Grape leaf disease
This is not a disease. The damage is from erineum mites, Colomerus vitis, an occasional pest of grapes. The mite feeding produces what appear to be blisters on the top surface of grape leaves. The corresponding depressions on the reverse are filled with a white cottony-appearing material which is deformed leaf tissue that will age to tan, then brown.
Erineum mites are very different in size and shape than the more common spider mites you may be familiar with. They’re microscopic, elongated, and pale yellow and, because they overwinter under bud scales, they’re able to feed on newly expanding leaves. Unfortunately, their populations have surged this year. We’re receiving numerous inquiries from gardeners. See http://www.ipm.ucdavis.edu/PMG/GARDEN/FRUIT/PESTS/grerineummte.html.
Fortunately, the damage is only esthetic – the foliage looks really ugly when on a patio overhead. Pesticides aren’t needed against these particular mites because their feeding doesn’t affect grape yield, quality, or flavor.
Naturally occurring predatory mites help limit erineum mites and are considered important biological control agents of erineum mites. Pesticides used for other grape pests usually control erineum mites.