Response of Phytophthora parasitica var. nicotianae to Metalaxyl Exposure. H. D. Shew, Assistant Professor, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616. Plant Dis. 69:559-562. Accepted for publication 17 November 1984. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/PD-69-559.

Isolates of Phytophthora parasitica var. nicotianae were collected from seven flue-cured tobacco fields before initial metalaxyl use and after one, two, or three consecutive years of use for control of tobacco black shank. Eight hundred seventy-seven isolates were screened for inhibition of mycelial growth on 5% V-8 juice agar amended with 0.1, 1, 10, or 100 μg of metalaxyl per milliliter of medium. Inhibition increased as fungicide concentration increased; however, at low fungicide concentrations, the range in inhibition was less for isolates collected after exposure and the percentage of isolates inhibited at the 0.1- and 1-μg a.i./ml concentrations of the fungicide decreased after 1–3 yr of exposure. The change in sensitivity resulted in mean ED₅₀ values of 0.4, 0.3, 0.7, and 1.2 μg/ml for isolates exposed to metalaxyl for 0, 1, 2, or 3 yr, respectively. An in vivo assay that measured mean inhibition of root infection on a susceptible tobacco cultivar gave results similar to those obtained in the in vitro test for inhibition of mycelial growth. Continuous use of metalaxyl in soil enhanced selection of P. parasitica var. nicotianae populations less sensitive to metalaxyl, and thus periodic removal of this selection pressure through crop or fungicide rotation should be considered.