leaves under moderately warm andmoist weather conditions. This disease is favored by somewhat coolerweather than southern leaf blight and has been quite severe in themountain counties. Northern corn leaf blight can cause prematuredeath and gray appearance of foliage that resembles frost or droughtinjury. As with southern corn leaf blight, control is by rotation,destruction of crop debris, and use of resistant hybrids. There are atleast three pathogenic races of the fungus, but moderate to goodresistance is available to all of them.Anthracnose (caused by the fungus Colletotn'chum grammicola).Symptoms of this disease vary widely, depending on the hybrid, ageof the leaf, and environment. Small, oval to elongate, water-soakedspots first appear on the leaves at any stage of growth. The spots may

enlarge up to one-half inch long and become tan at the center withred, reddish-brown, or yellow-orange borders. The lesions may growtogether, blighting the entire leaf. Leaf symptoms are most commonearly in the season on the lower leaves and late in the season on theupper leaves.Lesions on stalks (Fig. 4) usually appear initially as black linearstreaks under the epidermis. On susceptible plants the lesions maydevelop into large oval, black areas measuring 1/2 to 1 inch, or larger.In severe infections, large areas of the stalk may be blackened. Whenthe infected stalks are split, a mottled brown discoloration may beseen, particularly at the nodes. This discoloration may be presenteven when lesions are not apparent on the surface of the stalk. It iscommon withanthracnose for the upper one-third of the plant toprematurely die. Anthracnose is a very important cause of lodging inNorth Carolina.Anthracnose is favored by warm, moist conditions during thegrowing season. Plants are most susceptible in the seedling stage andlater as they approach maturity. There is a wide range of suscep-tibility in hybrids. The fungus overwinters on plant debris left aboveground. Thus, control of this disease is based upon crop residuedestruction, rotation, and use of tolerant or resistant hybrids.

Fig. 1

Fig. 3

Southern corn leaf blight onleaves.

Northern corn leal blight. Notethat the lesions are much largerthan those of southern corn lealblight.

Fig. 2 An ear of corn infected withBipolaris maydis (SCLB).

Fig. 4 Anthracnose damage on thestalk.

Southern Rust (caused by the fungus Puccimo polysora). Southernrust can be recognized by the bright orange or golden brown, circularto oval pustules which give a rusty appearance to the leaves (Fig. 5).The pustules are about the size of a pin head and are filled with pow-dery masses of orange spores which can be rubbed off. These sporesare readily dislodged and blown about in the wind. The spores cansurvive and infect plants after being transported hundreds of milesby the wind.The southern rust fungus has no known means of survival in theabsence of living susceptible plants. During the winter months it islimited to tropical areas where corn is grown year round. The extentto which it spreads into temperate areas depends upon weather pat-terns and the susceptibility of the corn along the path of spread.Southern rust is favored by the warm, humid conditions found inmany lowland tropical areas where corn is grown. However, even inthose areas, corn with good resistance suffers little or no damage. Intemperate areas less ideal for the growth of the fungus, damage canoccur in corn that lacks good resistance.Since southern rust cannot survive the winter in North Carolina,the initial infections must result from spores blown into NorthCarolina from the south. The fungus can multiply very rapidly onsusceptible corn, and the amount of damage that occurs depends uponhow early the first spores arrive. Epidemics may result from unusualweather patterns that cause mass air movements from the tropicswhere the rust is present.Common Rust (caused by the fungus Puccinia sorghi). Commonrust occurs in temperate to sub-tropical areas. It differs fromsouthern rust by the darker, more reddish-brown color of thepustules. Also, pustules of common rust tend to be longer than thoseof southern rust and they occur more often in scattered clumps on theleaves. Pustules of southern rust are usually quite uniformly dis-tributed over the surface of the leaf. Common rust is able to survivethe winters in temperate areas because it produces teliospores, whichare resistant to weathering. These spores germinate in the spring toproduce basidiospores. The basidiospores can infect wood sorrel(Oxalz's spp); the spores produced in infections on wood sorrel cancomplete the life cycle of the fungus by infecting corn.Common rust has been present for a great many years in all majorcorn producing areas of the world. It has not been regarded as a majorcause of damage in any of those areas. In 1951 in one of the heaviestoutbreaks of common rust known in the United States, estimatedaverage losses ranged from less than 1 percent to 3 percent.Resistance and tolerance to common rust are prevalent and effectivein corn hybrids throughout the world.Common Smut (caused by the fungus, Ustilago mag/dis). Commonsmut occurs wherever corn is grown. Losses to smut are generallylight, but may be important in some situations, particularly with

