Cross resistance of SDHIs in Botrytis cinerea and development of molecular diagnostic tools for SDHI resistance detection.

Succinate dehydrogenase inhibitors (SDHIs) are keystone synthetic fungicides used to manage Botrytis cinerea in several hosts. In this study, we investigated the cross-resistance between five new SDHIs, including pyraziflumid, isofetamid, benzovindiflupyr, fluxapyroxad, and pydiflumetofen with commonly used SDHIs, boscalid and fluopyram. Different SdhB mutations were detected in a B. cinerea collection from Michigan grapes, and their frequency and EC50 value were determined. Among 216 B. cinerea isolates, five different mutations were detected, including H272R/Y, P225F/H, and N230I at frequencies 82.6, 4.3, 11.5, 0.4, and 5.3 %, respectively. Five isolates of each genotype were used to screen the cross-resistance of the SDHIs. We have classified the resistant profile of our mutants into five patterns. We report that all tested mutants were sensitive to benzovindiflupyr, indicating that it can be used as an effective fungicide against all B. cinerea mutants. In addition, fluopyram, pydiflumetofen, and isofetamid can provide effective control according to which type of mutation is present in the field. We also developed and compared two molecular diagnostic tools, rhAMP and TaqMan assays, for rapid detection of SDHI resistance-associated mutants in B. cinerea. We report that the TaqMan assay was more successful than the rhAMP assay in detecting the B. cinerea mutant DNA at ≤ 10 pg, and in a single assay was capable of monitoring two amino acids positions. Our results provide essential information about new SDHIs and provide molecular tools for monitoring SDHI resistance mutations, which will assist in gray mold disease control.