Evaluation of the Risk of Development of Fluopicolide Resistance in Phytophthora erythroseptica

: Fluopicolide has shown effective pink rot (Phytophthora erythroseptica) control in potato disease management. To efficiently utilize this chemical, the risk of fluopicolide resistance in P. erythroseptica needs to be assessed. In this study, 34 isolates of P. erythroseptica were obtained from symptomatic potato tubers with pink rot in Maine. The sensitivity of these wild-type isolates to fluopicolide was assessed by culturing them on agar medium amended with fluopicolide at various concentrations. The 50% effective concentration (EC50) of fluopicolide for the inhibition of mycelial growth was determined and used to establish a baseline sensitivity of these P. erythroseptica isolates to fluopicolide. The wild-type isolates were sensitive to fluopicolide, with EC50 values ranging from 0.08 to 0.35 μg/ml. By exposing P. erythroseptica zoospores to agar medium containing 100 μg/ml fluopicolide, 6 out of the 34 wild-type isolates produced fluopicolide-resistant mutants. The mutants were transferred to fungicide-free V8 medium consecutively for 10 times, and the 10th transfer of mutants was examined for resistance stability and biological fitness. In general, the mutants had similar or slower growth rates compared with their wild-type parents, and the virulence of some mutants was significantly reduced. The results indicated a low to moderate risk of P. erythroseptica developing resistance to fluopicolide, and suggested a trade-off between fluopicolide resistance and biological fitness in P. erythroseptica.