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 during rains of 0.1 inch (2.5 mm) or
more 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 81ºF (20 to 27ºC). Wetness is not required for infection.
Temperatures above 95ºF (35ºC) inhibit new infections. Begin
monitoring early in the season, focusing on shaded leaves and clusters inside the canopy. |
