A comparison between antagonistic activity of Phenazine antibiotic producing and non-producing strains of Pseudomonas aureofaciens against Rhizoctonia solani, the causal agent of cotton seedling damping-off disease

Due to the importance of biological control of plant diseases, testing and introducing new biocontrol-active microorganisms is a major concern among plant pathologists. In this regard, we tried to investigate the antagonistic activities of Pseudomonas aureofaciens )chlororaphis( 30-84 (phenazine producing wild type and non-phenazine producing mutant) strains on Rhizoctonia solani, the causal agent of cotton seedling damping-off disease in comparison with some isolates of P. fluorescent under both In vitro (laboratory) and In vivo (greenhouse) conditions. In the laboratory experiment, inhibitory effects of all bacteria on the growth of R. solani were evaluated using dual culture procedure. Results showed that five isolates of P. fluorescent along with both strains of P. aureofaciens inhibited the growth of R. solani significantly. Effective bacterial antagonists were then evaluated in a greenhouse experiment where cotton seeds were coated with their suspensions and were sown in the pasteurized field soil pre-inoculated with a virulent isolate of R. solani. Efficacy of bacterial antagonists was evaluated by counting the number of survived seedlings in different treatments 15 and 60 days after sowing for determining pre- and post emergence damping-off incidence. According to the results of greenhouse experiment, at both intervals two isolates of P. fluorescent along with both strains of P. aureofaciens caused significant increases in the number of healthy seedlings in comparison with untreated control and commonly used fungicide (carboxin-thiram). The efficacy of phenazine producing wild type strsin of P. auerofaciens was higher than its non-phenazine producing mutant indicating that phenazine plays an important role in the antagonistic activity of P. auerofaciens. Effective bacterial antagonists were then studied for their antagonistic mechanisms. Results showed that all four bacteria employed different mechanisms including siderophore, volatile metabolites and non-volatile metabolites production in their antagonistic activities. The results of this study suggest that P. auerofaciens may be a new biocontrol agent for controlling cotton seedling mortality disease.