Sensitivity of Rhizoctonia spp. to flutolanil and characterization of the point mutation in succinate dehydrogenase conferring fungicide resistance

Rhizoctonia are the primary pathogens associated with damping-off disease of sugar beet seedlings in China. Flutolanil is an important member of the succinate dehydrogenase (SDH) inhibitors, and exhibits a high level of antifungal activity against Rhizoctonia. The sensitivity of Rhizoctonia associated with sugar beet seedling damping-off disease to flutolanil has not been comprehensively examined, nor has the underlying basis for resistance. In the present study, a total of 221 collected Rhizoctonia isolates were found to be extremely sensitive to flutolanil in vitro, having a mean median effective concentration (EC50) value of 0.1727 +/- 0.0074 mu g mL(-1). Two flutolanil-resistant isolates were generated using flutolanil-amended media. The resistant isolates, relative to the parental sensitive isolate, exhibited a slower rate of mycelial growth at certain temperatures and slightly decreased virulence on sugar beet seedlings. Flutolanil resistance was stable in the two generated resistant isolates after growing them for ten generations on potato dextrose agar (PDA) plates without fungicide. Flutolanil resistance was positively correlated with cross-resistance to thifluzamide, penthiopyrad, and tebuconazole. Molecular characterization and amino acid polymorphism analysis of the translated SDHA, SDHB, SDHC genes in Rhizoctonia indicated that there were no amino acid substitutions in the SDHA or SDHB subunits, however, a replacement of phenylalanine (Phe) by leucine (Leu) at position 48 (F48 L) in the SDHC subunit was observed. This study is the first documentation of flutolanil-resistance in R. solani anastomosis group (AG)-4HGI isolates with a point mutation in the SDHC subunit.