Black rot, caused by the fungus Guignardia bidwellii is one of the most serious diseases of cultivated grapes in the eastern United States, especially in warm, humid areas. Crop losses can be devastating, ranging from 5 to 80 percent depending on the weather, the variety of grape being grown, and the amount of disease in the vineyard. The fungus can infect all green parts of the vine but the most damaging effect is on the fruit, which shrivel up into dark-colored mummies.