Baseline sensitivity and resistance risk assessmemt of Rhizoctonia cerealis to thifluzamide, a succinate dehydrogenase inhibitor.

During 2010-2012, a total of 120 isolates of Rhizoctonia cerealis were collected from wheat with symptoms of sharp eyespot in four provinces (Henan, Shandong, Anhui and Jiangsu) in China. All the isolates were determined for baseline sensitivity to thifluzamide, a succinate dehydrogenase inhibitor (SDHI) with strong antifungal activity. The sampled pathogenic populations, never exposed to SDHIs, had similar sensitivity to trifluzamide (0.025-0.359 µg/ml) in the four regions and over the two years. The baseline sensitivity was distributed as a skewed unimodal curve with a mean EC50 value (effective concentrations for 50% inhibiting mycelial growth) of 0.064 ± 0.013 µg/ml. The resistance risk of R. cerealis to thifluzamide was further evaluated in vitro. Two thifluzamide-resistant mutants of R. cerealis were obtained by culturing on thifluzamide-amended plates. The resistance factors (RF = EC50 value of a mutant/EC50 value of the wild type progenitor of the mutant) were 120 and 40 for two R. cerealis mutants, respectively. All the mutants exhibited similar fitness after 10 successive transfers when compared to their wild-type parents in mycelial growth, sclerotia production, and virulence. However, the two thifluzamide-resistant mutants differed significantly in sensitivity to boscalid and flutolanil. Therefore, a low-to-moderate risk of resistance development was recommended for thifluzamide.