Effect of potting mix microbial carrying capacity on biological control ofRhizoetonia damping-off of radish and Rhizoctonia crown and root rot of Poinsettia

Potting mixes prepared with dark, highly decomposed Sphagnum peat, with lig ht, less decomposed Sphagnum peat, or with composted pine bark, all three o f which were colonized by indigenous microorganisms, failed to; consistentl y suppress Rhizoctonia damping-off of radish or Rhizoctonia crown and root rot of poinsettia. Inoculation of these mixes with Chryseobacterium gleum ( C299R2) and Trichoderma hamatum 382 (T-382) significantly reduced the sever ity of both diseases in the composted pine bark mix in which both biocontro l agents maintained high populations over 90 days. These microorganisms wer e less. effective against damping-off in the light and dark peat mixes, res pectively, in which populations of C299R2 declined. In contrast, crown and root rot, a disease that is severe late in the crop, was suppressed in all three types of mixes. High populations of T-382 in all three mixes late dur ing the cropping cycle may have contributed to control of this disease.