Now's the time to concentrate on a control program to take care of this disease. There's another plus for keeping black rot under control, because other diseases which affect bunch grapes should be less of a problem when recommended practices for black rot control are followed. Black rot is caused by a fungus which invades all parts of the grape vine, causing symptoms on stems, tendrils, leaves and fruit. Leaf infections appear as tiny, reddish-brown spots on the upper surface in early June. These increase in size and eventually blight the entire leaf.
Infections begin to appear on the fruit when the grapes are about one-half grown.
At first a small white spot forms on the surface of a grape berry. This spot enlarges rapidly until the entire grape is rotten. Affected grapes soon turn black, shrivel, and dry up. Within a few days most of the grapes within a cluster are infected and soon rot. Make sure you're spraying vines with a recommended fungicide every ten to fourteen days.