Though there are a number of diseases and insect pests of grapes, the most common problem growers encounter with grapes is black rot. Black rot is a disease caused by the fungus Guignardia bidwellii. The fungus infects the fruit while it is green, causes the berries to turn brown, and eventually shrivel to a hard black mummy. During wet years it can cause severe losses. The fungus survives the winter primarily on mummified fruit.
Sanitation is a key method of controlling the disease.
Research has shown that if the mummies are left hanging in the vine they will release spores all summer long. If, however, the mummies are removed from the vine and deposited on the ground they will produce and release spores for 3-4 weeks only during May and June. If the mummies are disposed of by burial, or are removed from the vineyard, then no spores will be released. So, by simply removing the old rotted clusters from the vine growers can greatly reduce the potential amount of disease. Despite your best attempts at sanitation, however, some chemical applications are likely to be necessary, especially during wet years.