Major Corn Diseases in North Carolina

Harry E. Duncan, Specialist in Charge, Extension Plant Pathology
K. J. Leonard, Professor (USDA), Plant Pathology
Gary Payne, Assistant Professor, Plant Pathology
Placed on the Web prior to 1/95 by the Center for Integrated Pest Management, NCSU


Table of Contents

  1. DISEASES
    1. Seed Rots and Seedling Blights
    2. Southern Corn Leaf Blight
    3. Northern Corn Leaf Blight
    4. Anthracnose
    5. Southern Rust
    6. Common Rust
    7. Common Smut
    8. Gray Leaf Spot
    9. Brown Spot
    10. Stalk Rots
    11. Ear and Kernel Rots
  2. NEMATODES
    1. Stubby-root
    2. Sting
  3. VIRUSES
    1. Maize Dwarf Mosaic Virus
    2. Maize Chlorotic Dwarf Virus
  4. BACTERIAL DISEASES
    1. Bacterial leaf blight
    2. Bacterial stalk rot

Each year diseases cause some losses in corn production in North Carolina. A number of diseases affect corn with individual fields often suffering severe losses.

All parts of the plant may be attacked -- the ears, leaves, stalks, and roots -- at various stages of development. Diseases reduce the value and quality of the grain and may increase harvesting costs when affected plants lodge.

Corn leaf, ear and stalk diseases generally are favored by warm, wet weather. Most root rots such as Pythium root rot are more severe in wet than in dry soils, but charcoal rot is favored by hot, dry conditions. Seedling diseases are favored by cool soil temperatures that delay emergence and growth of seedlings and are more severe when planting time is unusually wet. Symptoms of nematode damage are more pronounced in sandy soils, which can support large nematode populations, and during dry weather, when the effects of root 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 certain recommended practices. Growers should become acquainted with disease problems and factors affecting the severity of the disease.

DISEASES

  • Seed Rots and Seedling Blights (caused by species of Fusarium, Diplodia, Pythium, and other fungi). Germinating corn kernels may be attacked by a number of soilborne or seedborne fungi that cause seed rots and seedling blights. The terms "preemergence" and "postemergence damping-off" are often used to specify the affected growth stage. These diseases are more prevalent in poorly drained, excessively compacted, or cold, wet soils. Planting old or poor quality seed with mechanical injury to the pericarp will increase seed rot and seedling blight as will planting seed too deep in wet, heavy soils. Hybrids differ in genetic resistance to the fungi that cause seed rot and seedling blight. Seed treatment with a good fungicide is an important method for control of these fungi.

  • Southern Corn Leaf Blight (caused by the fungus Bipolaris maydis [Helminthosporium maydis]). Southern corn leaf blight occurs worldwide, but is particularly damaging in regions of warm, moist weather. Lesions on the leaves (Fig. 1) caused by Race 0 of the fungus are elongated between the veins, tan, up to one inch long, with limited parallel margins and buff to brown borders. Lesions produced by Race T (which was very prevalent in the early 1970s) are tan, 1/4 to 1/2 inch x 1/4 to 1 inch, spindle-shaped or elliptical, with yellow-green or chlorotic halos. Later, the Race T lesions often have dark, reddish-brown borders and may occur on all parts of the plants ( Fig. 2). Race T is commonly found only when hybrids with Texas male sterile cytoplasm are grown. Both races of the fungus overwinter on corn debris in the field. Thus, rotation and destruction of residue will reduce losses due to this disease. Resistant hybrids are also available.

  • Northern Corn Leaf Blight (caused by the fungus Exserohilum turcicum [Helminthosporium turcicum]). Symptoms of this disease ( Fig. 3) are long elliptical, grayish-green or tan lesions ranging from 1 to 6 inches in length, developing first on lower leaves and later causing severe damage to the upper leaves under moderately warm and moist weather conditions. This disease is favored by somewhat cooler weather than southern leaf blight and has been quite severe in the mountain counties. Northern corn leaf blight can cause premature death and gray appearance of foliage that resembles frost or drought injury. As with southern corn leaf blight, control is by rotation, destruction of crop debris, and use of resistant hybrids. There are at least three pathogenic races of the fungus, but moderate to good resistance is available to all of them.

  • Anthracnose (caused by the fungus Colletotrichum graminicola). Symptoms of this disease vary widely, depending on the hybrid, age of the leaf, and environment. Small, oval to elongate, water-soaked spots 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 with red, reddish-brown, or yellow-orange borders. The lesions may grow together, blighting the entire leaf. Leaf symptoms are most common early in the season on the lower leaves and late in the season on the upper leaves.

