Resistance Against the Damping-off Fungus Rhizoctonia solani Systemically Induced by the Plant-Growth-Promoting Rhizobacteria Pseudomonas aeruginosa (IE-6S+) and P. fluorescens (CHA0)

Bare-root-dip treatment or soil drench with Pseudomonas aeruginosa strain IE-6Sþ and P. fluorescens CHA0 significantly suppressed Rhizoctonia solani and the bacterial efficacy was more pronounced in the presence of low inoculum level of the fungus. The two bacteria afforded better protection to tomato roots against R. solani during early growth stages of the roots. There was a significant difference in the level of protection between root-dip treatment and soil drench with both the bacterial inoculants. The ability of the antagonistic bacteria to induce systemic resistance against R. solani was demonstrated in splitroot trials, where the root system of a single tomato plant was spatially divided into two separate parts. Root infection caused by R. solani was substantially lower in both bacterized and non-bacterized halves. However, the level of protection was greater in the bacteria-treated half compared with the bacteria-free half. Viable and dead cells of the two bacteria showed similar activity against R. solani, while the culture filtrate of P. fluorescens induced systemic resistance of a higher degree towards R. solani compared with P. aeruginosa. In general, populations of the two bacteria in the rhizosphere declined by tenfold at each harvest and colonization of P. aeruginosa was slightly greater compared with that by P. fluorescens. Initially (day 0), both the bacterial inoculants had almost a similar pattern of rhizosphere colonization, whereas at first (day 7) and second harvest (day 14) P. aeruginosa colonized more heavily than P. fluorescens. At final harvest (day 21), the two bacteria did not differ significantly in their colonization pattern. Whereas P. fluorescens specifically colonized the rhizosphere, P. aeruginosa also colonized the inner root tissues of tomato