It sounds as though your grapes have been attacked by black rot, a fungal disease of grapes that shows up in warm, humid weather. Infection starts as leaf spots followed by small light-brown spots on half-grown grape berries. The spots enlarge, soften, and overtake the grape in just a few days. The berry shrivels, turns black, hardens, and mummifies. These mummies remain attached to the cluster. By the time you notice the mummies, it is too late to control black rot that season. However, take steps early next season to prevent the disease from returning. Remove mummies and diseased tendrils from the vines. Make sure the plants are growing in a sunny area with good air movement. Apply a protective fungicide early in the season, beginning at bud break, all the way through bloom for good control of black rot.