Answer: Based on the symptoms you describe, the grape vines may have a fungal disease called Downey mildew. This caused by the fungus Plasmapara viticola and is common in grape growing areas of the world with high humidity during the growing season. Downey Mildew is first observed as small, pale yellow to reddish spots with indefinite borders on the upper leaf surface with white downy growth on the lower surface. Infected leaves are killed, turn brown and may fall. Young infected shoots and cluster stems may curl, distort, show thickening of the infected tissue and may be covered with a whitish fungal mass. Infected berries turn brown and eventually shrivel. Infective spores are produced during rainy periods if temperatures are above 50F and infections are triggered by warm, humid nights followed by rain.
Limited testing has shown copper fungicides to reduce infections.
Mites can cause similar symptoms on leaves, but their feeding doesn't generally cause the grapes to be deformed.
You might want to take a sample of the problem to your local Cooperative Extension office for positive identification and control measures. (330-533-2424)
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