SOIL BACTERIA SELECTED FOR SUPPRESSION OF RHIZOCTONIA-SOLANI, AND GROWTH PROMOTION, IN BEDDING PLANTS

Over 5000 bacterial isolates were obtained by dilution plating of potting media that were suppressive to damping-off disease caused by Rhizoctonia solani AG 8 in seedlings of Celosia argentea. These isolates were screened for inhibition of AG 8 in vitro, then 91 selected isolates were screened further for their ability to suppress the effects of R. solani AG 8 in seedlings of Capsicum annuum and Celosia argentea grown in pasteurized potting medium in a glasshouse. Fourteen isolates selected from the above screenings then were screened similarly for suppression of damping-off caused by R. solani AG 4 in seedlings of Capsicum, Celosia, Petunia and Viola. Three isolates reduced disease more consistently than the others. B. subtilis A13, an isolate previously reported to control R. solani in bedding plants, did not reduce damping-off in any host plant species. These three isolates were compared in two experiments over a range of doses for growth promotion, and suppression of AG 4, in Capsicum. Although there were few dose responses, when data for all dose rates were combined, all isolates increased dry weights of Capsicum shoots in the absence of R. solani. In one of the experiments, all but one isolate also increased seedling survival in the absence of R. solani. Two bacterial isolates and the fungicide quintozene reduced damping-off caused by AG 4. The selected bacterial isolates exhibit potential for biological suppression of damping-off in bedding plants in nursery potting media.