Corn diseases can affect all stages of production, from seed germination and seedling emergence through grain development, harvest, and storage. A wide range of pathogens, including fungi, oomycetes, bacteria, viruses, and nematodes, can cause diseases that reduce plant growth, yield, grain quality, and profitability. Disease development is influenced by environmental conditions, hybrid susceptibility, pathogen presence, and production practices. Because multiple diseases can occur within the same growing season, effective disease management requires an integrated approach that addresses multiple pathogens and production challenges. This includes selecting resistant hybrids, implementing appropriate cultural practices, monitoring disease development, and using chemical management strategies when economically justified.

Seedling diseases in corn can be 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 treatments with effective fungicide packages are an important method for controlling these fungi.

Tar Spot (caused by the fungus Phyllachora maydis).

Symptoms consist of small, round to irregular, raised black structures called stromata that develop on both upper and lower leaf surfaces. Stromata may also occur on leaf sheaths, husks, and tassels. Occasionally, tan to brown lesions with dark borders, commonly referred to as “fisheye” lesions, develop around the stromata. Unlike insect frass, stromata cannot be scraped off the leaf surface. Disease development is favored by moderate temperatures (60–80°F), high relative humidity (greater than 85%), and prolonged leaf wetness. Management includes selecting resistant hybrids, scouting for early disease development, and applying fungicides when warranted. Fungicide applications between tasseling (VT) and milk (R3) generally provide the greatest potential for disease suppression and economic return, while applications after dent (R5) are unlikely to provide economic benefits.

Gray Leaf Spot (caused by the fungus Cercospora zeae-maydis).

Gray leaf spot primarily affects leaf blades and, to a lesser extent, leaf sheaths. Symptoms consist of long, narrow, gray to pale-brown lesions with parallel sides restricted by leaf veins, giving them a rectangular appearance. Lesions typically measure approximately ¼ inch wide and 1 inch long and may merge under severe disease pressure, causing extensive leaf blight and premature leaf death. In North Carolina, gray leaf spot has historically been most severe in the mountains and western Piedmont, although it has also been observed in other regions. The fungus overwinters in infected corn residue, making the disease particularly problematic in continuous corn and no-till production systems. Management includes crop rotation, residue management, and planting resistant or tolerant hybrids.

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 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. 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 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).

Anthracnose affects both leaves and stalks, with symptoms varying depending on hybrid susceptibility, plant age, and environmental conditions. Foliar symptoms begin as small, oval to elongated, water-soaked lesions that enlarge to approximately ½ inch, developing tan centers with red, reddish-brown, or yellow-orange borders. Lesions may merge, causing extensive leaf blight. Symptoms are most common on lower leaves early in the season and upper leaves later in the season. Stalk infections initially appear as black linear streaks beneath the epidermis that may expand into large, blackened areas. Internally, infected stalks may exhibit mottled brown discoloration, particularly at the nodes, and severe infections can cause premature death of the upper third of the plant and lodging. Anthracnose is favored by warm, moist conditions, particularly during seedling development and near maturity. The fungus survives on infected crop residue, and management relies on crop rotation, residue management, and resistant or tolerant hybrids.

Southern Rust (caused by the fungus Puccinia polysora).

Southern rust is characterized by bright orange to golden-brown, circular to oval pustules approximately the size of a pinhead that give infected leaves a rusty appearance. These pustules contain powdery masses of orange spores that are readily dispersed by wind over hundreds of miles. The fungus requires living host plants to survive and cannot overwinter in North Carolina. Instead, initial infections result from spores transported northward from tropical regions where corn is grown year-round. Disease development is favored by warm, humid conditions, and the fungus can spread rapidly on susceptible hybrids. Disease severity depends largely on hybrid resistance, favorable weather conditions, and how early spores arrive during the growing season.

Common Rust (caused by the fungus Puccinia sorghi).

