Powdery Mildew
Causal Agent
Several Erysiphe species cause powdery mildew on beans and peas. It is currently not known which species are present in Utah.
Symptoms
The leaves show white, powdery spots that enlarge and eventually cover the entire leaf. Leaves may turn yellow and drop.
Disease Cycle
Infections occur in early summer from spores released from fruiting structure in plant debris, overwintering legumes, or spores blown in by wind from warmer areas further south. Spores can be carried by wind currents for hundreds of miles. In contrast to many fungi, powdery mildews do not spread in rain or free water. For infection, powdery mildews only need high humidity or dew for a few hours. After a spore lands on a suitable plant surface, it germinates and the germ tube penetrates the tissue and the fungus starts growing on the plant surface. It takes about a week after infection before the first spores are produced and dispersed. Once spores are produced in a field, powdery mildew spreads quickly from plant to plant by air movement and on worker’s clothes going through the rows.
Management
Powdery mildew must be controlled early on when the first lesion is seen. Once the fungus covers the entire leaves or the leaves are yellow, it is too late to control the disease. Remove infected plant debris from fields before planting a new crop and remove weeds in the legume family. There are resistant pea varieties for E. pisi. They are not resistant to other powdery mildews infecting peas. It is therefore necessary to identify the species present in Utah. Fungicide applications following the label may be necessary.