sweet corn. Young actively growing parts of the plant are susceptibleto infection. Large galls may appear on stalks at the nodes, on ears(Fig. 6), or rarely on tassels. Leaf infections may result in small in-conspicuous galls. On ears or stalks the galls expand rapidly and arecovered with a thin greenish-white or silvery-white tissue. As thegalls mature, the covering ruptures exposing masses of black sporeswithin. Individual galls on stalks may be up to 6 inches in diameter.On infected ears, a large number of galls originating from individualinfected kernels may combine to form the compound gall mass thatreplaces most of the ear.Smut is usually more severe on plants heavily fertilized withnitrogen. The severity is increased by injury from hail, cultivators,etc. Control involves avoiding highly susceptible varieties, avoidingmechanical injury to plants during cultivation and spraying, andproviding well-balanced soil fertility.Gray Leaf Spot (caused by the fungus Cercospom zeae-maydis).

The fungus can infect leaf blades and, to a much lesser extent, leafsheaths. The gray or pale brown lesions are long and narrow withparallel sides delimited by leaf veins (Fig. 7). The ends are usuallyblunt, giving the lesions a long rectangular shape. Lesions commonlyare about 1/1 inch wide by about 1 inch long. When the disease issevere, lesions merge into long stripes. Eventually the entire leaf maybe killed.Gray leaf spot was first reported on corn in Illinois in 1925.Although it has been said to be common in South America, it was notreported in the United States again until 1943 when it was foundcausing moderate to severe damage in some fields in eastern Ten-nessee and Kentucky. Gray leaf spot has caused moderate to severedamage to corn in the mountain valleys of the Appalachian region. InNorth Carolina, the disease is most severe in the mountains andwestern piedmont, but it has been observed on corn in Wake Countywhen infected debris was spread on the ground in a field.

The gray leaf spot fungus survives the winter as resistant myceliumin corn debris left in the field. The disease is usually more severe inno-till planted corn without rotation. Thus, rotation, debris destruc-tion, and resistant varieties offer the best methods of controlling thisdisease. It appears that the most tolerant hybrids are late maturinghybrids.Brown Spot (caused by the fungus, Physoderma mag/dis). Brown

spot is favored by high temperatures and high humidity. It attacksleaf blades, sheaths and stalks, producing small, reddish-brown topurplish-brown spots which may merge together to form large brownblotches (Fig. 8). Weakened stalks frequently lodge and leaf sheathsmay be reduced to shreds. Good cultural practices and the use oftolerant varieties offer the best control.Stalk Rots (caused principally by the fungi Diplodia zeae and

species of Fusarium as well as Colletotrichum grammicola). Stalk7

Fig. 5 Southernrust.

Fig. 7 Gray lea! spot.

rots are present each year and may cause considerable damage, par-ticularly if abundant rainfall occurs during the latter part of thegrowing season. Stalks previously injured by cold, leaf diseases, or in-sects are especially susceptible to attack by these fungi. Diseasedstalks ripen prematurely and are subject to excessive stalk breaking(Fig. 9). Stalk rots (Figs. 10, 11 and 12) not only add to the cost of har-vesting but also bring the ears in contact with the ground, increasingtheir chance of rotting.Ear and Kernel Rots (caused by species of Diplodia, Fusarium andmany other fungi). Ear and kernel rots (Fig. 13) may cause serious

losses under warm, wet conditions at harvest time. Severe infectionnot only reduces yield but also lowers the quality and grade of thegrain produced. In addition to these losses, the infected kernels makepoor feed and are worthless as seed. The two principal ear and kernelrot fungi found in North Carolina are Diplodia and Fusarium.Fusarium typically causes a pink or reddish rot of the ear, while earsinfected with Diplodia appear white to grayish brown. Moldy grainmay be toxic to livestock, especially to young animals.AFLATOXIN CONTAMINATION

