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Relationship of Matric Water Potential and Air-filled Porosity of Container Media to Development of Phytophthora Root Rot of Rhododendron. B. H. Ownley, Former graduate research assistant, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616, Current address: U.S. Department of Agriculture, Agriculture Research Service, Root Disease and Biological Control Research Unit, Pullman, WA 99164-6430; D. M. Benson, professor, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616. Phytopathology 81:936-941. Accepted for publication 25 March 1991. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/Phyto-81-936.

Matric water potential and air-filled porosity were determined at four sampling dates during the growing season for trickle-irrigated pine bark and peat-based media that differed in composition and particle size distribution. At low and high inoculum densities (nine and 30 infested oat grains per container) of Phytophthora cinnamomi, severity of Phytophthora root rot of Rhododendron sp. was significantly less in pine bark media than in peat/sand/soil. Differences in root rot severity between the pine bark media and the peat/sand/soil medium were best explained by differences in air-filled porosity and matric water potential among media. Matric water potential generally fluctuated from 1.0 to 3.0 kPa between irrigations in the pine bark media and from 0 to 1.0 kPa between irrigations in the peat/sand/soil medium. Pine bark media had volumetric air-filled porosities that fluctuated from about 40% just after irrigation to about 45% 48 h later, whereas peat/sand/soil had an air-filled porosity near 10% after irrigation and about 20% 48 h later. Irrigation water ponded on the surface of containers with peat/sand/soil immediately after trickle irrigation. This was not observed in containers with pine bark media. Relatively large pores that provide high air-filled porosity in pine bark media account for the rapidity with which pine bark media drain after irrigation, which resulted in matric potentials unfavorable for zoospore discharge and dispersal.

Additional keywords: moisture retention curve.