The Role of Microorganisms in the Suppression ofRhizoctonia solaniin Container Media Amended with Composted Hardwood Bark

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Abstract
Suppression of Rhizoctonia damping-off in container media amended with mature (> 11-wk-old) composted hardwood bark (CHB) was eliminated by heat (60.degree. C) and .gamma. radiation (275 krad). Media amended with green (< 11-wk-old) CHB were only slightly suppressive and heating had no effect on the resulting incidence of damping-off. Suppression could be reestablished in heated mature CHB media by adding 10% (vol/vol) unheated mature CHB or 107 colony-forming units (CFU) of Trichoderma harzianum per gram of container medium. Propagules of Rhizoctonia solani were killed rapidly in unheated media containing mature CHB and in heated mature CHB media amended with T. harzianum, but a high percentage of propagules remained viable in media amended with either green or heat-treated mature CHB. Results suggest that suppression of Rhizoctonia damping-off of radish in container media amended with mature CHB is induced by microbial activity, whereas the low level of suppression observed in green CHB-amended media may involve chemical inhibitors.