Shijian Zhuang, Shunping Ding,* Edgar Godoy Monterroso, Qun Sun, Matthew Fidelibus, and Philippe Rolshausen
*California Polytechnic State University-San Luis Obispo, 1 Grand Ave, San Luis Obispo, CA, 93407 (sding01@calpoly.edu)

Grapevine powdery mildew caused by Erysiphe necator and Botrytis bunch rot, caused by Botrytis cinerea, are the two of the most important fungal diseases for California grape production. Growers are increasingly interested in using biofungicides because of tightening state regulations and the development of fungicide resistance in fungal populations. We initiated biofungicide trials in 2023 in two CA regions: CA Central Coast (San Luis Obispo), using Chardonnay and Pinot noir, and the San Joaquin Valley (Fresno), using Chardonnay and Carignan. At each location, two blocks were selected per variety. Weather stations were installed in the vineyards before budbreak to monitor the real-time powdery mildew risk index. Fourteen treatments included three biofungicides (Bacillus subtilis, Streptomyces lydicus, and extract of Reynoutria sachalinensis) applied at different intervals (weekly, biweekly, and mildew risk index-based), biofungicides rotating with synthetics, synthetic standard only, and an untreated control. Treatments started at bloom. Disease incidence and severity were rated biweekly, approximately one week prior to veraison, and yield and berry chemistry were measured at harvest. Biofungicide by itself did not show a better efficacy than synthetic standard on disease incidence or severity. However, the rotation of bio-fungicide and synthetic showed similar efficacy against disease severity as synthetic standard alone on both varieties at each location. No significant difference in yield or berry chemistry was found between biofungicide and synthetic treatments. The ongoing project will be repeated in 2024.

Funding Support: CDFA-Specialty Crop Block Grant Program and California State University Agricultural Research Initiatives