Sensitivity determination and resistance risk assessment of Rhizoctonia solani to SDHI fungicide thifluzamide

Laboratory experiments were conducted to determine (a) the baseline sensitivity of Rhizoctonia solani to thifluzamide and (b) the risk of the fungus developing resistance to the fungicide. Thifluzamide sensitivity was assessed for 227 isolates of R. solani collected from 12 provinces of China from 2007 to 2011. One insensitive isolate GD‐1 was obtained from the field, and the EC₅₀ values of the 226 sensitive isolates had a unimodal frequency distribution with a mean of 0.0351 µg mL^?1. Nine resistant mutants were generated using thifluzamide‐amended media or UV radiation in the laboratory. The resistance was stable for all mutants after 10 transfers on PDA medium. Fitness of the most resistant mutants was lower than that of the sensitive isolates, implying a lower competitiveness of the mutants relative to sensitive isolates in field. Cross‐resistance was detected between thifluzamide and the Succinate dehydrogenase inhibitors (SDHIs) fenfuram, carboxin, penflufen and boscalid, but not between thifluzamide and difenoconazole, carbendazim, propiconazol, SYP‐2815 (quinone outside inhibitor (QoI) fungicide developed in China), fluazinam, jinggangmycin, pyrimorph or mepronil. The SDHI fungicide fluopyram did not inhibit R. solani. Taken together, these results suggest that the risk of R. solani developing resistance to thifluzamide is low to moderate.