University of Illinois

No. 19/July 29, 1998

Gray Leaf Spot Update

Gray leaf spot (GLS) of corn has been reported throughout much of Illinois. This leaf blight pathogen is favored by warm, humid weather and frequent rainfall. It typically appears first on the lower leaves and moves upward if weather conditions are favorable. Yield losses are greatest when the pathogen infects before tasseling, especially if the upper leaves develop lesions. Losses are minimal if lesions do not appear on the upper leaves until 4 to 6 weeks after tasseling.

Lesions of gray leaf spot are more rectangular in shape than those of the other leaf blights (northern or southern leaf blights, for example). If an infected leaf is held toward the light, the lesion margins will appear to be parallel, when compared to the oblong shape of the lesions from other leaf blights.

Control of GLS is based upon several factors, including selection of resistant hybrids, crop rotation, the selected use of tillage where appropriate, and foliar fungicides. Because this pathogen overwinters on corn debris, burial of infected material may be helpful but cannot substitute for other control measures. Spores are windblown and can easily travel long distances. Thus burial of residues in a single field cannot ensure reduction of disease levels unless combined with other control measures.

H. Walker Kirby (kirby@mail.aces., Extension Plant Pathology , (217)333-8414