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Spatial Pattern of Downy Mildew in Hop Yards During Severe and Mild Disease Epidemics. Dennis A. Johnson, Plant pathologist, Washington State University Irrigated Agriculture Research and Extension Center, Prosser 99350; J. Richard Alldredge, Jennifer R. Allen, and Rochelle Allwine. Associate professor and technical assistants, respectively, Program in Statistics, Washington State University, Pullman 99164. Phytopathology 81:1369-1374. Accepted for publication 19 July 1991. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/Phyto-81-1369.

Hop plants in hills in three or four hop yards were assessed visually during severe and mild epidemics of downy mildew in the Yakima Valley of Washington. Several methods of analysis were used to investigate the spatial pattern of hop plants systemically infected with Pseudoperonospora humuli during the two types of epidemics. When disease incidences were low during the mild epidemics and early in the development of the severe epidemics, distributions associated with aggregation of disease provided a good representation of the frequency distribution of systemically infected shoots. A frequency distribution that represented data at high disease incidences was not found. Aggregation of disease within hop hills was indicated by large variance-to-mean ratios and Morisita’s index during both types of epidemics. Disease among hills became more aggregated as disease incidence increased as shown by doublet and runs analyses. Evidence of aggregation between nearby hills was shown by the Greig-Smith index and by semi-variograms when incidence of disease was high during the severe epidemics. Spatial association between contiguous hills as well as more widely separated hills was not direction dependent. The statistical methods used were in general agreement and provided useful information about spatial structure at several spatial scales. There was an association between disease occurrence in one year and condition of the same hills in the next year.