P-331 - Unveiling Microbial Communities and Biofungicides Interactions: A Comparative Study of Grape ‘Chardonnay’ Microbiomes in Central Coast of California

Powdery mildew caused by Erysiphe necator is a major winegrape disease that requires management with fungicides. These fungicides may alter the berry surface microbiome, which could influence wine fermentation. This study used high-throughput sequencing to analyze microbiome of Chardonnay grape surface under various biofungicide and synthetic fungicide applications on the Central Coast of California. Berry samples were collected from a fungicide trial that involved weekly application of biofungicides (Reynoutria sp. (Rs), Streptomyces sp. (Sl), and Bacillus sp. (Bs)), rotations of these biofungicides with synthetic fungicides, a grower standard using all synthetics, and an untreated control (UTC). Disease severity was high in the trial, with UTC showing 92.6–100% in 2023 and 45.8–92.4% in 2024. Diversity analysis revealed that microbiome compositions varied among treatments where biofungicides were applied alone, whereas treatments involving synthetic fungicide rotations showed more similar microbiome profiles. In 2023, Bacillus velezensis dominated (77.9%) the Bs alone treatment, while Xanthomonas cassavae was most prevalent (83.4%) when rotating Rs with synthetics in 2024. Fungal communities were consistently led by Cladosporium cladosporioides in most of the treatments with 54.9% when rotating Bs with synthetics in 2023 and 71.0% in grower standard in 2024. These findings suggested that fungicide applications induced microbiome shifts on grape berry surfaces. However, further studies are needed to determine how these changes affect wine fermentation and quality.