Monitoring Resistance to SDHI Fungicides in Botrytis cinerea From Strawberry Fields.

Succinate dehydrogenase inhibitor (SDHI) fungicides have been used to control gray mold of strawberry for more than a decade, and selection for resistance in the causal agent Botrytis cinerea has become a threat to producers. In total, 2,570 B. cinerea isolates were collected from strawberry fields in the eastern United States across three seasons and their sensitivity to the SDHI materials boscalid, fluopyram, fluxapyroxad, and penthiopyrad was assessed. Assays were based on visual assessment of presence or absence of mycelial growth on media amended with discriminatory fungicide doses to distinguish sensitive from resistant isolates, respectively. Overall frequencies of isolates resistant to boscalid, fluopyram, fluxapyroxad, and penthiopyrad increased over the 3 years to 30.0, 1.0, 5.5, and 7.4%, respectively. Four resistance patterns, designated A, B, C, or D, were found. Pattern A isolates were resistant to boscalid with the allele H272R at locus sdhB; pattern B isolates were resistant to boscalid and penthiopyrad with the allele H272R or H272Y at locus sdhB; pattern C isolates were resistant to boscalid, fluxapyroxad, and penthiopyrad with the allele H272Y at locus sdhB; and pattern D isolates were resistant to boscalid, fluopyram, fluxapyroxad, and penthiopyrad with alleles P225F or N230I at locus sdhB. Isolates with alleles H272Y, N230I, or P225F were sensitive to a new SDHI, benzovindiflupyr, with mean effective dose that inhibits 50% of mycelial growth values of less than 0.5 μg/ml for each genotype, suggesting that this fungicide may be useful for resistance management. Our data show an increase of B. cinerea isolates resistant to SDHI fungicides over three consecutive production seasons. Resistance management practices must be implemented for the sustained efficacy of SDHI fungicides against gray mold of strawberry.