Answer: It sounds like your grapes may be afflicted with black rot, a fungal disease which winters over on affected fruit. To try to limit the spread, remove and destroy (do not compost) the "raisins". If this is a frequent problem in your area you might wish to grow a more resistant variety. You might wish to contact your County Extension for a definite diagnosis of the problem and suggested control, if any. Their telephone number is 292-5250.
Grapes are subject to a number of problems, so much so that most gardeners rely on a carefully timed spray regime, combined with proper pruning techniques and careful site sanitation to control pests and diseases. Your County Extension should also be able to provide you with both a list of recommended varieties and the suggested spray schedule for your area.
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