Sensitivity to trifluzamide and main biological characteristics of resistant mutants of Rhizoctonia cerealis

The sensitivity of 166 isolates of R. cerealis to trifluzamide collected from areas in Hebei, Henan, and Shandong provinces where no novel amides fungicides had been used before were determined by mycelial growth rate method. UV mutagenesis and fungicide adaptation test were conducted to acquire resistant mutants to trifluzamide and to clarify the biological characteristics of the resistant mutants. The results showed that the frequency of isolates with different sensitivity to trifluzamide distributed as a unimodal curve, and there was no sub-group with declining sensitivity. The mean EC50 value was 0.072±0.022 μg/mL and it could be considered as the sensitivity baseline. Five resistant mutants of R. cerealis to trifluzamide were obtained by UV mutagenesis with the resistance levels ranging from 5.5~18.9 fold. There was no significant difference on the growth rate of mycelium on PSA between the mutants and the parent isolates, but the dry weight of mycelium, the number of sclerotium and the dry weight of sclerotium were significantly lower than that of the parent isolates. The virulence of five resistant mutants were significantly weakened to Shixin828 and Liangxing 99 than that of the parent isolates, except for the virulence of resistant mutant HD7-7U to Shixin 828, and the resistance of five mutants could not be inherited stably. No cross-resistance existed between trifluzamide and triadimefon, jinggangmycin, tebuconazole, fludioxonil, propiconazole and carbendazim.