The major corn diseases can be grouped into four categories: leaf blights, stalk rots, ear rots, and viral diseases.
Leaf Blights: A number of leaf-blight diseases occur on corn. The most common are gray leaf spot, Stewart's bacterial leaf blight, and northern corn leaf blight. These diseases can be found in almost any field, depending on the year and susceptibility of the hybrid planted. Some leaf-blight diseases are most often found associated with continuous corn, especially in reduced-tillage, continuous corn fields. These are anthracnose, gray leaf spot, eyespot, and northern leaf spot.
All leaf blight diseases cause loss of green leaf tissue, resulting in fewer kernels and lightweight grain. Plants may be predisposed to stalk-rot diseases when leaf damage is severe. The amount of yield loss is usually related to the time when the plant's upper leaves become infected. The most severe yield loss occurs when the upper leaves, the ear leaf, and those above the ear, become infected at or soon after tasseling. Yield losses will be minimal if disease does not occur on these leaves until six to eight weeks after tasseling.
Leaf blight diseases are most effectively controlled by selecting hybrids with genetic resistance. Contact your seed dealer for information on hybrids with resistance to gray leaf spot, Stewart's bacterial leaf blight and other leaf diseases important in your area. A one-to two-year rotation away from corn and destruction of old corn residues by tillage may be helpful if susceptible hybrids must be grown. Fungicides are also available for control of leaf diseases, but are economically viable only under severe disease pressure.
Stalk Rot: Stalk rots are the most important and common diseases of corn. Annual losses are estimated at 5 to 10 percent. There are several stalk-rot diseases, but Gibberella stalk rot and anthracnose stalk rot currently are the most prevalent. Both are fungal diseases that result in premature ripening, chaffy ears, and lodging of plants before harvest. The interior of the stalk becomes rotted, tissues break down, and the stalk is easily broken. Anthracnose stalk rot is usually associated with continuous corn and is recognized by the blackening of the outer surface of the stalk late in the season. Stalks with Gibberella stalk rot can be found in nearly any field. Affected stalks often have pink to reddish discolored internal tissues. Control of stalk rot diseases is based on reducing plant stress from factors such as lack of moisture, leaf diseases, insect injury, and nutritional stress. To reduce the affects of stalk rot diseases, follow as many of the following practices as possible:
Ear Rot: Gibberella, Fusarium, and Diplodia ear rot diseases occur in Ohio, but Gibberella ear rot is the most important. The Gibberella ear rot fungus is the same fungus that causes Gibberella stalk-rot disease. Gibberella enters from the silk end of the ear when cool, wet weather persists for several weeks through late silking of the crop. The occurrence of a whitish to pinkish mold on the ear tip is diagnostic, but extensive mold growth may not occur. On shelled grain, the symptoms may be seen as a pinkish coloration in some of the kernels. Even though extensive rotting does not always occur, the disease is serious because the fungus frequently produces toxins that makes the corn unfit for feeding. Hogs are particularly sensitive to the toxins produced in moldy grain and may refuse to eat it even when hungry. If hogs refuse to eat grain, have a mycotoxin analysis run to determine the kinds and levels of toxin present. Some corn hybrids are less susceptible than others to Gibberella ear rot. Ears with tight husks which mature in an upright position often have more ear rot than those maturing in a declined position. Diplodia ear rot appears to be more common in continuous corn under reduced tillage. Ears affected by Diplodia are covered with a thick mat of white fungal growth. Fusarium ear rot is common, but only individual kernels are affected on ears. Plant hybrids known to be less susceptible to these ear diseases. Grain with evidence of ear or kernel rot should be dried to 14 percent moisture before storage and maintained at this level until used. Feeding less than 5 percent moldy kernels may prevent feeding problems, but grain should be tested for mycotoxin levels prior to feeding to avoid any problems. For more information on mycotoxins, refer to Ohio State University Extension Bulletin 735 Moldy Grains, Mycotoxins, and Feeding Problems or contact Ohio State's Plant and Pest Diagnostic Clinic.
Virus Diseases: Maize dwarf mosaic and maize chlorotic dwarf, are potentially destructive diseases where johnsongrass is established. The two viruses that cause these diseases are able to survive in this perennial weed grass. Aphids and leafhoppers feeding on johnsongrass in the spring pick up the virus and inoculate nearby corn. Control is achieved by planting resistant or tolerant hybrids. Efforts also should be made to eradicate johnsongrass.
Additional information on corn disease and their control is in Ohio State University Extension Bulletin 802, Corn Disease Control in Ohio, available from your local county Extension office or the Ohio State University Extension website Ohioline (http://ohioline.ag.ohio-state.edu).