Control of Botrytis cinerea from Chilean Grapevines by Pydiflumetofen: Baseline and Carboxamide-Mutant Sensitivity

The gray mold (Botrytis cinerea; Botrytis) is the main disease affecting grapevine production in Chile. Succinate dehydrogenase inhibitors belonging to the carboxamide fungicide family are a key tool for the control of Botrytis in grapevines from the Chilean Central Valley. This study aimed to determine the sensitivity of the Chilean Botrytis population to the new generation carboxamide pydiflumetofen. Conidial germination (CG) and germ-tube elongation (GTE) sensitivity assays were conducted on 200 single-spore isolates collected during the 2016 to 2017 season. The mean effective concentration that inhibited 50% (EC₅₀) of CG in the Botrytis population was 0.0545 μg/ml, with mean values of 0.066 and 0.042 μg/ml for table and wine grapes, respectively. The mean EC₅₀ value of GTE was 0.000245, 0.0003, and 0.0019 μg/ml for the total, table grape, and wine grape populations, respectively. The comparison between pydiflumetofen and fludioxonil, a highly efficient fungicide carrying a different mode of action, showed 87.5 and 97.5% of Botrytis control with an EC₅₀ threshold of 0.1 μg/ml in table grape and wine grape populations, respectively. No cross-resistance between pydiflumetofen and fludioxonil was detected. For nine isolates with reduced pydiflumetofen sensitivity, we evaluated SdhB mutations using a qPCR-HRM diagnostic system. Two isolates carried the sdhB^P225/H272R genotype, and two isolates carried the sdhB^P225/H272Y genotype. Additional analysis of SdhB mutant isolates determined that pydiflumetofen controls wild-type as well as sdhB^P225/H272R and sdhB^P225H/H272 mutants. Pydiflumetofen does not control CG in the sdhB^P225/H272Y mutant but is effective in the GTE control. Pydiflumetofen significantly controls Botrytis independently of the SdhB genotype in wounded berry assays. This condition resembles the berry cracking due to heavy rainfall right before harvest, as seen in recent years in the Chilean Central Valley. The findings demonstrate that pydiflumetofen effectively controls the grapevine Botrytis population, suggest a moderate risk of pydiflumetofen resistance, and highlight the significance of incorporating genetic data into the design of control programs.