In Vitro Growth Response of Phytophthora parasitica var. nicotianae Isolates to Metalaxyl. H. D. Shew, Assistant Professor, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616. Plant Dis. 68:764-766. Accepted for publication 18 May 1984. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/PD-68-764.

Three hundred seventeen isolates of Phytophthora parasitica var. nicotianae were screened in vitro for sensitivity to metalaxyl. Sensitivity to metalaxyl was decreased by increasing the concentration of nutrients in the basal medium. Growth decreased linearly with metalaxyl concentrations from 0.1 to 10 μg/ml for the isolates screened resulting in a mean ED₅₀ of 0.4 μg/ml. Isolates from six flue-cured tobacco fields varied greatly in their response to metalaxyl within as well as among fields. The range of mean inhibition in the six fields was 08–57, 46–85, 89–95, and 85–96% at 0.1, 1, 10, and 100 μg/ml of metalaxyl, respectively.