As spots enlarge they become yellow-brown to dark brown. A grayish downy tuft of sporangiophores develops on the lower leaf surface during periods of moist weather. Severely infected leaves drop prematurely. Cultivars with some resistance are available and should be used. However, many races of the pathogen have been identified, and cultivars which are resistant to all known races have not yet been developed.
Powdery mildew occurs sporadically on soybeans.
Yield reductions are possible if foliage is affected during pod fill or extensive defoliation occurs. Plants are susceptible at any growth stage, but symptoms are rarely seen in the field until the mid to late reproductive stages. Disease incidence is greater in seasons with cooler than normal temperatures. The first symptom is usually a circular area of white to light-gray, superficial powdery growth on the upper leaf surface. All green plant parts may be covered by the powdery growth.