Seasonal and Spatial Analysis of Populations of Phytophthora parasitica in Citrus Orchards in Florida. L. W. Timmer, Professor, University of Florida, IFAS, Citrus Research and Education Center, 700 Experiment Station Road, Lake Alfred 33850. S. E. Zitko, H. A. Sandler, and J. H. Graham. Biological Scientist II, Assistant in Plant Pathology, and Associate Professor, University of Florida, IFAS, Citrus Research and Education Center, 700 Experiment Station Road, Lake Alfred 33850. Plant Dis. 73:810-813. Accepted for publication 3 May 1989. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/PD-73-0810.

Populations of Phytophthora parasitica were determined five times per year for a 3-yr period in four citrus orchards. Season of the year, soil temperature, and soil moisture at sampling did not greatly affect measured propagule densities. Horizontal spatial pattern of propagules of P. parasitica in each of eight orchards was determined in 49 three × three tree quadrats. Means varied from 0.6 to 16.3 propagules/cm³, and frequency counts data for populations were best described by a negative binomial probability distribution. Variance-to-mean ratios varied from 2.5 to 12.5, k values from the negative binomial distribution varied from 0.11 to 6.97, and Lloyd’s index of patchiness varied from 1.12 to 20.17. All indices of dispersion indicated varying degrees of aggregation of propagules. The greatest aggregation was observed in orchards where mean propagule densities were lowest. However, spatial lag correlation analysis did not detect clusters with similar populations in most of the orchards. Sampling of citrus orchards to measure propagule densities can be conducted without regard to time of year or within-orchard location of sample collection, but populations may change from year to year.