Comparative Efficacy of Three Tomato-Associated Rhizobacteria used Singly or in Combination in Suppressing Rhizoctonia Root Rot and Enhancing Tomato Growth

Abstract

Three indigenous tomato-associated rhizobacteria strains -Bacillus subtilis str. B2 KT921327, B. thuringiensis str. B10 KU158884 and Enterobacter cloacae str. B16 KT921429 - were tested singly and in combination as substrate drench for Rhizoctonia Root Root suppression and plant growth promotion on two tomato cultivars during two cropping seasons. All bacteria-based treatments were found to be more effective in suppressing disease than the fungicide on both cultivars and in both cropping seasons. The disease-suppression and growth-promotion abilities of the treatments tested varied significantly depending on pathogen presence or absence, bacterial strains, tomato cultivars and cropping years. Overall, for all trials and cultivars combined, disease suppression potential, as compared to the untreated controls, ranged between 74.72 and 83.94% using three-strain mixture relative to 60.46-85.01% achieved using single strains. Height increment in disease free plants achieved with mixtures varied between 17.02 and 45.69% compared to 7.55 and 44.76% noted using single strains. Plants grown in R. solaniinoculated peat and challenged with three-strain mixture were 49.46 to 76.74% higher than controls whereas those grown in peat amended with single strains showed 42.28-83.58% height increase. Increment of aerial parts and root fresh weights on disease free plants were 42.31-78.09% and 45.03-91.21% for plants treated with mixture compared to 33.70-82.48% and 20.52-92.39% recorded using strains singly, respectively. On inoculated plants, these parameters were enhanced by 61.2-95.44% and 59.13-98.5% using mixed treatment and by 48.41-97.02% and by 51.5-99.05%, respectively, using single-train-based treatments. Analysis of the microbial populations revealed no differences between Single Strand Conformational Polymorphism (SSCP) profiles when neither the rhizobacteriabased treatment nor the pathogen inoculation was considered. The microbial communities differed only depending on cultivars grown.