Resistance to Metalaxyl in Isolates ofPseudoperonospora cubensis,the Downy Mildew Pathogen of Cucurbits

Abstract

Metalaxyl-resistant P. cubensis was isolated from infected cucumber [Cucumis sativus cv. Delila] 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 .mu.g a.i.[active ingredient]/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.