Fungicide Sensitivity Profiles of Botrytis Isolates from Blueberry and Red Raspberry Fields in Washington and Oregon

Botrytis spp. cause major economic losses for small fruit growers in Washington and Oregon, the top blueberry- and red raspberry-producing states in the U.S. Pre- and post-harvest fruit rots are conventionally managed through repeated within-season applications of fungicides from various Fungicide Resistance Action Committee (FRAC) groups. As such, monitoring fungicide resistance profiles of Botrytis spp. from small fruit fields is crucial. To this end, in 2022 and 2023 a total of 708 and 305 Botrytis isolates were obtained from blueberry (WA and OR) and red raspberry (WA) fields, respectively. These isolates were screened against technical grade fungicides at discriminatory doses using in vitro conidial germ tube growth assays. Across years, isolates from blueberry showed varying resistance frequencies to fungicides tested: boscalid (60%; FRAC 7), fluopyram (49%; FRAC 7), fluxapyroxad (38%; FRAC 7), isofetamid (21%; FRAC 7), pyrimethanil (19%; FRAC 9), cyprodinil (4%; FRAC 9), pyraclostrobin (68%; FRAC 11), azoxystrobin (62%; FRAC 11), and fenhexamid (34%; FRAC 17). Botrytis isolates from red raspberry displayed the highest resistance frequency to fluopyram (79%), followed by pyrimethanil (31%), boscalid (30%), fluxapyroxad (22%), isofetamid (21%), and cyprodinil (6%). Cross-resistance was prevalent for fungicides in FRAC group 11 and for various fungicide combinations within FRAC group 7. The unique fungicide resistance patterns seen here compared to previous studies in the Pacific Northwest and elsewhere highlight the need for ongoing fungicide resistance monitoring in small fruits.