Resistance to acylalanines in Pseudoperonospora cubensis1

Resistance to acylalanine fungicides in Pseudoperonospora cubensis, the downy mildew of cucurbits, is reviewed. Insensitive strains of the fungus predominated during the second year of metalaxyl application in cucumber glasshouses in Greece and Israel. Resistance was detected either on detached cucumber leaves floated on metalaxyl solutions or on potted plants treated (sprayed or drenched) with this fungicide. Metalaxyl-resistant strains of P. cubensis showed good pathogenicity and fitness, and competed favorably with the sensitive strains in the absence of the fungicide. A synergism between these two biotypes resulted in an increased virulence of the resistant forms on metalaxyl-treated plants. Metalaxyl-resistant strains of the pathogen exhibited cross resistance to other acylalanine fungicides. Strains resistant to acylalanines and to the chemically unrelated oomycete-fungicides fosetyl-Al and propamocarb were isolated in Israel, as well. In Greece, however, protective sprays with fosetyl-Al, cymoxanil and mancozeb gave good control of cucumber downy mildew when the pathogen was resistant to metalaxyl. P. cubensis is the first case of fungus which developed resistance to acylalanines in the field.