The grapes in the photo looks like they were subject to black rot, a fungal disease. Black rot cannot be cured once the grapes are infected, but it can be prevented by adhering to a fungicide spray schedule beginning at bud-break and continuing every 7-10 days through the growing season and until the grapes change color. Minimize black rot problems by removing infected fruit, cleaning up mummies, and pruning out leaves that surround fruit clusters. See fruit links from Va Tech http://www.virginiafruit.ento.vt.edu/ and Penn State http://extension.psu.edu/plants/gardening/fphg/grapes
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