Response ofPhytophthora parasiticavar.nicotianaeto Metalaxyl Exposure

Publisher Website
Google Scholar
Abstract
Isolates of P. parasitica var. nicotianae were collected from 7 flue-cured tobacco fields before initial metalaxyl use and after 1, 2 or 3 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 .mu.g of metalaxyl/ml 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-.mu.g a.i. [active ingredient]/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 .mu.g/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 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.