The fungus Erysiphe polygoni causes powdery mildew of beans and peas.

The disease causes white powdery patches to appear mainly on the older leaves and stems of plants.

SYMPTOMS OF POWDERY MILDEW

White powdery patches appear on both sides of the leaves. Young leaves are almost immune. A large part of the talc-like powder on the leaf surface is composed of spores. These spores are easily blown by winds to nearby susceptible plants.

Heavily infected leaves become yellow, then become dry and brown, before dropping off. If the leaves drop off, the beans will not fill much further. The pods are covered with white patches which may develop purplish spots and become distorted .

The powdery mildew fungus can reproduce under relatively dry conditions. Increased humidity can increase the severity of the disease, and infection is worse during periods of heavy dew.

CONTROL OF POWDERY MILDEW

• Attacks of powdery mildew occur in dry seasons and are worst in sheltered gardens.
• It is not known how the fungus survives between crop seasons, & crop rotation and many other cultural practices seem to have little effect on the incidence of powdery mildew.
• Peas & beans need fertile soil. Plants grown in poor soil, without correct applications of fertiliser, will develop powdery mildew before well fed plants. Healthy, vigorous leaves and stems are less prone to infection.
• Mildew resistant varieties of peas & beans reduce the problem of the disease.
• Chemical sprays are available to control mildew. Always follow the instructions on the packet.