Evaluation of Fungicide Programs for the Management of Grape Summer Bunch Rot in California, 2025

Summer bunch rot (SBR) is a late-season grape disease complex caused primarily by Botrytis cinerea, with secondary colonizers including Alternaria, Aspergillus, Cladosporium, Penicillium, and Rhizopus species. Symptoms develop after berry ripening and increase in severity when berries are injured. Affected clusters exhibit berry decay, shriveling, and juice leakage, which attract vinegar flies and may lead to sour rot development. Masses of black, brown, or green spores form on the surface of decayed berries, providing visual signs of the fungal groups present. This report evaluates the efficacy of fungicide programs consisting of experimental and registered biological and synthetic products for SBR management in California. The trial was conducted in 2025 in Clarksburg, California, in a mature 13-year-old Riesling vineyard. The outcomes of this research provide practical guidance for U.S. grape growers in optimizing disease management strategies under regional production conditions.