Thanks Karen, my guess would be grape black rot because you said they go from green to rotten before ever ripening. Summer Bunch Rot will only appear when sugars are above 8% and the grapes start ripening. The timing is right for black rot to start showing up, it likes it hot and humid. Frequent rains and temperatures in the high 80s are perfect for black rot infection. Black rot affects all parts of the plant, but the most damaging effect is to the fruit. Most infections start when the fruit is half to almost full size. Early varieties with large grapes are the most susceptible. In general, grapes that ripen late in the season are affected the least.
Black rot spores love moisture, so you want to reduce the amount of moisture held in the canopy by providing great air circulation. During dormancy, prune heavily keeping only a few healthy, strong canes from last year's growth to allow better air to flow. Prune out any diseased parts of the vine and keep the vine tied up and off the ground. If all else fails there is also numerous fungicides you can use if you want to go that route. Good luck and please keep us updated.
