It sounds like you have the vintner’s ancient scourge, grape black rot. It usually starts with small spots on the foliage that enlarge and are surrounded by a darker brown border. Spots also appear on the fruit, but, as you noticed, not until they are about half grown. They enlarge quickly, rotting the entire grape in a few days. The diseased fruits turn black, shrivel, and dry up; they look very much like raisins and are known as mummies.

Grape black rot is caused by a fungus, Guignardia bidwellii, and is a serious problem for grape growers, since all cultivars are susceptible. Wayne Wilcox, a specialist in grape diseases at Cornell University in Ithaca, New York, explains that sanitation is of utmost importance for control. The fungus produces two types of spores: The overwintering spores survive on mummies and these are airborne, thus any infected fruit left on the ground or on the canes becomes the primary source of infection. Later, the disease is further spread through waterborne spores that develop on infected fruit. Remove all mummies from the vines and from the ground beneath. Mulching to cover any remaining overwintering spores creates a physical barrier that will help reduce infection.