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Spatial Pattern of Downy Mildew in Hop Yards. Dennis A. Johnson, Associate and extension plant pathologist, Washington State University Irrigated Agriculture Research and Extension Center, Prosser 99350; J. Richard Alldredge, and Rochelle Fisher. Associate professor and technical assistant, Program in Statistics, Washington State University, Pullman 99164. Phytopathology 78:1389-1395. Accepted for publication 5 May 1988. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/Phyto-78-1389.

Hop plants in hills in six hop yards in the Yakima Valley of Washington were assessed visually to determine the spatial and temporal pattern of hop downy mildew caused by Pseudoperonospora humuli. Hop shoots infected systemically with downy mildew were aggregated within the hill. Patterns of distribution were best described by the Negative Binomial distribution, and values of variance-to-mean ratio and Morisita’s index were greater than one. Small clumps of hills with downy mildew were identified with doublet analysis in all yards. Evidence of clumps of diseased hills seemed to depend on the time since the yard was established. The youngest yards exhibited more aggregation of diseased hills than the older yards. Disease-incidence clumps consisting of a large number of hills were not found in any of the yards. There was no evidence for an increase in aggregation in the direction of the prevailing winds.