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Qualitative and Quantitative Soil Assays for Phytophthora cinnamomi. H. D. Shew, Graduate research assistant, Department of Plant Pathology, North Carolina State University, Raleigh 27650; D. M. Benson, associate professor, Department of Plant Pathology, North Carolina State University, Raleigh 27650. Phytopathology 72:1029-1032. Accepted for publication 14 January 1982. Copyright 1982 The American Phytopathological Society. DOI: 10.1094/Phyto-72-1029.

Leaf disks of azalea and hybrid rhododendron and 4-wk-old seedlings of Fraser fir used as baits in a qualitative assay provided better recovery of Phytophthora cinnamomi from soil than did blue lupine radicles and Deodar cedar needles. A quantitative soil assay utilizing a wet-sieving technique and a selective agar medium (PCH) was developed for enumeration of population densities of P. cinnamomi in naturally and artificially infested soil. The selective PCH (pimaricin, chloramphenicol, and hymexazol) medium improved recovery of P. cinnamomi from soil in comparison to other selective media tested. A modified McCains wet-sieving procedure was used for assaying soil samples up to 50 g; samples up to 200 g were assayed by using a semiautomatic elutriator. P. cinnamomi colonies were macroscopically identifiable on PCH after 4872 hr of incubation at 20 C in the dark. In assays of naturally infested soils, 87% of the colonies originated from chlamydospores free in the soil, 7% from pieces of colonized organic matter, and 6% from unidentified sources. Many chlamydospores from naturally infested soil had thick walls (mean thickness of 1.75 μm and a range of <1.0 to 2.6 μm).