We have been receiving multiple grape samples with black rot in our diagnostic clinic.  Only one or two infected grapes can provide a vast number of spores for secondary infections of clusters going forward.  Prevalent rainfall has made control difficult. The disease cycle will slow as grapes mature, but fruit can still be infected till late in the season.  The infections observed on the attached photos occurred within the last 10-21 days, as some of these may be secondary to initial infections. Black rot control is critical from bloom till 4-6 weeks after. If you see any infected grapes in your vineyards at this time, you should tighten your spray programs and make sure to include the most efficacious fungicides for management of this disease going forward (see the IPM guide for grapes at www.smallfruits.org). Many grape diseases, especially rots, can have similar symptoms, so microscopy may be needed for accurate diagnosis. Local county agents can help with identification of this and other diseases, so please feel free to contact your local county agent should you have questions relative diagnostics or management of grape diseases.

Black rot of grape.
Various stages of fruit decay from black rot on an individual cluster.
Pycnidia (fruiting structures of black rot).
Conidia (spores) oozing from and individual pycnidium. These will splash or blow to adjacent fruit, setting up secondary infections.