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Downy mildew has been present in the U.S. since 1923 and has become one of the most common foliar diseases of soybean. The disease rarely causes severe damage to a crop, but there are some reports that it has reduced seed size and quality if present early in the season and at high levels. This disease occurs annually in Ohio, most often appearing after flowering. There are differences in the level of susceptibility among varieties.
Light green to yellow spots that, in some areas, have coalesced into larger lesions on the upper leaf surface (top left). Seeds can also be affected, giving a dusty appearance to the seed surface (top right). The underside of the leaf will have fuzzy gray-light purple areas where the pathogen is producing inoculum on the leaf surface (bottom).