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Control of Hop Downy Mildew with Systemic Fungicides. R. M. Hunger, Graduate Research Assistant, Department of Botany and Plant Pathology, Oregon State University, Corvallis 97331. C. E. Horner, Professor, Department of Botany and Plant Pathology, Oregon State University, Corvallis 97331. Plant Dis. 66:1157-1159. Accepted for publication 25 March 1982. Copyright 1982 American Phytopathological Society. DOI: 10.1094/PD-66-1157.

Four systemic fungicides (metalaxyl, M 9834, propamocarb, and efosite aluminum) were evaluated for control of the systemic infection phase of hop downy mildew (Pseudoperonospora humuli) during 19791981. Metalaxyl applied to the soil surface over the perennial hop crown area in April resulted in nearly complete control and increased yields significantly. Treatment with M 9834 resulted in control comparable to metalaxyl up to 4 wk after application, but disease incidence increased sharply after 6 wk. Propamocarb and efosite aluminum were less effective than metalaxyl or M 9834 but gave substantial disease control. The possibility of using metalaxyl and a broad-spectrum fungicide in alternate years to avoid selection of fungal strains tolerant to metalaxyl is discussed.

Keyword(s): fungicide resistance, fungicide tolerance.