Field evaluation of biofungicides to control powdery mildew and botrytis bunch rot of wine grapes in California

Abstract

Grapevine powdery mildew caused by Erysiphe necator and Botrytis bunch rot caused by Botrytis cinerea are two of the most important fungal diseases in California grape production. Growers are increasingly interested in using biofungicides because of the tightening of state regulations and high risk of fungicide resistance in the pathogen populations. During 2023 and 2024, we conducted two biofungicide field trials on Chardonnay grapevines located in the Central Coast (San Luis Obispo) and San Joaquin Valley (Fresno) of California, respectively. At each location, two experimental blocks were selected. Weather stations were placed in the vineyards prior to budbreak to monitor powdery mildew disease pressure using the Gubler-Thomas risk index model. Fourteen treatments, including three biofungicides (Bacillus subtilis, Streptomyces lydicus, and an extract of Reynoutria sachalinensis) applied at different intervals (weekly, biweekly, or based on the powdery mildew risk index), biofungicides in rotation with synthetic fungicides, a grower standard program with only synthetic fungicides, and a non-treated control, were applied to grapevines starting at bloom time and until veraison. Disease incidence and severity were rated biweekly until veraison; yield and berry chemistry were measured at harvest. Standalone biofungicides demonstrated higher disease control efficacy when applied weekly. Biofungicides provided effective disease control under low disease pressure (2024) but were less effective when pressure was high (2023). Overall, programs rotating biofungicides and synthetic fungicides achieved the best disease control, comparable to synthetic fungicides-only treatments. No significant differences in yield or berry chemistry were observed between the biofungicide and synthetic fungicide treatments.