Establishment of sensitivity baseline of Phytophthora infestans to valifenalate and risk assessment of resistance

In order to establish the baseline sensitivity of Phytophthora infestans (Mont.) de Bary to valifenalate, the sensitivities to valifenalate of 105 P.infestans isolates collected from Hebei, Heilongjiang, Guizhou, Sichuan and Inner Mongolia Autonomous Region, where valifenalate had never been used,were determined by mycelial growth rate method. To clarify the difficulty degree of acquisition of valifenalate-resistant mutants in P. infestans, UV mutagensis and fungicide adaptation test were conducted. In order to clarify the patterns of cross-resistance between valifenalate and fungicides regularly used for late blight control, the sensitivities to valifenalate and six fungicides of eight resistant mutants and their six parent sensitive strains were tested. The results showed that the EC₅₀ values ranged from 0.0594 mg/L to 0.159 mg/L and the mean EC₅₀ value of 105 isolates was (0.102±0.024) mg/L. The frequency of sensitivity to valifenalate distributed as a unimodal curve, and there was no subgroup with sensitivity declined, so the mean EC₅₀ value could be regarded as the sensitivity baseline to valifenalate in P. infestans. Four mutants resistant to valifenalate with the resistance factors ranging from 3.2 to 14.9 were obtained through UV-irradiating mycelia of strains sensitive to valifenalate and the resistance mutagenesis frequency was 0.54%. Two mutants resistant with the resistance factors of 8.1 and 8.2 were obtained through UV-irradiating sporangia of strains sensitive to valifenalate and the resistance mutagenesis frequency was 1.33 × 10⁻⁷. Two mutants resistant to valifenalate with the resistance factors from 3.1 and 9.4 were obtained through 11 generation subcultures. No cross resistance relationship existed between valifenalate and azoxystrobin or fluopicolide or metalaxyl or cymoxanil, but cross resistance existed between valifenalate and mandipropamid or dimethomorph. It was suggested that P. infestans could have a low to moderate resistance risk to valifenalate. Therefore, valifenalate should be used for late blight in mixtures or rotation with other fungicides of different action mechanisms in production to avoid or delay the buildup of valifenalate resistance in P. infestans.