Answer: There are a number of diseases that can cause grape rot but without knowing which one specifically is causing problems with yours, I can't really make a recommendation. Botrytis is a common problem but so is Black Rot. The controls are different. I think the best course of action is for you to take samples of the affected grapes and leaves to your cooperative extension office for positive diagnosis. You can contact UMASS through the following website, which also offers a diagnostic tool online: http://www.umass.edu/agland/diagnostics/index.html
Hope this information is helpful.
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