Combining potential oomycete and bacterial biocontrol agents as a tool to fight tomato Rhizoctonia root rot

[Display omitted] •P. oligandrum associated with 3 bacteria suppressed tomato Rhizoctonia root rot.•The microbial association was more effective than the chemical fungicide tested.•The microbial association enhanced the growth of tomato plants.•Native rhizospheric bacterial communities depend on the tomato cultivar (SSCP). Biological control of Rhizoctonia root rot and growth-promoting potential on two tomato cultivars was performed using Pythium oligandrum in combination, or not, with three tomato-associated rhizobacterial strains of Bacillus subtilis, B. thuringiensis and Enterobacter cloacae. The three bacterial strains displayed antifungal activity against Rhizoctonia solani, with pathogen growth inhibition up to 60%. P. oligandrum also destroyed R. solani cells by antibiosis and mycoparasitism processes. In a three-month greenhouse assays, for all trials and the two tomato cultivars used, disease suppression potential, as compared to the controls, was up to 80% using the microorganisms mixtures and 74% by using P. oligandrum alone. Increased height in disease-free plants was obtained with the microorganism mixtures (59%) compared to P. oligandrum (49%). Plants grown in R. solani inoculated peat and challenged with microorganism mixtures were higher than controls, whereas those amended with P. oligandrum showed 46 to 87% height increase. All treatments with the microorganisms were more effective in suppressing the disease than chemical fungicide. Increment of aerial parts and root fresh weights on disease-free plants were until 51% for those treated with microorganism mixtures compared to 46% recorded using P. oligandrum. On inoculated plants, these parameters were enhanced using the mixed treatment. No differences in the rhizosphere-bacterial populations were observed by Single Strand Conformational Polymorphism (SSCP) when, either the rhizobacteria, with or without P. oligandrum, or the pathogen, were inoculated. This suggests that the native bacterial communities were not significantly impacted by the various microbial-inoculations. Only differences were obtained for the two tomato cultivars that the native bacterial microflora colonize.