Reappearance and Control of Onion Downy Mildew Epidemics in New York. R. W. Smith, Graduate Research Assistant, Department of Plant Pathology, New York State College of Agriculture and Life Sciences, Cornell University, Ithaca 14853. J. W. Lorbeer, Professor, and A. A. Abd-Elrazik, Visiting Professor, Department of Plant Pathology, New York State College of Agriculture and Life Sciences, Cornell University, Ithaca 14853. Plant Dis. 69:703-706. Accepted for publication 2 January 1985. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/PD-69-703.

Epidemics of downy mildew of onion, caused by Peronospora destructor, severely damaged commercial onion fields in western and central New York during the 1977–1979 growing seasons. On greenhouse-grown plants, when applied to onion leaves 24 hr before inoculation with P. destructor, chlorothalonil and mancozeb at rates equivalent to 1.07 L/ha (1.5 pt/acre) and 3.54 kg/ha (1 lb/acre) of formulated material, respectively, completely prevented infection. Greenhouse-grown plants were less susceptible to infection by P. destructor than outdoor-grown plants. Chlorothalonil and mancozeb at formulated rates equivalent to 3.08 L/ha (4.25 pt/acre) and 7.08 kg/ha (2 lb/acre), respectively, differed greatly in their residual effectiveness against infection of outdoor-grown onion plants. Mancozeb at that relatively low rate provided superior control over chlorothalonil at that relatively high rate when applied 2, 4, 7, or 10 days before inoculation. During 1980, onion growers in western and central New York generally used mancozeb to control downy mildew, and the incidence and severity of the disease was greatly reduced. From 1981 to 1984, incidence was nil when mancozeb was used.