Baseline Sensitivity to and Succinate Dehydrogenase Activity and Molecular Docking of Fluxapyroxad and SYP-32497 in Rice Sheath Blight (Rhizoctonia solani AG1-IA) in China

Rice sheath blight caused by Rhizoctonia solani occurs worldwide and is mainly controlled by fungicides. SYP-32497 is a novel succinate dehydrogenase inhibitor, which interferes with the succinate ubiquinone reductase in the mitochondrial respiratory chain of fungi. This study aimed to evaluate the baseline sensitivity of R. solani from 13 major rice producing areas in China to SYP-32497 and fluxapyroxad. The study also explored the cause for the activity discrepancy between SYP-32497 and fluxapyroxad via an enzyme activity inhibition test and molecular docking. A total of 360 R. solani isolates were sensitive to SYP-32497 and fluxapyroxad. Baseline sensitivities were unimodally distributed with mean values of 0.00667 ± 0.00475 and 0.0657 ± 0.0250 μg mL−1, respectively, for SYP-32497 and fluxapyroxad. Enzyme activity assays and molecular docking results revealed that SYP-32497 exhibited a much higher SDH inhibition (IC50 = 0.300 μg mL−1) than to fluxapyroxad (IC50 = 1.266 μg mL−1) because of its excellent SDH binding ability via hydrogen bonding, π-cation, and hydrophobic interactions. These results suggest that SYP-32497 is a good suitable control agent for alternative rice sheath blight.