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Septoria Brown Spot in Soybeans

The most common foliar disease on soybeans to date has been Septoria brown spot. Varieties differ in susceptibility so if you have multiply varieties make note of the differences in their response to the disease.

Figure 1. septoria brown spot in soybeans

Symptoms appear first on the primary unifoliate leaves shortly after trifoliolate leaves have developed. In some cases depending on the amount of rain and splashing that has occurred the trifoliate leaves may have more lesions then the unifoliates. Disease symptoms begin as small, dark brown, irregular spots, 1-2 mm in diameter with or without a yellow halo which develop on upper and lower surfaces of lower leaves. Initial infections on primary leaves and cotyledons produce secondary inoculum that infects upper leaves as they develop. The fungus produces a toxin that contributes to yellowing.

Lesions may enlarge and coalesce, and frequently they are concentrated along the leaf veins or at the leaf margin (Fig. 1). The disease is more cosmetic then anything but development early in the season can lead to significant defoliation o very susceptible varieties but in most cases as the weather turns warmer and drier symptoms often disappear. Symptoms may be difficult to distinguish from those of bacterial blight, soybean rust and downy mildew. As is always the case a good rotation with non-host crops such as wheat and corn will lower disease levels.