Resistance risk assessment of the novel SDHI fungicide fenopyramid in <i>Rhizoctonia solani</i>: moderate resistance risk and synergistic strategies with DMIs

Fenopyramid is a new succinate dehydrogenase inhibitor (SDHI), which will be soon launched on the market. Sensitivity detection and resistance risk assessment should be performed promptly to track the emergence of fenopyramid resistance and establish resistance management strategies. In this study, the sensitivity of 78 <i>R. solani</i> isolates to fenopyramid was evaluated, along with resistance risk, cross-resistance patterns, fitness costs, and resistance mechanisms in the lab. Three low-level resistant mutants and three high-level resistant mutants were obtained by repeated exposures of wild-type sensitive isolates to fenopyramid. The fitness in mycelial growth, mycelial biomass, and pathogenicity of the low-level resistant mutants were less than or similar to that of the parental isolate. There was no fitness cost in the high-level resistant mutants in comparison with that of the parental isolate. Fenopyramid had moderate to strong cross-resistance with commonly used SDHIs thifluzamide, boscalid, carboxin, flubeneteram, and non-SDHIs fluazinam. Overall, the results suggest that the resistance risk of <i>R. solani</i> to fenopyramid is moderate. Mixtures of fenopyramid with four DMIs showed synergistic effects. Therefore, it is recommended to mix fenopyramid with DMIs in a proper mass ratio to avoid and delay the development of fenopyramid-resistance. No mutations or significant expression changes were detected in the target-site genes <i>SDHA</i>, <i>SDHB</i>, <i>SDHC</i>, and <i>SDHD</i>. Other nontarget-related resistance mechanism may be responsible and require further investigation.