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Competition Between Metalaxyl-Resistant and -Sensitive Strains of Pseudoperonospora cubensis on Cucumber Plants. Yigal Cohen, Department of Life Sciences, Bar-Ilan University, Ramat-Gan 52100, Israel; Moshe Reuveni(2), and Yair Samoucha(3). (2)(3)Department of Life Sciences, Bar-Ilan University, Ramat-Gan 52100, Israel. Phytopathology 73:1516-1520. Accepted for publication 17 May 1983. Copyright 1983 The American Phytopathological Society. DOI: 10.1094/Phyto-73-1516.

The infectivity of two pairs of metalaxyl-resistant and -sensitive strains of Pseudoperonospora cubensis was compared in the absence of metalaxyl selection pressure by single strain inoculations, passing mixed (1:1, 1:6, and 1:20 resistant:sensitive in initial sporangial suspension inoculum) strains through two inoculation cycles on intact cucumber cotyledons in growth chambers, and exposing cucumbers growing in plastic houses to both strains (at a ratio of 1:1 or 1:4). With single strain inoculation, strains were equally infective to cucumbers. In mixed strain inoculations in growth chambers, after one sporulation cycle (two successive inoculations) the original 1:1, 1:6, and 1:20 ratio of resistant:sensitive components changed essentially to 1:0. In mixed strain inoculations in plastic houses, the original 1:1 and 1:4 ratio (resistant:sensitive) changed essentially to 1:0 after 12- 17 and 23 days, respectively. The results indicate that, with the technique used to monitor strain frequency, the two metalaxyl-resistant strains of the fungus compete favorably with the two metalaxyl-sensitive strains in the absence of the fungicide.

Additional keywords: Cucumis sativus.