    Lesions on stalks ( Fig. 4) usually appear initially as black linear streaks under the epidermis. On susceptible plants the lesions may develop into large oval, black areas measuring 1/2 to 1 inch, or larger. In severe infections, large areas of the stalk may be blackened. When the infected stalks are split, a mottled brown discoloration may be seen, particularly at the nodes. This discoloration may be present even when lesions are not apparent on the surface of the stalk. It is common with anthracnose for the upper 1/3 of the plant to prematurely die. Anthracnose is a very important cause of lodging in North Carolina.

    Anthracnose is favored by warm, moist conditions during the growing season. Plants are most susceptible in the seedling stage and later as they approach maturity. There is a wide range of susceptibility in hybrids. The fungus overwinters on plant debris left above ground. Thus, control of this disease is based upon crop residue destruction, rotation, and use of tolerant or resistant hybrids.

  • Southern Rust (caused by the fungus Puccinia polysora). Southern rust can be recognized by the bright orange or golden brown, circular to 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 powdery masses of orange spores which can be rubbed off. These spores are readily dislodged and blown about in the wind. The spores can survive and infect plants after being transported hundreds of miles by the wind.

    The southern rust fungus has no known means of survival in the absence of living susceptible plants. During the winter months it is limited to tropical areas where corn is grown year round. The extent to which it spreads into temperate areas depends upon weather patterns and the susceptibility of the corn along the path of spread.

    Southern rust is favored by the warm, humid conditions found in many lowland tropical areas where corn is grown. However, even in those areas, corn with good resistance suffers little or no damage. In temperature areas less ideal for the growth of the fungus, damage can occur in corn and lacks good resistance.

    Since southern rust cannot survive the winter in North Carolina, the initial infections must result from spores blown into North Carolina from the south. The fungus can multiply very rapidly on susceptible corn, and the amount of damage that occurs depends upon how early the first spores arrive. Epidemics may result from unusual weather patterns that cause mass air movements from the tropics where the rust is present.

  • Common Rust (caused by the fungus Puccinia sorghi). Common rust occurs in temperature to sub-tropical areas. It differs from southern rust by the darker, more reddish-brown color of the pustules. Also, pustules of common rust tend to be longer than those of southern rust and they occur more often in scattered clumps on the leaves. Pustules of southern rust are usually quite uniformly distributed over the surface of the leaf. Common rust is able to survive the winters in temperate areas because it produces teliospores, which are resistant to weathering. These spores germinate in the spring to produce basidiospores. The basidiospores can infect wood sorrel (Oxalis spp.); the spores produced in infections on wood sorrel can complete the life cycle of the fungus by infecting corn.

    Common rust has been present for a great many years in all major corn producing areas of the world. It has not been regarded as a major cause of damage in any of those areas. In 1951 in one of the heaviest outbreaks of common rust known in the United States, estimated average losses ranged from less than 1 percent to 3 percent. Resistance and tolerance to common rust are prevalent and effective in corn hybrids throughout the world.

  • Common Smut (caused by fungus Ustilago maydis). Common smut occurs wherever corn is grown. Losses to smut are generally light, but may be important in some situations, particularly sweet corn. Young actively growing parts of the plant are susceptible to infection. Large galls may appear on stalks at the nodes, on ears (Fig. 6), or rarely on tassels. Leaf infections may result in small inconspicuous galls. On ears or stalks the galls expand rapidly and are covered with a thin greenish-white or silvery-white tissue. As the galls mature, the covering ruptures exposing masses of black spores within. Individual galls on stalks may be up to 6 inches in diameter. On infected ears, a large number of galls originating from individual infected kernels may combine to form the compound gall mass that replaces most of the ear.

    Smut is usually more severe on plants heavily fertilized with nitrogen. The severity is increased by injury from hail, cultivators, etc. Control involves avoiding highly susceptible varieties, avoiding mechanical injury to plants during cultivation and spraying, and providing well-balanced soil fertility.

  • Gray Leaf Spot (caused by the fungus Cercospora zeae-maydis). The fungus can infect leaf blades and, to a much lesser extent, leaf sheaths. The gray or pale brown lesions are long and narrow with parallel sides delimited by leaf veins (Fig. 7). The ends are usually blunt, giving the lesions a long rectangular shape. Lesions commonly are about 1/4 inch wide by about 1 inch long. When the disease is severe, lesions merge into long stripes. Eventually the entire leaf may be 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 not reported in the U.S. again until 1943 when it was found causing moderate to severe damage in some fields in eastern Tennessee and Kentucky. Gray leaf spot has caused moderate to severe damage to corn in the mountain valleys of the Appalachian region. In North Carolina, the disease is most severe in the mountains and western piedmont, but it has been observed on corn in Wake County when infected debris was spread on the ground in a field.

