Molecular mechanism and risk assessment of Rhizoctonia solani resistance to the SDHI fungicide isopyrazam

Rice sheath blight is one of the three major rice diseases worldwide, and the succinate dehydrogenase inhibitor (SDHI) fungicide isopyrazam has not yet been registered for its control in China. This study investigated the sensitivity of Rhizoctonia solani to isopyrazam and assessed its resistance risk. Specifically, the sensitivity of 113 R. solani isolates to isopyrazam was evaluated, with EC₅₀ values ranging from 0.0018 to 0.0336 μg/mL and an average of 0.0101 ± 0.0064 μg/mL. Ten isopyrazam-resistant mutants were generated by ultraviolet mutagenesis, including 2 highly resistant (RF > 100), 6 moderately resistant (100 > RF > 20), and 2 lowly resistant (20 > RF > 5) mutants. Fitness assays showed that resistant mutants had significantly reduced mycelial growth and pathogenicity. Positive cross-resistance was observed between isopyrazam and thifluzamide, but no cross-resistance was detected with non-SDHI fungicides. Sequence alignment of the SDH protein indicated that the H249L mutation in the SDHB contributed to moderate resistance, while the I78F mutation in the SDHC was associated with moderate to high levels of resistance. Notably, the SDHC-I78F was identified as a novel mutation site. Molecular docking indicated that these mutations reduced the binding affinity between isopyrazam and the SDH protein. Therefore, we assessed the resistance risk of Rhizoctonia solani to isopyrazam as ranging from low to moderate. These findings indicate that isopyrazam has potential for controlling rice sheath blight in the future.