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Synergy Between Metalaxyl-Sensitive and Metalaxyl-Resistant Strains of Pseudoperonospora cubensis. Yair Samoucha, Graduate student, Department of Life Sciences, Bar-Ilan University, Ramat-Gan 52100, Israel; Yigal Cohen, professor, Department of Life Sciences, Bar-Ilan University, Ramat-Gan 52100, Israel. Phytopathology 74:376-378. Accepted for publication 16 November 1983. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/Phyto-74-376.

Metalaxyl-sensitive (MS) strains of Pseudoperonospora cubensis increased the infectivity of metalaxyl-resistant (MR) strains when mixtures of the two were inoculated into cucumber plants treated with 5 mg a.i. metalaxyl per pot (0.4 kg of air-dried soil). The synergy factor ([percent infection with MR + MS]/[percent infection with MR + H2O]) in various experiments ranged from 3 to 6. Synergy was also observed when inoculation with the sensitive strain preceded that of the resistant strain. This phenomenon may have enhanced the spread of resistant strains of the fungus in commercial plastic greenhouses in Israel from 1978 to 1980.