The two most likely possibilities are either black rot or bitter rot. Black rot develops early in the season, generally from about the time from pollen shed up to the time when the grape berries start to turn color. At this point, the grapes are no longer susceptible tot he black rot fungus. infected berries will eventually dry up and look like raisins on the cluster. Bitter rot, on the other hand, begins to develop at the time of color change in the berries and continues until the fruit is fully ripe. Both of these are fungal diseases and may overwinter on mummified berries on the vine or on the ground under vines, from leaf litter and from infected woody tissue on the vines. Therefore, sanitation in the fall and winter is critical for control. Good pruning to remove infected canes and to allow greater light, air and spray penetration throughout the canopy of the vine during the growing season, as well as for promoting new fruiting wood, should be done every winter. Fungicide sprays should begin at bloom. Your state Extension Service (County Agent's office) should have information on what to spray with and how to use it.