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Sampling Citrus Orchards in Florida to Estimate Populations of Phytophthora parasitica. L. W. Timmer, Professor, University of Florida, Institute of Food and Agricultural Sciences, Citrus Research and Education Center, 700 Experiment Station Road, Lake Alfred 33850; H. A. Sandler, J. H. Graham, and S. E. Zitko. Assistant in plant pathology, Associate professor, and Biological scientist II, respectively, University of Florida, Institute of Food and Agricultural Sciences, Citrus Research and Education Center, 700 Experiment Station Road, Lake Alfred 33850. Phytopathology 78:940-944. Accepted for publication 9 February 1988. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/Phyto-78-940.

Soil sample collection and soil handling methods were evaluated and the number of samples needed for reliable estimation of populations of Phytophthora parasitica were determined for Florida citrus orchards. Propagule densities of P. parasitica in field soil measured by assay on a selective medium were consistent for up to 8 days after collection if soil was maintained moist at 2124 C, but they were reduced drastically by air drying. Assay of soils removed from storage at 8 C or collected from the field in late winter yielded low propagule densities, but incubation of soils 1 to 2 days at room temperature maximized recovery of the fungus. Collection of soil samples beneath the tree canopy within the drip line gave consistent recovery of P. parasitica. Propagule densities were lower and more erratic outside of the canopy, and root and fungal propagule densities diminished sharply with increasing depth. Mean propagule densities in 100 single-core samples from each of four 4-ha blocks of trees were 11.2, 0.8, 2.0, and 4.7, and frequency distributions best fit a negative binomial distribution (chi squares of 20.1, 3.88, 14.71, and 9.89 and k values of 0.93, 0.14, 0.12, and 0.62, respectively). The number of single core per tree samples needed to estimate the population with a standard error-to-mean ratio of 0.25 varied from 19 for sites with 11.2 propagules/cm3 to 149 for sites with 2.0 propagules/cm3. In an experiment to evaluate among-tree and among-sample variability, the number of trees that needed to be sampled varied from 23 when one core per tree was collected to five trees when 10 cores per tree were collected. Thus, if populations are moderate to high (above 5 propagules/cm3) and about 2040 samples per 4-ha block are collected from the surface soil beneath tree canopies in an orchard and processed as indicated above, populations can be estimated reliably.