VIEW ARTICLE

Disease Control and Pest Management

Suppression of Phytophthora cinnamomi in a Composted Hardwood Bark Potting Medium. H. A. J. Hoitink, Associate Professor, Department of Plant Pathology, Ohio Agricultural Research and Development Center, Wooster, OH 44691; D. M. VanDoren, Jr.(2), and A. F. Schmitthenner(3). (2)Professor, Department of Agronomy, Ohio Agricultural Research and Development Center, Wooster, OH 44691; (3)Professor, Department of Plant Pathology, Ohio Agricultural Research and Development Center, Wooster, OH 44691. Phytopathology 67:561-565. Accepted for publication 9 November 1976. Copyright © 1977 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-67-561.

Concentrations of Phytophthora cinnamomi propagules that caused severe root rot of lupine seedlings in peat-sand potting media produced little damage to similar seedlings growing in hardwood bark-sand compost. The low disease level in the suppressive bark mix was not related to drainage since all media drained rapidly and the air-filled pore space at various negative pressures (7.5, 22.5, 50, and 100 millibars) was lower in bark compost than in the peat media. Mycelium of P. cinnamomi did not grow through any of the media, but a significant inhibition of sporangium production occurred in the bark compost. Inhibitors in leachates from fresh bark composts lysed zoospores and cysts. These inhibitors were not detected in leachates from bark compost in which rhododendrons had been grown for 2 yr. Therefore, the suppressive effect of the bark compost probably is due to chemical and biological rather than physical factors.