This fungus grows on all above ground parts of the vine. Powdery mildew causes curling and withering of young leaves and dark staining on the surface of mature leaves. Other symptoms include dropping, discoloration or splitting of berries, browning, and poor maturation of canes. American grape varieties are rarely damaged by this disease. Since the fungus is favored by low humidity and can grow at 90 degrees F., this disease is more common in West Texas.
Prevention is the best means to control powdery mildew.
It should be controlled by fungicides applied when foliage first develops and repeated at two to three week intervals until berries are full size. If newer fungicides are not available, sulfur dust applied at the rate of 5 to 10 pounds of dusting sulfur per acre will prevent powdery mildew from developing. Make additional applications every two weeks until fruit matures. Reapply sulfur after rains. Do not spray table grapes with this mixture if berries are more than one-third full size.