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.
Phil Brannen is a Professor in the Plant Pathology Department at the University of Georgia. He attended the University of Georgia for his undergraduate degree in Plant Protection and Pest Management, where he also received an M.S. in Plant Pathology, followed by a Ph.D. in Plant Pathology from Auburn University. He has extensive experience with disease management programs in numerous cropping systems. He serves as the extension fruit pathologist for Georgia – conducting research and technology transfer for multiple fruit commodities. His efforts are directed towards developing IPM practices to solve disease issues and technology transfer of disease-management methods to commercial fruit producers. He also teaches the graduate level Field Pathology Course, the History of Plant Diseases and their Impact on Human Societies Course, team-teaches the IPM Course, coordinates the Viticulture and Enology in the Mediterranean Region Course (Cortona, Italy), and guest lectures in numerous other courses throughout the year.