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