Interactions Between Thermophilic Fungi andTrichoderma hamatumin Suppression of Rhizoctonia Damping-Off in a Bark Compost-Amended Container Medium

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Abstract
The ability of Trichoderma hamatum to induce suppression of Rhizoctonia damping-off in a container medium prepared with mature, composted, hardwood tree bark removed from various temperature zones in compost piles varied significantly. Efficacy of the biocontrol agent in medium amended with compost from a 40-50 C zone was reduced significantly as compared with that of the medium containing higher-temperature composts. Population densities of T. hamatum or Rhizoctonia solani did not differ in the medium prepared with compost from the various temperature zones. Population densities of thermophilic bacteria and fungi were significantly higher in the 40-50 C compost. Humicola spp. were the predominant fungal taxa isolated from compost in this temperature zone, but they were unable to grow on potato-dextrose agar (PDA) at 25 C. Some isolates of Humicola significantly reduced efficacy but not population densities of T. hamatum in the medium. Bacteria isolated from the 40-50 C zone did not reduce efficacy of the biocontrol agent. Damping-off was suppressed by T. hamatum if the medium containing 40-50 C compost or compost fortified with Humicola spp. was incubated for three or more weeks at 25 C before bioassays were performed. We propose that some thermophilic fungi naturally present in 40-50 C compost temporarily interfered with the biocontrol activity of T. hamatum after compost-amended medium was first prepared.