Gray Leaf Spot in Corn

[General Information] [Symptoms] [Management] [Action Steps] [Before You Apply Fungicide]
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General Information

Gray leaf spot is caused by the fungus Cercospora zeae-maydis. The fungus survives overwinter in corn debris and may be especially damaging in reduced tillage systems where corn follows corn. Typically, disease is most severe in the mountains and piedmont of North Carolina, but also has caused yield losses in the coastal plain and tidewater regions. High humidity is required for the fungus to infect leaves.

Symptoms

The symptoms of gray leaf spot are rectangular lesions on the leaf surface 1 to 5 mm in width by as much as 6 cm long, that are usually restricted by small leaf veins (Fig. 1). Lesions are yellow to gray, and as they mature the gray lesions remain opaque. The fungus generally produces spores on the underside of the leaf and the spore bearing structures may appear as small black specks. As disease progresses the lesions may coalesce and the entire leaf may turn brown.

Management

Resistance to gray leaf spot is controlled by many genes (polygenic resistance) Hybrids with high levels of resistance to gray leaf spot are available and should be considered in systems using minimal tillage with limited rotation. Because corn hybrids change so frequently, it best to consult with seedsmen for information on hybrid resistance to this and other diseases in high yield environments.

Action Steps to Minimize Gray Leaf Spot

1. Tillage can reduce the initial amount of disease, but windblown spores for other areas may still infect the crop.
2. Rotation can also aid in management of this disease, but susceptible hybrids may still be infected from spores from other areas.
3. Selection of resistant hybrids can minimize disease and yield losses.

Before You Apply Fungicide

1. Field corn within two weeks of black layer is unlikely to benefit.
2. Estimate yield potential – spraying corn with a yield potential of less than 150 bushels/ acre is unlikely to provide a profitable return.
3. If more than 10% of the leaf area is affected, spraying may increase yields.
4. Continued warm humid weather may cause disease to accelerate in susceptible hybrids.
5. Cost of fungicide and application is likely to be $15.00 to $20.00/acre including application costs, thus a 10 bushel increase is needed to pay for treatment costs.

Other Resources

For assistance with a specific problem, contact your local North Carolina Cooperative Extension Service Office. Outside North Carolina, look for your state extension service partners.