Abstract: In order to determine the sensitivity of metalaxyl-resistant strains of Phytophthora infestans to fungicides flumorph, azoxystrobin, pyraclostrobin, dimethomorph and cymoxanil, the toxicity of fungicides to metalaxyl- sensitive and metalaxyl-resistant strains of P.infestans were tested by measuring radial growth on agar medium amended with fungicides. The results showed that the five tested fungicides had high inhibitory activity against mycelium growth and sporangia formation of metalaxyl-resistant and metalaxyl-sensitive isolates of P.infestans. Flumorph exhibited the strongest toxicity to metalaxyl-resistant strains with the mean EC₅₀ of 0.037 6 μg·mL^-1.There was obvious difference in sensitivity of tested isolates to cymoxanil, which EC₅₀ values ranged from 0.008 4 μg·mL^-1 to 0.868 1 μg·mL^-1,the mean EC₅₀ value was 0.386 6 μg·mL^-1 in inhibiting the mycelium growth of metalaxyl-resistant strains. There is high correlation of tested results between in vitro method and the floating-leaf-disk method, and showed that the tested fungicides had no cross-resistance with metalaxyl. These fungicides maybe substitute metalaxyl for controlling potato and tomato late blight disease.

Key words: Phytophthora infestans, Metalaxyl-resistant, Fungicides, Sensitivity test