Resistance to Metalaxyl in Isolates of Pseudoperonospora cubensis, the Downy Mildew Pathogen of Cucurbits. Talma Katan, Division of Plant Pathology, Agricultural Research Organization, The Volcani Center, Bet Dagan 50 250, Israel. Esther Bashi, Division of Plant Pathology, Agricultural Research Organization, The Volcani Center, Bet Dagan 50 250, Israel. Plant Dis. 65:798-800. Accepted for publication 27 January 1981. Copyright 1981 American Phytopathological Society. DOI: 10.1094/PD-65-798.

Metalaxyl-resistant Pseudoperonospora cubensis was isolated from infected cucumber leaves in a greenhouse in Israel where metalaxyl failed to control downy mildew. The resistant isolate caused typical infections on cucumber plants sprayed with metalaxyl (up to 500 µg a.i./ml) 2 days before or 2 days after inoculation. Disease symptoms and severity on sprayed plants were similar to those on unsprayed inoculated control plants. The resistance was stable through 10 successive inoculations in the absence of metalaxyl and did not affect pathogenicity.

Keyword(s): acylalanine fungicides, tolerance.