University of Maryland Extension

Powdery Mildew on Vegetables

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powdery mildew infected cucumber plant
Powdery mildew on cucumber plant
 

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. 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.

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 older leaves. Disease spread occurs as spores are released from the surface layer of mycelium.

powdery mildew on pumpkin
Powdery mildew infection on pumpkin foliage

powdery mildew on muskmelon
Closeup of powdery mildew on muskmelon
leaf

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.

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