Abstract: The effect of salicylhydroxamic acid(SHAM) on mycelium growth of Phytophthora infestans in vitro was investigated using mycelium growth rate method, and the impact of SHAM on the sensitivity of P.infestans to azoxystrobin at different developmental stages was also investigated. The results showed that SHAM increased the mycelium growth of P.infestans at low concentrations( 10 μ g/mL). The inhibition rate was less than 10% when the concentration of SHAM was 10 μ g/mL, but was greater than 70% when the concentration was 50 μ g/mL. The sensitivity of P.infestans to azoxystrobin amended with 10 μ g/mL SHAM was not significant different to that without amendments of SHAM, with mean EC50 values of 0.08 and 0.10 μ g/mL in mycelium growth, respectively, and 0.65 and 0.67 μ g/mL in sporangium germination, respectively. In addition, sporangium production, zoospore release, and cystospore germination of P.infestans in azoxystrobin treatments were not affected significantly by SHAM supplementation at 10 μ g/mL; However, at 50 μ g/mL, SHAM supplementations inhibited significantly P.infestansa at all developmental stages except mycelium growth. Therefore, salicylhydroxamic acid may not be added when the sensitivity of P.infestans to azoxystrobin was determined in vitro.