MICROBIAL RECOLONIZATION AND SUPPRESSION OF RHIZOCTONIA-SOLANI IN A BEDDING-PLANT POTTING MIX AMENDED WITH RECYCLED MIX BEFORE AERATED-STEAM TREATMENT

Some South Australian bedding-plant nurseries add recycled potting mix to fresh potting mix before aerated-steam treatment in an attempt to decrease fungal root diseases. To test the hypothesis that this addition of recycled potting mix affects microbial colonization and, consequently, the disease-suppressiveness of mixes treated with aerated steam, recycled potting mix was added to or withheld from fresh potting mix. The mixes were then treated with aerated steam at 60-degrees-C for 35 min or not treated. Their recolonization by actinomycetes, heat-resistant organisms, pseudomonads, and fungi was followed for 25 days. The addition of recycled potting mix frequently increased the numbers of actinomycetes and heat-resistant microbes recovered from the mixes, whether treated with steam or not, but decreased the numbers of aerobic bacteria. Fungal populations were rarely influenced by the addition of recycled potting mix. Pseudomonad populations dropped to undetectable levels after the aerated-steam treatment and failed to reach levels comparable to those in treatments without steam during he next 25 days. Organisms from each group were tested for in vitro antagonism to Rhizoctonia solani Kuhn (AG-8). The proportion of inhibitory organisms varied with the microbial group, the mix treatment, and the time since mix treatment. Where aerated-steam treatment was applied, the addition of recycled potting mix rarely affected the development of Rhizoctonia disease symptoms on seedlings of Celosia argentea. The potential for improving the suppressiveness of aerated-steam-treated nursery mixes via microbial inoculation is discussed.