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Occurrence and Incidence of Metalaxyl Resistance in Pseudoperonospora humuli in the Pacific Northwest. R E. KLEIN, Assistant Plant Pathologist, Department of Plant Pathology, Washington State University—Prosser, Prosser 99350. Plant Dis. 78:161-163. Accepted for publication 6 October 1993. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/PD-78-0161.

Basal spikes, systemically infected with Pseudoperonospora humuli, were collected from hop (Humulus lupulus) plants in Oregon, Washington, and Idaho in 1992. Zoosporangial suspensions from each spike were tested for metalaxyl resistance in a floating leaf disk assay. Washington samples had an average EC-50 (the metalaxyl concentration which reduced the incidence of sporulation by 50%) of 0.15 ?g/ml, while the average for Oregon samples was 74 jig/ml. None of the zoosporangial suspensions from 50 Washington basal spikes were metalaxyl resistant at 25 ?g/nil metalaxyl, while six of 10 Idaho and 81 of 94 Oregon suspensions exhibited resistance. This is the first report of metalaxyl resistance in P. humuli in North America.