Leaf Blights of Corn

July 16, 1999
Management of corn leaf diseases relies on several factors. First is a working knowledge of what are the common as well as unusual diseases in your area. Then the disease must be properly identified based on signs, symptoms, and field distribution patterns, and observation of the severity of the disease must be done. And finally, familiarity with the damage threshold for a given disease is necessary so that management decisions can be made before the economic injury level is reached.

The critical period for scouting leaf diseases is from 2 weeks before pollination to 2 weeks after pollination. During this time, yield losses generally occur when whole plant infection reaches 15 percent severity. Leaf blights caused by fungi generally begin on the lower leaves. Some fungal leaf blight diseases that can be found on corn in Illinois include northern corn leaf blight (NCLB), southern corn leaf blight (SCLB), northern corn leaf spot (NCLS), and gray leaf spot (GLS).

Symptoms of NCLB are long, elliptical, grayish or tan lesions. Race 1 is usually not a problem since most commercial hybrids have the Ht1 gene for resistance. Race 2 is in some areas and can infect hybrids that have only the Ht1 gene for resistance. Other genes for resistance to Race 2 have been incorporated into many commercial hybrids. Heavy dews, frequent light showers, high humidity, and moderate temperatures (64° to 84°F) favor NCLB.

Symptoms of SCLB are not as distinct as those of NCLB. Generally, lesions are small, tan in color, and somewhat rounded, and have buff to brown borders. However, lesion appearance may vary greatly depending on the genetic background of the corn hybrid. SCLB is most common in the southern third of the state but can appear in the southern half of Illinois. High humidity and warm temperatures (69° to 90°F) favor SCLB. Long periods of sunny weather between rains are unfavorable for disease development.

Lesions from NCLS on susceptible inbreds are tan and oval to circular but can vary depending on the race in the field. Race 2 produces oblong, brownish spots up to 1 inch in length. However, this type of infection is rare. Race 3 is the most common race in the Midwest and produces narrow, linear to oblong lesions on leaves, sheaths, husks, and ears. Lesion appearance varies with the genotype of the plant and the isolate of the fungus. Lesions are dark grayish tan and are commonly surrounded by a darker border. Moderate temperatures (68° to 80°F) and high humidity favor NCLS. Spore production occurs primarily in damp weather.

GLS lesions are blocky to rectangular when compared to other leaf blights. They are pale brown or gray to tan in color and are characteristically restricted by leaf veins. Lesions may coalesce, resulting in large, dead areas of leaves or the killing of entire leaves. Continuous corn, no-till corn, warm to hot temperatures (71° to 85°F), high humidity, wet weather, and cloudy days favor GLS. GLS is usually found in low-lying fields, particularly in river bottoms, which tend to have more favorable environments for this pathogen.

Control of leaf blights is best accomplished by selection of resistant hybrids. Choose hybrids with resistance to the most common leaf blights in your area. Tillage to bury infected residues may also be helpful where erosion is not a problem.

Fungicides are useful when conditions favor diseases, especially if you are producing hybrid seed. Apply during the 14-day period before and after tasseling. Multiple applications may be necessary when disease pressures are high. However, during periods of high temperatures or dry conditions, disease pressures will be low and additional applications may not be necessary.

Crop rotation is also helpful because all leaf blights tend to increase in continuous culture. However, because spores of these fungi are windblown, rotation alone will not provide the necessary protection. Combine several pest-management techniques for the best results when managing corn leaf blights.--Joe Toman

Author: Joe Toman