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Spatial Patterns of Verticillium dahliae Propagules in Potato Field Soils of Oregonís Columbia Basin. K. B. Johnson, Department of Botany and Plant Pathology, Oregon State University, Corvallis 97331. J. D. Apple, and M. L. Powelson. Department of Botany and Plant Pathology, Oregon State University, Corvallis 97331. . Plant Dis. 72:484-488. Accepted for publication 21 January 1988. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/PD-72-0484.

Frequency distributions of propagules of Verticillium dahliae were determined in three potato fields. Eighty or 100 soil cores were analyzed from each of eight 10 ◊ 10 m sites. Taylorís power law, an empirical measure of aggregation, indicated clumping of propagules when it was applied to the site means and variances (Taylorís b = 2.51, R2 = 0.87). The variance-to-mean ratio and Lloydís index of patchiness also indicated aggregation of propagules at each site. The Poisson distribution model did not fit the observed population frequencies, and conclusions about aggregation derived from the negative binomial distribution model provided little additional information beyond that obtained from the other indices. Moranís I statistic indicated a random pattern of spatial autocorrelation at each site. For all sites, means and variances of the samples were greatly influenced by a small number of extremely high propagule counts. The relative precision of confidence intervals for both the mean and the median were compared for each site. In some situations, the median may be a better estimator of central tendency than the mean. Two subsamples per soil core adequately described the number of propagules in a single soil sample.

Keyword(s): index of dispersion, potato early dying disease.