Black rot, caused by the fungus Guignardia bidwellii, is a serious disease of cultivated and wild grapes. The disease is most destructive in warm, wet seasons. It attacks all green parts of the vine – leaves, shoots, leaf and fruit stems, tendrils, and fruit. The most damaging effect is to the fruit. Note: Guignardia bidwellii forma parthenocissi causes a leaf spot on Boston ivy and Virginia creeper. Control is as for black rot of grapes.
Warm, muggy weather in the spring and summer, along with unsprayed fruit of susceptible varieties, may cause fruit to become almost completely rotted by harvest time. Black rot is not difficult to control if good cultural practices are followed along with the use of protective fungicide sprays.