    The gray leaf spot fungus survives the winter as resistant mycelium in corn debris left in the field. The disease is usually more severe in no-till planted corn without rotation. Thus, rotation, debris destruction, and resistant varieties offer the best methods of controlling this disease. It appears that the most tolerant hybrids are late maturing hybrids.

  • Brown Spot (caused by the fungus Physoderma maydis). Brown spot is favored by high temperatures and high humidity. It attacks leaf blades, sheaths, and stalks, producing small, reddish-brown to purplish-brown spots which may merge together to form large brown blotches ( Fig. 8). Weakened stalks frequently lodge and leaf sheaths may be reduced to shreds. Good cultural practices and the use of tolerant varieties offer the best control.

  • Stalk Rots (caused principally by the fungi Diplodia zeae and species of Fusarium as well as Colletotrichum graminicola). Stalk rots are present each year and may cause considerable damage, particularly if abundant rainfall occurs during the latter part of the growing season. Stalks previously injured by cold, leaf diseases, or insects are especially susceptible to attack by these fungi. Diseased stalks 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 harvesting but also bring the ears in contact with the ground, increasing their chance of rotting.

  • Ear and Kernel Rots (caused by species of Diplodia, Fusarium and many other fungi). Ear and kernel rots ( Fig. 13) may cause serious losses under warm, wet conditions at harvest time. Severe infection not only reduces yield but also lowers the quality and grade of the grain produced. In addition to these losses, the infected kernels make poor feed and are worthless as seed. The two principal ear and kernel rot fungi found in North Carolina are Diplodia and Fusarium. Fusarium typically causes a pink or reddish rot of the ear, while ears infected with Diplodia appear white to grayish-brown. Moldy grain may be toxic to livestock, especially to young animals.

    Aflatoxin Contamination

    Toxic metabolic by-products of fungi known as mycotoxins have received considerable attention during the past several years. The mycotoxin found in corn harvested in North Carolina that has received the most attention is aflatoxin, produced by the fungus Aspergillus flavus. Aflatoxins can occur in several chemical forms. In corn the most common aflatoxins are aflatoxin B1 and aflatoxin B2. Aflatoxins are known to cause serious health problems in animals including reduced weight gain, capillary fragility, reduced fertility, suppressed disease resistance, and even death. No animal is known to be resistant, but in general, older animals are more tolerant than younger animals. Aflatoxins have been implicated in deaths from acute toxicoses in young animals, particularly poultry, as well as several animal health problems, including reduced fertility and growth rate. Aspergillus flavus is widely distributed in nature and is favored by high temperature. Temperatures ranging from 80 to 100 degrees F and a relative humidity of 85 percent (18 percent moisture in the grain) are optimum for A. flavus growth and aflatoxin production. Growth of the fungus does not occur below 12 to 13 percent moisture in the grain.

    Aflatoxin contamination is higher in corn that has been produced under stress conditions. Thus, drought, heat, insect, and fertilizer stress are all conducive to high levels of aflatoxins. Therefore, in order to minimize the level of aflatoxins, the following practices should be followed:

    1. Use recommended production practices.
    2. Plant early.
    3. Irrigate to reduce drought stress, especially during pollination.
    4. Harvest early.
    5. Avoid kernel damage during harvest.
    6. Dry and store corn properly.
    7. Keep storage facilities clean.
    8. Keep feeding facilities clean.

    NEMATODES

    Nematodes attack corn roots, hereby limiting their development and restricting the uptake of water and nutrients. Thus, affected plants are stunted and appear deficient of nutrients. Since nematodes do not occur in a uniform population throughout the field, stunted plants likewise are not uniformly distributed. They often appear in roughly circular areas in the field ( Fig. 14). Nematode damage occurs most often when the preplant densities of certain nematodes are high and 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 the stubby-root and sting nematodes.

    Stubby-root - (Paratrichodorus minor) - The stubby-root nematodes do not enter the roots of corn plants, but remain outside the roots and feed on the growing root tips. Their feeding prevents the further development of the root tip, resulting in short, stunted or stubby roots ( Fig. 15). The damage to the root system by stubby-root nematodes resembles that caused by several herbicides. A plant heavily parasitized with these nematodes is stunted, turns yellow, often exhibits magnesium deficiency, and produces a small ear. Since these nematodes are so widespread in the coastal plain area, they may very well be the most damaging nematodes on corn in North Carolina.

