Metalaxyl sensitivity of Phytophthora spp. associated with red raspberry in British Columbia

Root rot and wilting complex (RRWC), caused by Phytophthora spp., is the most devastating disease of red raspberry in British Columbia (BC). Metalaxyl is the most commonly used fungicide to manage the disease in the province. P. rubi and P. gonapodyides were isolated from red raspberry samples showing symptoms of RRWC. The metalaxyl sensitivity of strains of Phytophthora spp. (12 strains of P. rubi and six strains of P. gonapodyides) isolated from diverse raspberry farms of BC were evaluated in vitro using the radial growth method. Metalaxyl was amended to 20% clarified V8 agar to yield final concentrations of 0, 0.001, 0.01, 0.1, 1, 10 μg mL−1. Each strain was tested in triplicate for treatments and experiments were repeated twice independently. Fifty percent effective concentration (EC50) values and dose response curves were determined for each strain. Results showed that ~ 83% of P. rubi and 100% of P. gonapodyides strains were highly sensitive to metalaxyl with mean EC50 values of 0.0576 (range from 0.0375 to 0.0738) for P. rubi and 0.0542 (range from 0.0244 to 0.0758) for P. gonapodyides. A detailed sensitivity study of a large number of Phytophthora isolates is in progress.