Black rot, caused by the fungus Guignardia bidwellii, is the most serious disease of grapes in Illinois. The disease is most destructive in warm and wet seasons. The fungus attacks all green parts of the vine - the leaves, shoots, leaf and fruit stems, tendrils, and fruit. The most damaging effect is to the fruit. When warm, muggy weather in the spring and summer is prolonged, unsprayed fruit on very susceptible varieties may become almost completely rotted by harvest time.