Toxic metabolic by-products of fungi known as mycotoxins, havereceived considerable attention during the past several years. Themycotoxin found in corn harvested in North Carolina that hasreceived the most attention is aflatoxin, produced by the fungusAspergillus flames (shown on front cover). Aflatoxin can occur inseveral chemical forms. In corn, the most common aflatoxins areaflatoxin B1 and aflatoxin B2. Aflatoxins are known to cause serioushealth problems in animals including reduced weight gain, capillaryfragility, reduced fertility, suppressed disease resistance, and evendeath. No animal is known to be resistant, but in general, olderanimals are more tolerant than younger animals. Aflatoxins havebeen implicated in deaths from acute toxicoses in young animals, par-ticularly poultry, as well as several animal health problems, in-cluding reduced fertility and growth rate. Aspergillus flavus is widelydistributed in nature and is favored by high temperature. Tem-peratures ranging from 80 to 100°F and a relative humidity of 85 per-cent (18 percent moisture in the grain) are optimum for A. flawsgrowth and aflatoxin production. Growth of the fungus does not oc-cur below 12 to 13 percent moisture in the grain.Aflatoxin contamination is higher in corn that has been producedunder stress conditions. Thus, drought, heat, insect, and fertilizerstress are all conducive to high levels of aflatoxins. Therefore, in or-der to minimize the level of aflatoxins, the following practices shouldbe followed:

Use recommended production practices.Plant early.Irrigate to reduce drought stress, especially during pollination.

. g 14WFig. 8 Brown spol. Fig. 9 Stalk rot and lodging.

Fig. 11 Charcoal rot—a closeup View.

Fig. 10 Charcoal rot—a general view.

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Fig. 12 Fusarium stalk rot.

Harvest early.Avoid kernel damage during harvest.Dry and store corn properly.Keep storage facilities clean.Keep feeding facilities clean.

NematodesNematodes attack corn roots, thereby limiting their development

and restricting the uptake of water and nutrients. Thus, affectedplants are stunted and appear deficient of nutrients. Since nematodesdo not occur in a uniform population throughout the field, stuntedplants likewise are not uniformly distributed. They often appear inroughly circular areas in the field (Fig. 14). Nematode damage occursmost often when the preplant densities of certain nematodes are highand corn seedlings get off to a slow start because of unfavorable 'growing conditions. Damage is most severe in the coastal plain area.The two most damaging nematodes on corn in North Carolina are thestubby-root and sting nematodes.

Stubby-root - (Paratrichodorus minor) - The stubby-root nema-todes do not enter the roots of corn plants, but remain outsidethe roots and feed on the growing root tips. Their feeding prevents thefurther development of the root tip, resulting in short, stunted orstubby roots (Fig. 15). The damage to the root system by stubby—rootnematodes resembles that caused by several herbicides. A plantheavily parasitized with these nematodes is stunted, turns yellow, of—ten exhibits magnesium deficiency, and produces a small ear. Since

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Fig. 14 Afieldsceneshowingnematodedamage.

Fig. 15 Damageproduced bystubby-rootnematodes.' swaav «om

these nematodes are so widespread in the coastal plain area, theymay very well be the most damaging nematodes on corn in NorthCarolina.Sting - (Belonolaimus sp.) - The sting nematodes feed from the out-side without penetrating or becoming attached to roots. They feed atroot tips and along the sides of succulent roots. Injured roots ShOWblackened, sunken dead areas along the root and at the root tip. Theseareas may girdle the root causing it to die. Sometimes the damagedone to young plants is quite severe and infected plants may obtain aheight of only 8 to 10 inches. The population appears to increase anddecrease very rapidly on corn. They are found in soils that contain atleast 80 percent sand. This nematode, especially when combined withthe stubby-root nematode, causes severe yield losses.

