Department of Plant Pathology

Botrytis bunch rot is an important grapevine disease in California. Twenty eight fungicide programs were evaluated for control of bunch rot in a field experiment in a Chardonnay (clone 4) vineyard in the Carneros region of Napa Valley, California during 2009. Four fungicide applications were made from bloom to harvest with the final application made just prior to heavy rainfall. Disease was rated three weeks following the final application. Disease developed rapidly during the month of October, approaching 60% infected berries in untreated vines. Most fungicide programs failed to give a large (>3 fold) reduction in disease severity, but many partially lowered disease severity. Several products reduced disease to less than 10% severity: fenhexamid (alone and with tetraconazole), fluopyram + tebuconazole and fluopyram + tebuconazole alternated with trifloxystrobin + tebuconazole. Tetraconazole and cyprodinil alone, mineral oil, other soft chemistry products, and boscalid + pyraclostrobin did not effectively control disease.