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Influence of Pine Bark, Matric Potential, and pH on Sporangium Production by Phytophthora cinnamomi. D. M. Benson, Professor, Department of Plant Pathology, North Carolina State University, Raleigh 27695; Phytopathology 74:1359-1363. Accepted for publication July 17, 1984. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/Phyto-74-1359.

Sporangium production on mycelial mats of Phytophthora cinnamomi was compared in pine bark and in a Cecil clay soil at controlled matric potentials of 0 to - 150 mb. Optimum sporangium production occurred between - 15 and - 25 mb in both substrates. Sporangium production was markedly less at - 10 mb and at - 150 mb in both pine bark and soil. Matric potential, not water content of the medium, regulated sporangium production. Maximum numbers of sporangia were produced 1 day after mats of mycelium were placed in the tensiometers at a constant matric potential of - 25 mb. Incubation of mats for an additional 1- 3 days did not increase sporangium production. Pine bark at pH 3.7 suppressed sporangium production. At pH 3.5, no sporangia were observed on mats incubated at a constant matric potential of - 25 mb. Air-drying pine bark prior to saturation in Büchner funnel tensiometers completely suppressed sporangium production by P. cinnamomi.

Additional keywords: moisture retention curve, physical properties, summation curve.