The involvement of phenazines and cyclic lipopeptide sessilin in biocontrol of Rhizoctonia root rot on bean (Phaseolus vulgaris) by Pseudomonas sp CMR12a is influenced by substrate composition

Pseudomonas sp. CMR12a is an effective biocontrol agent that produces various antifungal metabolites including phenazine antibiotics and cyclic lipopeptides. In this study, we wanted to investigate the influence of substrate composition on the role of phenazines and the cyclic lipopeptide (CLP) sessilin in biocontrol of bean root rot caused by Rhizoctonia solani AG 2-2. Disease severity and spreading rate of R. solani were determined in three substrates, containing different ratios of potting soil and sand, inoculated with Pseudomonas sp. CMR12a or its mutants impaired in phenazine and/or sessilin biosynthesis. In the mixtures containing 50 or 75 % of potting soil, the presence of either phenazines or sessilin was sufficient to suppress bean root rot. However, in the mixture containing only 25 % of potting soil, the involvement of both compounds was required to provide better protection. The ratio between potting soil and sand also determined the spreading rate of Rhizoctonia hyphae. The mixture containing 75 % of potting soil was the most suppressive against R. solani invasion. The use of various potting soil/sand combinations affected the development of bean root rot disease but Pseudomonas sp. CMR12a effectively controlled the disease in all substrates. Depending on the disease pressure, however, the production of either phenazines or sessilin, or both compounds is required for effective biocontrol.