Answer: What you describe sounds like powdery mildew, a fungal disease. Once the plant is fully involved it is difficult to control. Lime sulfur fungicides can help protect the unaffected plant parts but should not be applied when temperatures are over 85F. Pruning the plant back can help remove the fungal spores. Or you can contact your cooperative extension office for other suggestions: University of California Riverside County, 82-675 Highway 111, Rm. 118 Indio, CA 92201. Or phone (951)683-6491 x 0 and ask to speak to a Master Gardener. Best wishes with your grapevines.
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