Why do grape berries turn black and shrivel up in summer, even before they ripen?
Black rot, caused by the fungus Guignardia bidwellii, consistently destroys grapes, especially during wet seasons. Although black rot is the most common, other grape diseases such as bitter rot, ripe rot, and anthracnose can also cause fruits to turn black and shrivel up. Diagnosing these fruit diseases might require the services of a professional diagnostic laboratory.
Fruit infection begins as a dark rotted spot, but soon spreads to the entire berry and to other fruits in the cluster. Black rot disease is also seen on grape leaves as circular, tan spots with dark margins.
Black rot management begins in winter with sanitation. Shriveled fruit left over from the summer must be removed from the vines and picked up off the ground and destroyed. Because of black rot, many grapes are difficult to grow without the benefit of regular fungicide applications from bud break until just before harvest. Contact your local Cooperative Extension office for fungicide recommendations.