    Sting - (Belonolaimus sp.) - The sting nematodes feed from the outside without penetrating or becoming attached to roots. They feed at root tips and along the sides of succulent roots. Injured roots show blackened, sunken dead areas along the root and at the root tip. These areas may girdle the root causing it to die. Sometimes the damage done to young plants is quite severe and infected plants may obtain a height of only 8 to 10 inches. The population appears to increase and decrease very rapidly on corn. They are found in soils that contain at least 80 percent sand. This nematode, especially when combined with the stubby-root nematode, causes severe yield losses.

    Control of Nematodes

    In order to determine whether or not a field should be treated with a nematicide to control nematodes, a soil sample should be collected in September-November and sent to the N.C. Department of Agriculture for an assay. Based upon this service, about 1/3 of the corn acreage in eastern North Carolina should be treated to control nematodes. Where the population density is high enough to justify treatment, a grower can expect an increase of about 20 to 25 bushels per acre. Nematodes are controlled by use of a nematicide, rotation, and crop destruction. For a more complete discussion on nematodes, please refer to Plant Pathology Information Note 214 titled Nematodes on Field Corn, which is available from County Extension offices.


    VIRUSES

    There are two major viruses of corn in North Carolina, maize dwarf mosaic virus (MDMV) and the maize chlorotic dwarf virus (MCDV). These two virus diseases can cause serious yield reductions, with reported losses ranging from 5 to 90 percent in some fields. Much of the loss due to these two diseases in North Carolina is confined to the piedmont section of the state, although losses in the coastal plain and mountain areas have been reported. This may be due to two factors: (1) there is less johnsongrass in the coastal plain area, and (2) the johnsongrass in the piedmont is infected with the two viruses while the johnsongrass in other sections of the state is not as heavily infected, or not infected at all. The two viruses are transmitted from infected johnsongrass to corn by insects. The MDMV is transmitted by aphids (principally the corn leaf aphid, Aphid maidis and the MCDV is transmitted by leafhoppers (Graminella nigrifrons).

    Maize Dwarf Mosaic Virus - Symptoms of MDMV first appear on the youngest leaves as an irregular, light and dark green mottle or mosaic which may develop into narrow streaks along veins that appear as dark green "islands" on a lighter green background. As infected plants mature, leaves become yellowish-green. Plants with these symptoms are sometimes stunted with excessive tillering, multiple ear shoots and poor seed set. Early infection may predispose corn to root and stalk rots and premature death. Symptoms can appear in the field within 30 days after seedling emergence.

    Maize Chlorotic Dwarf Virus - MCDV, which used to be called corn stunt, causes more severe stunting than does MDMV. Infected leaves become yellow, but no mosaic pattern develops. Such leaves usually develop a deep, reddish discoloration later in the season. The internodes of infected plants fail to elongate, resulting in very stunted plants ( Fig. 16). Quite often infection occurs late in the season. Thus, the lower portion of the plant develops normally with the upper portion being red and stunted. Infection can result in severe reduction in ear size if susceptible varieties are grown and infection occurs early enough in the development of the plant.

    Losses from these two viruses can be reduced by growing varieties that are resistant, or tolerant, to these viruses. There are several varieties adapted to North Carolina that are resistant to both viruses.


    BACTERIAL DISEASES

    There are two major bacterial diseases of corn in North Carolina, bacterial leaf blight (sometimes called Stewart's bacterial wilt) and bacterial stalk rot.

    Bacterial leaf blight (caused by the bacterium Erwinia stewartii is more of a problem with sweet corn than it is with field corn; however, it can be a problem with certain hybrids. The symptoms are short 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 entire leaves die and dry up. When leaves die prematurely, yield is reduced 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 disease in sweet corn, it is not a sound practice for field corn producers. Resistance to the disease, which is available in many hybrids, is the preferred method of control.

    Bacterial stalk rot (caused by the bacterium Erwinia chrysanthemi pv. zeae) can be a problem where overhead irrigation is used and the water is pumped from a lake, pond, or slow-moving stream. Quite often the infection occurs at about ear height, and the upper portion of the plant breaks over due to a collapse of the stalk ( Fig. 18). Often, an unpleasant odor is associated with this disease. The bacteria usually do not spread from plant to plant, so diseased plants are quite often found 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.