Gray Leaf Spot Evident

August 13, 1999
Gray leaf spot (GLS) has been reported throughout much of Illinois. I have seen fields with various degrees of severity. Gray leaf spot of corn is caused by a fungal organism that is most damaging in continuous corn, and can totally destroy a crop where no-till continuous corn is planted. These practices provide an overwintering site for the pathogen and a source of early inoculum the following season. However, spores are windblown and can easily travel long distances. Thus cornfields under a corn-soybean rotation can also be affected by late season with GLS.

Gray leaf spot is a disease that is favored by high humidity and frequent rains. Fungal spores can survive at humidities as low as 60 percent, but infection and colonization of the host does not occur unless relative humidities rise above 85 percent. The pathogen has a long latent period when no symptoms are visible. This period may last from 2 to 4 weeks. Thus farmers are now seeing infections that occurred during earlier rainy periods. However, fogs and heavy dews may provide enough moisture to produce an epidemic in susceptible hybrids. River-bottom fields or low-lying areas are most affected by GLS.

Scouting is important to determine whether the disease is present and how severe it is. This disease, as with many of the other leaf blight diseases, requires regular field monitoring 2 weeks prior to and 2 weeks after tasseling. Yield losses are greatest when the pathogen infects before tasseling, especially if the upper leaves develop lesions. The disease also predisposes the plant to stalk rot, causing reduced production during the grain-filling period and lodging. Losses are minimal if lesions do not appear on the upper leaves until 4 to 6 weeks after tasseling.

The disease symptoms include pale brown or gray-to-tan, long, narrow, rectangular lesions that are characteristically restricted by the leaf veins. Gray leaf spot typically appears on the lower leaves because of the spores being either windblown or rain splashed from previous crop debris. The lesions may coalesce, killing the leaves.

Control of GLS is based on several factors, including selection of resistant hybrids, crop rotation, the selected use of tillage where appropriate, and foliar fungicides. With the current corn prices, foliar fungicide treatments in most situations are not economical. Because this pathogen overwinters on corn debris, burial of infected material may be helpful but cannot substitute for other control measures.--Joe Toman

Author: Joe Toman