Black rot is an important disease of grapes in the northeastern United States. The fungus overwinters in old berries. Ascospores are ejected and conidia ooze from the old mummified berries during rains beginning in spring, usually after bud break. Leaves, blossoms, and young fruit are susceptible to infection. Small circular reddish-brown spots up to 1/4 inch in diameter appear usually on the lower leaves, in mid- to late-June.

Spray programs for black rot typically begin once grape shoots are 1" in length and end by the time grapes start to color (veraison). The most important control period is from 1" shoot length to two weeks after bloom. If early season infections are not controlled, additional treatments are needed to protect fruit from spores produced by infected leaves. Mature leaves and coloring fruit are not susceptible to infection (after approximately late July in a typical Michigan summer).

Infection require wetness, with the duration of wetness needed for infection dependent on the temperature. 

Are conditions right for black rot?
Forecast models for black rot available at Enviro-weather. Select a weather station from the map that is closest to your location. Then click on “fruit” for a list of weather resources and models for fruit production.

Another resource on black rot: http://grapes.msu.edu/blackrot.htm