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Gray Leaf Spot in Illinois Cornfields

July 10, 2003

The corn crop in the northern half of Illinois is generally doing very well, and the crop in southern Illinois is starting to catch up after late planting this spring. We have received a few unconfirmed reports of leaf diseases of corn--principally gray leaf spot (GLS) and anthracnose. The recent warm weather (above 76°F) and forecast for continuing warm temperatures suggest common rust will not be a problem this year. However, we should be on the lookout for southern rust, which, unlike common rust, is favored by warm temperatures. Southern rust pustules are most common on the upper leaf surfaces, whereas common rust pustules are common on both leaf surfaces.

The severity and incidence of gray leaf spot and other corn leaf diseases will be affected to a great extent by rainfall, humidity, and temperature in July. Because corn leaf diseases generally cause greatest yield losses when infection occurs early in the growth stages of the plant, it now is time to consider scouting. Leaf diseases may be of particular concern in southern Illinois, where the corn is behind its normal development stage at this time of the year. Hence, there may be increased risk of losses from foliar diseases.

Gray leaf spot (caused by the fungus Cercospora zeamaydis) is usually the most common corn leaf disease across Illinois. This disease can take off quickly when the conditions are favorable. GLS infections are initiated from spores that are produced on infested corn residue remaining from the previous year. Large numbers of spores of the GLS pathogen usually are produced on residue beginning in June. After the spores are disseminated by wind and rain to the new crop, there may be a 1- to 2-week lag period before lesions become visible. GLS generally requires high humidity (>95%) for 24 hours, warm temperatures (75°F to 85°F), and susceptible inbreds or hybrids in order to cause significant damage. The symptoms of GLS first appear on lower leaves. Initial lesions are small, tan spots; 1 to 3 millimeters long; and irregular to rectangular in shape. As the lesions mature, they turn gray to tan in color and are rectangular in shape parallel to leaf veins.

Gray leaf spot symptoms on corn.

Gray leaf spot symptoms on corn.

GLS is managed by planting resistant hybrids, avoiding fields that were planted with corn the previous year, and making selective and timely applications of fungicides. Fungicides for management of GLS should be considered only for susceptible inbreds in most cases. In some fields where a susceptible hybrid is planted, the yield potential is high, disease pressure is high, and the environment is favorable for GLS. Under those conditions, then fields should be scouted and perhaps fungicide application should be considered. Fungicides should be applied in the time period of 1 to 2 weeks be-fore tasseling to about 1 week after tasseling.

Several different fungicides are available for control of GLS, including propiconazole (Tilt and Propimax), azoxystrobin (Quadris), a mixture of trifloxystrobin and propiconazole (Stratego), chlorothalonil (Bravo), and mancozeb (Dithane, Manzate, and Penncozeb). Always read and follow product label recommendations when using any fungicide. Additional information on control of corn leaf diseases can be found in the University of Illinois Field Crop Scouting Manual and the 2002 Illinois Agricultural Pest Management Handbook.--Dean Malvick

Author: Dean Malvick

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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