Common rust occurs in temperate to subtropical regions and is characterized by reddish-brown pustules that are generally darker, longer, and more irregularly distributed than those of southern rust. Unlike southern rust, common rust produces weather-resistant teliospores and has an alternate host, wood sorrel (Oxalis spp.), which is involved in its life cycle. Common rust is widespread in corn-producing regions but generally causes minimal economic losses. Even during a major U.S. outbreak in 1951, estimated average yield losses ranged from less than 1% to 3%. Management primarily relies on resistant or tolerant hybrids, which are widely available and effective against the disease.

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. 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.

Bacterial Leaf Blight (Stewart's Wilt) (caused by the bacterium Erwinia stewartii).

Bacterial leaf blight is more common in sweet corn than field corn, although susceptible field corn hybrids can also be affected. Symptoms include irregular, pale-green to yellow streaks on leaves that originate from corn flea beetle feeding sites. As lesions develop, affected tissues become straw-colored and necrotic, potentially causing premature leaf death, reduced yield, and increased susceptibility to stalk rots. The bacterium overwinters in corn flea beetles, which also transmit the pathogen. Management primarily relies on resistant hybrids, while insect control may be beneficial in sweet corn but is generally not economically justified for field corn.

Ear and stalk rots are caused by several fungal pathogens that survive in soil and crop debris. Spores spread through wind, rain, and insects, infecting ears through silks or wounds and contributing to stalk decay. These diseases can reduce yield, grain quality, and stalk integrity, increasing the risk of lodging and harvest losses. Some ear rot fungi also produce mycotoxins that pose health risks to humans and livestock. Disease severity is influenced by environmental conditions, pathogen pressure, crop stress, and insect injury. Management focuses on reducing inoculum, minimizing plant stress and injury, selecting resistant hybrids, and implementing timely harvest and proper grain storage practices. See Corn Ear Rots: Managing Mold and Mycotoxins

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, 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.

Bacterial stalk rot (caused by the bacterium Erwinia spp.)

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. 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.

Nematodes are soilborne, plant-parasitic roundworms that may cause significant damage in corn. Diagnosis of nematodes can be confirmed through assessment of soil and root samples for the presence of the nematode by a plant disease diagnostic laboratory or nematode assay laboratory. Soil samples for nematode assay services may be submitted to the North Carolina Department of Agriculture & Consumer Services, Agronomic Services Division (NCDA&CS), Nematode Assay Laboratory, 4300 Reedy Creek Road, Raleigh, NC 27607.

In order to determine nematode pressure, soil samples should be taken from several areas in the affected field. Using a soil probe, collect 20 to 30 soil cores across a 5 acre block, at a depth of 6 to 8 inches from each area using a grid-like or “zig-zag” pattern across the area. Combine soil cores in a clean plastic bucket and remove a smaller portion of soil from this larger sample to submit to the Nematode Assay Laboratory. Detailed sampling instructions can be found at the NCDA&CS Nematode Assay Laboratory sampling guidelines.

Nematode Resources:

• Sting Nematode in Corn
• Southern root-knot nematode in corn
• Lesion Nematode in Corn
• Stubby-Root Nematode in Corn

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. 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.

1. Use high-quality, treated seed to reduce seedborne and soilborne diseases.
2. Rotate crops with nonhost species to reduce pathogen buildup.
3. Manage crop residue to reduce survival of disease-causing pathogens.
4. Plant resistant or tolerant hybrids to minimize disease severity and yield losses.
5. Follow recommended planting dates to promote crop establishment and reduce disease risk.
6. Maintain balanced soil fertility and avoid excessive or deficient nutrient levels.
7. Manage water and soil conditions to minimize drought stress, waterlogging, and compaction.
8. Scout fields regularly to identify diseases early and monitor their development.
9. Apply fungicides when warranted, considering disease pressure, crop growth stage, and economic return.
10. Monitor nematode populations through soil sampling and implement management practices when economically justified.
11. Manage insect pests to reduce plant injury and the transmission of certain pathogens.
12. Harvest at the appropriate time to minimize lodging, ear rots, and grain deterioration.
13. Dry and store grain properly to limit fungal growth and mycotoxin contamination.
    
    See Other Disease Management Resources