Control of onion downy mildew in the presence of phenylamide-resistant strains of Peronospora destructor (Berk.) Caspary

Abstract

Two isolates of Peronospora destructor differed in their sensitivities to phenylamide fungicides. An isolate (3014) from the Lockyer Valley did not respond to increasing concentrations of metalaxyl, benalaxyl and oxadixyl between 0.1 and 200 mg a.i./L. An isolate (2967) from Bundaberg was sensitive to these fungicides, with EC50 values (concentrations which reduce disease severity by 50%) of 1.6, 6.0 and 12.1 mg a.i./L, respectively. Isolates 3014 and 2967 were both sensitive to dimethomorph, with EC50 values of 4.3 and 1.4 mg a.i./L, respectively. Field trials compared the efficacy of fungicides for the control of onion downy mildew in the Lockyer Valley in 1989 and 1990. In both trials, treatments containing mancozeb gave the best control, resulting in yield increases of 60% in 1990. There was no difference from the check treatment in either disease severity or yield when metalaxyl was applied, suggesting that phenylamide-resistant strains were prevalent. Other fungicide treatments which did not significantly reduce the severity of downy mildew in the 1990 trial included copper hydroxide, chlorothalonil, fosetyl-Al, and propamocarb. The experimental compound dimethomorph, although less effective than mancozeb, significantly reduced disease severity and increased yield by 24%.