From Brooke Warres, graduate student in Plant Pathology at the University of Georgia.

Powdery mildew, caused by the fungus Erysiphe necator, has been found at the UGA Mountain Research Station in Blairsville, GA. The disease was observed on an untreated vine in a fungicide trial. Make sure that your current spray program includes active fungicides that protect against powdery mildew; resistance to QoIs has been frequently observed, so please be cautious. This fungus infects both leaves and fruit of grapes. If the fruit is infected early, the berries may crack as seen in one of the pictures below. Make sure you are scouting regularly for this and other diseases! Contact your local county agent if you have questions about this disease or any other aspects of grape production.

Powdery mildew of grape.
Powdery mildew was first observed in Blairsville on June 28th.
Powdery mildew of grape with cracking of skins.