Description and Symptoms
Gray leaf spot (Cercospora zeae-maydis) of corn occurs virtually every growing season in Iowa. If conditions favor disease development, economic losses can occur. Symptoms first appear on lower leaves about 2 to 3 weeks before tasseling. The leaf lesions are long (up to 2 inches), narrow, rectangular, and light tan colored. Later, the lesions can turn gray. They are usually delimited by leaf veins, but can join together and kill entire leaves.
The fungus survives in corn residue, and consequently, the disease is often more severe in corn following corn. Spores are dispersed by wind and splashing water. Infection of corn leaves and disease development are favored by warm (80s°F), humid (>90% for 12+ hours) weather. Disease severity depends on hybrid susceptibility and environmental conditions.
The best time to scout is V15 through R4; earlier in seed production fields. Gray leaf spot (GLS) may be more prevalent in fields with infected corn residue. Lesions always start in the lower canopy.
Resistant hybrids and inbreds are available. Crop rotation and tillage reduce survival of the fungus. Foliar fungicides labeled for gray leaf spot are available. The cornerstone of eyespot and GLS management is resistance. Rotation can also reduce risk of disease. Fungicides are also effective at reducing disease and protecting yield.
To determine if a fungicide application is necessary, the following factors should be considered:
• disease pressure in the field
• hybrid susceptibility
• predicted weather conditions during grain fill
• price of corn and cost of fungicide plus application
• previous crop
• field history of disease
Photo by Adam Sisson