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Effects of Fungal Antagonists and Compost Age on Suppression of Rhizoctonia Damping-Off in Container Media Amended with Composted Hardwood Bark. E. B. Nelson, Former graduate research associate, Department of Plant Pathology, Ohio Agricultural Research and Development Center, Ohio State University, Wooster 44691, Present address of senior author: Dept. Seed and Vegetable Sciences, New York State Agricultural Experiment Station, Cornell University, Geneva 14456; G. A. Kuter(2), and H. A. J. Hoitink(3). (2)(3)Research scientist, and professor, respectively, Department of Plant Pathology, Ohio Agricultural Research and Development Center, Ohio State University, Wooster 44691. Phytopathology 73:1457-1462. Accepted for publication 18 May 1983. Copyright 1983 The American Phytopathological Society. DOI: 10.1094/Phyto-73-1457.

A total of 331 fungal isolates from container media amended with composted hardwood bark (CHB) were tested for their ability to suppress Rhizoctonia damping-off. Members of the genera Trichoderma, Gliocladium, Penicillium, Mortierella, Paecilomyces, Geomyces, and Ophiostoma were among the most effective fungi. Isolates of Trichoderma hamatum and Trichoderma harzianum, the most abundant taxa isolated from suppressive container media, were generally the most efficaceous antagonists. However, isolates of Trichoderma and other genera obtained from conducive media were also capable of restoring suppression to heated media (60 C) amended with mature CHB. Induction of suppression by T. harzianum was influenced by the degree of decomposition (age) of the organic component in the container medium. In container media amended with mature CHB, the population levels of T. harzianum increased from 102 colony-forming units (CFU)/g (dry weight) to 104- 106 CFU/g (dry weight) after 14 days and high levels of disease suppression were observed. However, similar inoculum levels did not induce suppression in a Canadian peat medium or in media amended with fresh or green CHB even though T. harzianum reached high population levels in these media. It was concluded that the production of container media that were consistently suppressive to R. solani required not only the addition of antagonists, but also the introduction of the antagonist into an environment that favored antagonistic activity.