No. 15 Article 7/July 2, 2004

Disease Update for Field Crops in Illinois

The corn crop appears to be doing well, with only minor disease problems in most of the state; the wheat situation is still focused on widespread head scab in the northern half of Illinois (see "Wheat Scab and Testing for Mycotoxins" in this issue of the Bulletin); scattered root rot and foliar disease problems have been reported for alfalfa; and foliar and root rot diseases have been reported for soybean.

Corn. Corn is tasseling now in central Illinois, and the time is right to scout for foliar diseases. Common rust has been reported in localized areas, but it should not become a major problem if the weather trends move toward normal warm late-June and early-July temperatures. Gray leaf spot could be developing now when dews are heavy and long lasting, primarily in susceptible hybrids or inbreds. Anthracnose leaf blight has been reported in northern Illinois. Poor root growth has been reported in some areas, but this is likely caused primarily by saturated soils earlier in the season and insect damage, with disease possibly playing a secondary role. Crazy top may be developing in some fields that were flooded in May and June; it may be seen as excessive tillering, twisting of the upper leaves, and proliferation and deformation of the tassel.

Crazy top on corn.

Soybean. Frequent disease problems are being reported for soybean, especially Septoria brown spot and root rots. By far the most frequently reported problem is Septoria brown spot on soybean leaves. This common disease has been favored by the warm, moist weather in much of Illinois. Septoria brown spot can cause premature defoliation, which is usually restricted to leaves in the lower part of the plant. Yield losses from Septoria brown spot are usually considered insignificant, although in severe cases it can cause losses of 5% to 15%.

Septoria brown spot on soybean leaf

Classic dark lesion caused by Phytophthora that extends up a soybean stem from the soil level

Root rots caused by a complex of pathogens, including Phytophthora, Fusarium, Pythium, and Rhizoctonia are causing damage across the state. In some cases, plants have died in patches in wet areas of fields. In other cases, the infected plants are surviving, but the lateral and tap roots and nodules are rotted and damaged to the point that plants may "limp along" and suffer from stresses throughout the season and have reduced yields. Phytophthora rot alone will be another problem to watch for now and as the season progresses. Wet soil conditions have been favorable in much of Illinois for initial infection and subsequent development of this disease. Additional information on these and other soybean, corn, and alfalfa diseases can be found at the University of Illinois Field Crop Diseases Web site. --Dean Malvick

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