University of Illinois

No. 14/June 27, 1997

Weather Conditions Favorable for Corn Leaf Blights

With the recent change in the weather to extremely humid conditions as well as very high temperatures, corn leaf blights may begin to appear in many fields. Already, we have reports of anthracnose on lower leaves, particularly in fields with previous crop residues. Gray leaf spot may also appear earlier than anticipated if this weather pattern continues.

Anthracnose is not at all that uncommon when we have high temperatures and humid conditions at this time of year. Typically, small-to-elongate brownish lesions develop on the lower leaves of plants at about the 3- to 4-leaf stage. These lesions are commonly chocolate brown in color and parallel the midvein. They generally do not exceed an inch or so in length.

Anthracnose has three components in the disease process: a leaf blight, a stalk rot, and a topkill phase. Only the stalk-rot phase is of major consideration to producers. Leaf blight and topkill appear in some fields each year, depending on weather, but are not nearly as serious as the stalk rot.

Resistance to the leaf blight phase is at very low levels until the plants are about knee high. Thus, lesions may develop on lower leaves but do not cause economic loss, nor are controls recommended. The spores overseason on infected crop residues, and when corn is planted into fields where residues remain from a previous corn crop, anthracnose can be a common early season nuisance. The activity of this fungus is governed to a great extent by weather patterns. Frequent rainfall or high humidity increases the activity of this pathogen and can result in more leaf blighting. However, commercial corn hybrids carry resistance genes to this phase of the disease and it should not be a concern to producers.

H. Walker Kirby, Extension Plant Pathology, (217)333-8414