A number of foliar diseases can be observed on soybeans in a given year, but not all of these generally cause large yield decreases, and not all can be controlled with a foliar fungicide.
Septoria brown spot and frogeye leaf spot are common in Illinois and can be controlled with foliar fungicides. Septoria brown spot symptoms appear as small brown spots on leaves. These spots can eventually coalesce and cause the entire leaf to turn yellow and senesce prematurely. Frogeye leaf spot symptoms appear as circular tan to gray spots on the leaves, surrounded by a very pronounced dark purple margin.
Foliar fungicides can be very effective in controlling both Septoria brown spot and frogeye leaf spot, but the two do not always cause yield losses. Septoria brown spot can cause yield loss only in very wet conditions. Generally, Septoria brown spot will remain in the lower portion of the canopy and not progress to the upper third of the canopy. Under very wet conditions, however, it can progress to the upper canopy and cause yield reductions. Frogeye leaf spot has historically been referred to as a southern Illinois disease, but it has been prevalent throughout the entire state and caused yield losses in northern Illinois as recently as last year.
Results from 2007 University of Illinois soybean fungicide research trials (supported by a grant from the Illinois Soybean Association) indicated that yield returns with a fungicide ranged from 6-8 bu./acre, with an average of 1 bu./acre The fungicide trials conducted in the southern Illinois region averaged -1 bu./acre return, in the central Illinois region averaged 1 bu./acre return, and in the northern Illinois region averaged 3 bu./acre return. Higher yield returns were related to higher precipitation amounts, as the southern, central, and northern Illinois locations averaged 4.4, 6.6, and 12.7 in. of rainfall, respectively, for July and August. The greater rainfall in northern Illinois made more conducive conditions for frogeye leaf spot and Septoria brown spot, which were prevalent in those trials.
Downy mildew and bacterial blight are also common in Illinois, but downy mildew is not listed on product labels as a disease controlled with a foliar fungicide, and bacterial blight is caused by a bacterium rather than a fungus, so fungicides have no affect on it.
Downy mildew symptoms appear as light yellow "blotches" on the upper leaf surface, with gray "tufts" appearing on the lower leaf surface directly under the yellow blotches. Bacterial blight symptoms appear as angular brown spots on the leaves, with a pronounced yellow "halo" around each spot.