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Disease Control and Pest Management

Occurrence of Metalaxyl-Resistant Isolates of Phytophthora infestans in Potato Fields in Israel. Yigal Cohen, Professor, Department of Life Sciences, Bar-Ilan University, Ramat-Gan 52100, Israel; Moshe Reuveni, postgraduate student, Department of Life Sciences, Bar-Ilan University, Ramat-Gan 52100, Israel. Phytopathology 73:925-927. Accepted for publication 2 December 1982. Copyright 1983 The American Phytopathological Society. DOI: 10.1094/Phyto-73-925.

Isolates of Phytophthora infestans resistant to metalaxyl were collected from potato fields in the spring of 1982. An isolate collected 27 April from Nahal-Oz (northwestern Negev) was infective on potato plants drenched with up to 0.5 mg a.i. of metalaxyl per plant, whereas isolates collected from the same location at 2 wk or later were infective on plants drenched with up to 5 mg a.i. of metalaxyl per plant. Disease induced by nine metalaxyl-sensitive isolates of the pathogen collected during 1977- 1981 was controlled in plants drenched with 0.05 mg a.i. of metalaxyl per plant. Growth chamber studies showed that after four infection cycles, conducted on plants treated with increasing dosages of the fungicide, the isolate collected on 27 April was infective on plants drenched with 15 mg a.i. per plant.