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. Heavily infected leaves turn yellow, then brown, and drop prematurely.
Powdery mildew is controlled with resistant cultivars.
Most determinate soybean cultivars are highly resistant to powdery mildew, while many indeterminate cultivars are susceptible. Aerial blight is found in several states, but is primarily of economic importance in Louisiana where extensive yield losses have been reported. The fungus survives in the soil and on plant debris, as well as on certain weed hosts. Infection usually begins at flowering during prolonged periods of high humidity and warm temperatures. Symptoms include leaf spots, leaf blighting, or defoliation.