Efficacy of cyclobutrifluram in controlling Fusarium crown rot of wheat and resistance risk of three Fusarium species to cyclobutrifluram

Cyclobutrifluram (TYMIRIUM (R) technology), a new succinate dehydrogenase inhibitor (SDHI) fungicide, is currently being registered by SYNGENTA for controlling Fusarium crown rot (FCR) of wheat in China. The application of 15 or 30 g of active ingredient/100 kg seed of cyclobutrifluram significantly reduced pre-emergence damping-off, discoloration on the stem base and formation of whiteheads caused by FCR. The EC50 values of cyclobutrifluram for 60 isolates of F. pseudograminearum, 30 isolates of F. asiaticum and 30 isolates of F. graminearum ranged from 0.016 to 0.142 mg L-1, 0.010 to 0.041 mg L-1 and 0.012 to 0.059 mg L-1, respectively. One hundred and seven cyclobutrifluram-resistant (CR) mutants were obtained from three Fusarium species isolates, with ten types of mutations identified in Sdh genes. Three Fusarium species isolates exhibited similar resistance mechanisms, with the most prevalent mutations, SdhC(1)(A83V) and SdhC(1)(R86K), accounting for 61.68% of mutants. The CR mutants possessed comparable or slightly impaired fitness compared to the corresponding parental isolates. The CR mutants carrying FpSdhB(H248Y/Q/D) exhibited increased sensitivity to fluopyram. An overall moderate risk of resistance development in three Fusarium species was recommended for cyclobutrifluram.