Sensitivity to fluopicolide and biological characteristics of resistant mutants of Phytophthora capsici in Chongqing

In order to clarify sensitivity of Phytophthora capsici to fluopicolide in Chongqing, the sensitivities of 110 isolates of P. capsici collected from areas in Chongqing without a history of fluopicolide appilication were determined by mycelial growth rate method. Resistance induction methods were conducted and main biological characters of the laboratory-induced fluopicolide-resistant mutants of P. capsici were studied. The results showed that the sensitivity frequency of isolates to fluopicolide distributed as a unimodal curve, and no sub-group with declining sensitivity was detected. The mean EC₅₀ value was (0.32 ± 0.11) μg/mL, which could be considered as the sensitivity baseline of P. capsici to fluopicolide and could be used for the field resistance monitoring. Three stable genetic fluopicolide-resistant mutants were acquired by UV irradiating mycelia. The resistance ratio ranged from 69.5 to 98.5 and the mutation frequency was 0.86%. It was found that the resistant strain BS11-5-1 showed similarity in mycelial growth rate, temperature adaptability, sporulation, pathogenicity, compared with its sensitive parent BS11-5. But biological characteristics of resistant mutants JLP11-4-2 and JLP11-4-3 were significantly lower than their parental isolate JLP11-4. The sensitivities of different resistant mutants to osmotic pressure were different from that of their parents. Utilization of 95 kinds of carbon sources of 3 resistant mutants was similar with their respective parents. No cross resistance existed between fluopicolide and metalaxyl, cymoxanil, dimethomorph, pyrimorph and azoxystrobin. It is suggested that the fluopicolide should be used alternately or as a mixture with other not cross-resistant fungicides.