Studies on baseline sensitivity to mandipropamid and biological characteristics of resistant mutants of Pseudoperonospora cubensis

The objectives of the study were to establish the baseline sensitivity of P.cubensis to mandipropamid and to assess the resistance risk of P.cubensis to mandipropamid.The sensitivity to mandipropamid of 69 P.cubensis strains collected from the areas in Hebei and Shandong province where no mandipropamid had been used before were determined by leaf disc floating test,UV mutagensis and fungicide adaptation test were conducted to acquire resistant mutants to mandipropamid and to clarify the biological characteristics of the resistant mutants.The results showed that the frequency of strains with different sensitivity to mandipropamid distributed as a unimodal curve,there was no sub-group with sensitivity declined,and the mean EC50 value was (0.358± 0.144) μ g/mL.Six mutants of P.cubensis resistant to mandipropamid were obtained by UV mutagensis with the resistance factors ranging from 5.74 to 22.96 fold,and the mutation frequency was 1.09×10-7,the fitness of six mandipropamid-resistant mutants was significantly lower than that of their respective parental strains,and the resistance could not be inherited stably.No cross resistance relationship existed between mandipropamid and metalaxyl,azoxystrobin,cymoxanil or fluopicolide,but the cross resistance was existed between mandipropamid and dimethomorph.In a word,it was suggested that P.cubensis had a low to medium risk of developing resistance to mandipropamid.