CONTROL OF NEMATODESIn order to determine whether or not a field should be treated witha nematicide to control nematodes, a soil sample should be collected

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in September-November and sent to the North Carolina Departmentof Agriculture for an assay. Based upon this service, about one-thirdof the corn acreage in eastern North Carolina should be treated tocontrol nematodes. Where the population density is high enough tojustify treatment, a grower can expect an increase of about 20 to 25bushels per acre. Nematodes are controlled by use of a nematicide,rotation, and crop destruction. For a more complete discussion onnematodes, please refer to Plant Pathology Information Note 214titled “Nematodes on Field Corn,” which is available from County Ex-tension Offices.VIRUSES

There are two major viruses of corn in North Carolina, maize dwarfmosaic virus (MDMV) and the maize chlorotic dwarf virus (MCDV).These two virus diseases can cause serious yield reductions, withreported losses ranging from 5 to 90 percent in some fields. Much ofthe loss due to these two diseases in North Carolina is confined to thepiedmont section of the state, although losses in the coastal plain andmountain areas have been reported. This may be due to two factors:(1) There is less johnsongrass in the coastal plain area, and (2) thejohnsongrass in the piedmont is infected with the two viruses whilethe johnsongrass in other sections of the state is not as heavily in-fected, or not infected at all. The two viruses are transmitted frominfected johnsongrass to corn by insects. The MDMV is transmittedby aphids (principally the corn leaf aphid, Aphis maidis) and theMCDV is transmitted by leafhoppers (Grammella nigm'frons).Maize Dwarf Mosaic Virus - Symptoms of MDMV first appear onthe youngest leaves as an irregular, light and dark green mottle ormosaic which may develop into narrow streaks along veins that ap-pear as dark green “islands” on a lighter green background. As in-fected plants mature, leaves become yellowish-green. Plants withthese symptoms are sometimes stunted with excessive tillering,multiple ear shoots and poor seed set. Early infection may predisposecorn to root and stalk rots and premature death. Symptoms canappear in the field within 30 days after seedling emergence.Maize Chlorotic Dwarf Virus - MCDV, which used to be calledcorn stunt, causes more severe stunting than does MDMV. Infectedleaves become yellow, but no mosaic pattern develops. Such leavesusually develop a deep, reddish discoloration later in the season. Theinternodes of infected plants fail to elongate, resulting in very stunt-ed plants (Fig. 16). Quite often infection occurs late in the season.Thus, the lower portion of the plant develops normally with the upperportion being red and stunted. Infection can result in severe reductionin ear size if susceptible varieties are grown and infection occursearly enough in the development of the plant.Losses from these two viruses can be reduced by growing varietiesthat are resistant, or tolerant, to these viruses. There are severalvarieties adapted to North Carolina that are resistant to both viruses.

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Bacterial DiseasesThere are two major bacterial diseases of corn in North Carolina,

bacterial leaf blight (sometimes called Stewart’s bacterial wilt) andbacterial stalk rot.Bacterial leaf blight (caused by the bacterium Eminia stewartii) ismore of a problem with sweet corn than it is with field corn.However, it can be a problem with certain hybrids. The symptoms areshort to long, irregular, pale green to yellow streaks in the leaves(Fig. 17). The streaked areas, which die and become straw-colored,originate from feeding marks of the corn flea beetle. Sometimes en-tire leaves die and dry up. When leaves die prematurely, yield isreduced and weakened plants become more susceptible to stalk rots.The bacteria overwinter in corn flea beetles, which also spread the

bacteria. Although insect control is important in controlling this dis-ease in sweet corn, it is not a sound practice for field corn producers.Resistance to the disease, which is available in many hybrids, is thepreferred method of control.Bacterial stalk rot (caused by the bacterium Erwinia chrysanthemipv. zeae) can be a problem where overhead irrigation is used and thewater is pumped from a lake, pond, or slow-moving stream. Quite of-ten the infection occurs at about ear height, and the upper portion ofthe plant breaks over due to a collapse of the stalk (Fig. 18). Often, anunpleasant odor is associated with this disease. The bacteria usuallydo not spread from plant to plant, so diseased plants are quite oftenfound scattered throughout the field.

To reduce losses due to disease:1. Rotate crops.2. Destroy crop residue.3. Plant resistant varieties.4. Observe proper planting dates.5. Fertilize properly.6. Harvest at proper time.7. Store corn properly.8. Treat seed.9. Control nematodes with nematicides.

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16 Symptoms produced bymaize chlorotic dwarl virus.the

Fig. 17 Bacterial leal blight.

Fig. 18 Bacterialstalk rot.

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