White coating on leaves
Powdery mildew grows as a white powdery coating over the surfaces of leaves. Pea, bean, okra, cucumber, squash, muskmelon, and pumpkin are common host plants. Different powdery mildew species infect different plant families. Leaves around the base of the plant are first affected. This fungal disease is favored by warm weather and can be destructive in dry as well as hot seasons. It can grow rapidly with high humidity but, unlike many other plant pathogens, is not encouraged by wet foliage.
Powdery mildews produce their sexual spores within a round dark-colored structure called a cleistothecium. These structures observed under a hand lens appear as very small dark spheres with attached appendages. These fungi can overwinter as spores on fallen leaves or in buds. Spores are carried to the leaves by air currents. The first symptoms of infection usually occur as a superficial white coating of mycelium on the upper sides of older leaves. Disease spread occurs as spores are released from the surface layer of mycelium.
Photo: Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org
Photo: Oliver T. Neher, The Amalgamated Sugar Company, Bugwood.org
Management
Select resistant cultivars. Pull up and remove all plant debris at the end of the season. Infections often start when plants are mature and are rarely a problem for gardeners. There are horticultural oil products labeled for powdery mildew control on cucurbits.