The host epidermis is ruptured and powdery masses of spores are produced. Spots may range from minute dots to one-half inch or more in diameter. With mild temperature and high humidity, the disease spreads quickly. Extreme hot or cold temperatures and dry weather will slow the disease progress. The fungus attacks the cotyledons, leaves, and petioles of the host plant.
Lesions are circular with gray, brown or slightly off-white centers with slightly darkened margins.
Severe infection can cause yellowing of foliage and eventual defoliation. Young seedlings are killed whereas later infection causes a reduction in size of the edible portion. The fungus is soilborne and can live in several weeds or old volunteer plants. Stromata are formed in the leaves and these overwinter in the fallen foliage. When weather conditions favor fungal development, growth resumes.