Powdery white spots appear on leaves when the plants are flowering and, under rain-free conditions spread rapidly to cover all leaves, stem, tendrils and pods. As foliage ages, pinhead-sized overwintering bodies of the pathogen speckle the mildewed surface. Powdery mildew is often most conspicuous on late maturing crops or groups of plants.
Powdery mildew reduces seed yield. In heavily mildewed crops, the uptake of desiccants is restricted and seed may have a musty odor.