Powdery mildew on peas and beans—Erysiphe spp.
Powdery mildew usually appears first as yellow spots on the upper leaf surface of older leaves; these spots develop the characteristic powdery growth and symptoms spread to the undersides of leaves and stems. The leaves may also curl and dry out. Affected leaves may turn completely yellow, die, and fall off and the fruit beneath may become sunburned. Pea pods may get brownish spots on them.
All powdery mildew fungi require living plant tissue to grow. Most powdery mildew fungi grow as thin layers of mycelium on the surface of the affected plant part. Spores, which are the primary means of dispersal, make up the bulk of the powdery growth and are produced in chains that can be seen with a hand lens. Spores are carried by the wind to new hosts. All powdery mildew species can germinate and infect in the absence of water. Moderate temperatures and shady conditions are generally the most favorable for powdery mildew development. Spores are sensitive to extreme heat and direct sunlight.
Powdery mildew is favored by warm, dry days and cool, damp nights. In most cases, planting resistant varieties or avoiding the most susceptible varieties and following good cultural practices will adequately control powdery mildew. Plant in unshaded areas as much as possible. Provide enough water and avoid excess fertilizer. Because spores cannot germinate when there is free moisture and may be killed, plantings with overhead sprinkler systems or frequent water sprays may have reduced incidence of powdery mildew. Where conditions are most favorable for mildew, you may consider application of fungicides such as highly refined oils, sulfur soaps, or biologicals. Compost or bury infected residues to destroy overwintering fungus.
For more information, see the Powdery
Mildew Pest Note.
Pea pod infected with powdery mildew
Spores of powdery mildew on foliage