MSU Extension Integrated Pest Management

Powdery mildew

Disease

Powdery mildew

Uncinula necator

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Powdery mildew can infect all green tissues and give them a white to gray, powdery appearance. Colonies occur mostly on the upper leaf surface. Early berry infections can result in split berries, secondary rots and undesirable flavors in wine. Late berry infections are less obvious but can still predispose the berries to rots. In late summer, the fungus produces small, brown to black fruiting bodies (cleistothecia) on infected plant parts. Cleistothecia overwinter in bark crevices and release ascospores when exposed to moisture between bud break and fruit set. In regions with mild winters, the fungus can also survive in dormant buds, which develop into “flag shoots.” Powdery mildew is favored by high humidity and temperatures of 68 to 81 degrees Fahrenheit (20 to 27 C). Wetness is not required for infection. Temperatures above 95 F (35 C) inhibit new infections. Begin monitoring early in the season, focusing on shaded leaves and clusters inside the canopy.