Resistance risk and mechanism of Ustilaginoidea virens to pydiflumetofen

Rice false smut, caused by Ustilaginoidea virens, is a devastating fungal disease in rice that not only leads to yield reduction but also poses a serious threat to food safety and human health due to the production of numerous mycotoxins. Pydiflumetofen, one of the most promising SDHI fungicides widely used for controlling various plant diseases, lacks available information regarding its antifungal activity against U. virens and the potential risk of resistance development in this pathogen. In this study, we evaluated the sensitivity of 33 field-isolated strains of U. virens to pydiflumetofen using mycelial growth inhibition method and assessed the potential for resistance development. The EC₅₀ values for pydiflumetofen against the tested strains ranged from 0.0032 to 0.0123 μg/mL, with an average EC₅₀ value of 0.0056 ± 0.0025 μg/mL. In addition, four strains of U. virens were randomly selected for chemical taming to evaluate their resistance risk to pydiflumetofen, resulting in the successful generation of eight stable and inheritable resistant mutants at a frequency of 1 %. These mutants exhibited significant differences in biological fitness compared to their respective parental strains. Cross-resistance tests revealed a correlation between pydiflumetofen and fluxapyroxad as well as fluopyram, but no evidence of cross-resistance was observed between pydiflumetofen and boscalid or tebuconazole. Therefore, we can conclude that the risk of resistance development in U. virens to pydiflumetofen is moderate. Finally, the target genes SDHB, SDHC, and SDHD in U. virens were initially identified, cloned, and sequenced to elucidate the mechanism underlying U. virens resistance to pydiflumetofen. Three mutation genotypes were found in the mutants: SDHB-H239Y, SDHB-H239L, and SDHC-A77V. The mutants carrying SDHB-H239Y exhibited low resistance, while SDHC-A77V showed moderate resistance, but the mutants with SDHB-H239L demonstrated high resistance. These findings contribute significantly to our comprehensive understanding of molecular mechanisms involved in the resistance of U. virens to pydiflumetofen, and provide an important reference for chemical control strategies against rice false smut in the field.