P-103 - Multiple fungicide resistance in Botrytis cinerea populations from table grapes in the Central Valley of California

Gray mold, caused by Botrytis cinerea, significantly reduces grapes (Vitis vinifera), yield and quality. Fungicide application is a primary management strategy; however, B. cinerea is a high resistance risk pathogen, with resistance to multiple fungicides detected across various hosts in California. To assess fungicide sensitivity, we evaluated 1,078 B. cinerea isolates collected from flowers (n = 950) and berries (n = 128) across 23 table grape vineyards in Fresno, Kern, and Tulare Counties. Resistance was determined using a mycelial growth assay at specific discriminatory doses for eight fungicides from seven chemical classes. All isolates were sensitive to fludioxonil (FRAC 12). However, resistance was common among the other fungicides, with high resistance (HR, >50% growth) detected for pyraclostrobin (FRAC 11), thiophanate-methyl (FRAC 1), fluopyram (FRAC 7), boscalid (FRAC 7), and iprodione (FRAC 2), with average resistance frequencies of 88, 77, 48, 42, and 39%, respectively. Moderate HR prevalence was observed for cyprodinil (FRAC 9, 9%) and fenhexamid (FRAC 17, 11%). Most isolates were resistant to at least one fungicide, with multi-resistance to two, three, and four fungicides being most common, at frequencies of 22, 24, and 25% respectively. Notably, no isolates showed simultaneous resistance to all eight fungicides. These findings highlight the widespread fungicide resistance in B. cinerea populations in California table grapes, emphasizing the need for strategic fungicide rotation and integrated disease management to mitigate resistance development.