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Issue No. 17, Article 4/July 20, 2007

Soybean Foliar Disease Update

Personal observations and reports from the field and from sentinel plots indicate that a number of soybean diseases are present throughout Illinois.

Septoria brown spot. Septoria brown spot is very common this year; it can be observed in almost any soybean field in the state. Symptoms appear as brown spots that begin on the lower leaves; these leaves may turn chlorotic after time. Septoria brown spot generally remains in the lower canopy, where it eventually causes defoliation of the lower leaves. This disease generally does not develop into the upper one-third of the soybean canopy, and most research has shown that yield loss due to brown spot is minimal.

Septoria brown spot on a soybean leaf.

Septoria brown spot causing leaf chlorosis.

Frogeye leaf spot. Frogeye leaf spot has been observed in sentinel plots and/or commercial soybean fields in McLean, Tazewell, Logan, DeWitt, Jersey, Ogle, Gallatin, DeKalb, Whiteside, Bureau, and Lee counties. Symptoms appear as circular tan to gray spots on the leaves, surrounded by a very pronounced dark purple margin. Scouting for frogeye leaf spot should occur during the R1 to R2 growth stages. If frogeye leaf spot can be easily found on a susceptible variety and conditions have been favorable for disease (wet, humid weather), then consider a fungicide application at R3 to control the disease. Fungicides that are registered on soybean that will control frogeye leaf spot include Topsin-M, Headline, Quadris, and Domark. Although these fungicides have good efficacy on frogeye leaf spot, yield benefit is not a certainty.

Frogeye leaf spot on soybean, characterized by a tan lesion with a distinctive dark border.

Downy mildew. Downy mildew has been observed in sentinel plots and/or commercial fields in Pope, White, Jackson, Gallatin, and Hamilton counties in southern Illinois. It has also been observed sporadically across the rest of the state. Downy mildew symptoms begin as light yellow "blotches" on the upper leaf surface. Gray "tufts" may appear on the lower leaf surface under the yellow blotches as the disease progresses. Foliar fungicide labels do not claim any control of downy mildew. The best management practices are resistant varieties, crop rotation, and avoiding planting downy mildew-infected seed.

Upper soybean leaf surface affected by downy mildew.

Lower soybean leaf surface affected by downy mildew.

Bacterial blight. This disease is beginning to show up in susceptible varieties across the state. Bacterial blight symptoms generally begin in the upper canopy, appearing as angular brown spots on the leaves with a pronounced yellow "halo" around each spot. This disease is generally not viewed as causing economic yield reductions in soybean. Fungicides will not control bacterial blight. Planting resistant varieties, avoiding "bin-run" seed, and crop rotation are the recommended management practices used to control bacterial blight.

Bacterial blight on soybean. Note yellow "halos" around the spots.

Soybean rust. Thus far in 2007, soybean rust has been confirmed in Texas, Louisiana, Mississippi, Alabama, Georgia, and Florida. Rust has been somewhat active in Louisiana and has been found in new sites there recently. The risk for Illinois soybean growers is still low, but growers should nonetheless keep track of the movement of soybean rust in the U.S. Visit www.sbrusa.net for a map that shows the current distribution in the U.S. to the county level. Updated management guidelines and tactics are also available at that site.--Carl A. Bradley

Carl A. Bradley

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