Abstract:In order to investigate the resistance level and geographical distribution of resistant isolates in Phytophthora capsici to metalaxyl in Anhui, about 200 samples of pepper Phytophthora blight were collected in major vegetable growing areas among 16 cities and counties of the province during 2005-2006, and 125 strains of P.capsici were isolated and purified from these samples. The sensitivity of these strains to metalaxyl was determined by mycelial growth rate method in vitro and biological test in vivo, respectively. The results showed that 79 isolates were sensitive to metalaxyl (<40% growth at 5.0μg/mL of metalaxyl), 38 isolates intermediate in sensitivity to metalaxyl (>40% growth at 5.0μg/mL of metalaxyl and <40% growth at 100μg/mL of metalaxyl), and 8 isolates resistant to metalaxyl (>40% growth at 100μg/mL of metalaxyl) according to Parra & Ristaino’s (2001) criterion. The proportion of metalaxyl-sensitive isolates (MS), metalaxyl-intermediate isolates (MI) and metalaxyl-resistant isolates (MR) covered 63.2%, 30.4% and 6.4% in P.capsici isolate population, respectively. There was also a significant difference in the sensitivity of the strains from different areas to metalaxyl. The strains from Shouxian were the most sensitive, with the minimum and mean EC50 value being 0.10μg/mL and 0.81±0.20μg/mL, respectively, while the strains from Dangshan and Hexian represented the resistance, with the mean EC50 value being more than 8.32±0.19μg/mL, and the maximum 25.47μg/mL, which suggested the highest resistant level was about 90 folds.