Determination on Sensitivity of Phytophthora nicotianae var. nicotianae to Two Unsystemic Fungicides

The sensitivities to chlorothalonil(B) and mancozeb(Z) of 38 strains of Phytophthora nicotianae Breda de haan var.nicotianae Waterhouse collected from tobacco plants in Yunnan,Guizhou,Shandong China in 2005 were determined with the method of colonial growth-rate.The results showed that the EC50 values of B and Z to all the strains ranged from 2.33 μg/mL to 49.47 μg／mL and from 3.51 μg／mL to 82.05 μg／mL,respectively;and the EC50 mean values were 9.35(±2.77) μg／mL and 10.59(±4.06) μg／mL, respectively.The frequency distributions of EC50 values of B and Z to 37 strains except of XR001 were bimodal with evident main peak and unimodal,respectively.The EC50 mean value (5.99±0.78 μg/mL) of B to 30 strains, which composed the normal main peak of sensitivity distribution,and the EC50 mean value (8.66±1.09 μg/mL) of Z to 37 strains,which composed the normal sensitivity distribution could be used as the baseline-sensitivity of Phytophthora nicotianae var.nicotianae to B and Z.