Answer: I suspect your grapes have botrytis mold, which is a fungus that forms a fuzzy coating on fruit and is most prevalent in cool, moist weather. The other possibility is powdery mildew, but this fungus commonly affects the leaves first.
Baking soda has been found to help control certain fungus diseases, and this is the basis for a product called Remedy Fungicide, available from Gardener's Supply in Burlington, VT (800) 863-1700. Remedy is registered for use on fruits and ornamentals.
Also, fermented compost tea can help control fungus diseases. You can make your own by adding one part mature compost that contains some manure to five parts water. Let the mixture sit in the shade for about 2 weeks, then filter and spray on your plants. Reapply after 2-3 weeks. The leftover residue can be spread around your plants to supply nutrients.
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