Well I'm certainly not an expert, but since no one else has chimed in yet I'll try to help you out. I'm thinking it might be black rot. Try googling black rot or grape diseases. You can also contact your county extension agent. If it is black rot, there's not too much you can do about it this year, but I think you can spray with a fungicide next year. Be sure to clean up the area carefully. Don't leave the shriveled grapes on the vine or ground around them. Good luck.

I'm a little late with this, but I agree it probably is "black rot". My grapes were beautiful for a couple years, three years ago this happened, bloomed nicely, set grapes and about the end of July the grapes turned black and hard ! Disdgusting! Someone said to spray with soda water.....didn't work. This year the same thing happened but I went to the "Grape" forum, too late to help this year. They say to spray with "Mancozel" , spray early in the year, then one week before blooming and a week after. Good Luck, hope this helps.