Suppressiveness of root-knot nematodes mediated by rhizobacteria

Plant growth-promoting rhizobacteria (PGPR) are beneficial bacteria that colonize the rhizosphere and plant roots resulting in enhancement of plant growth or protection against certain plant pathogens. Studies were conducted to test the hypothesis that induction of soil suppressiveness against Meloidogyne incognita using rhizobacterial inoculants is related to soil microbial activity and rhizosphere bacterial populations. Commercially-available rhizobacterial inoculants (Equity®, BioYield®, and AgBlend®) and FZB42, strain in the product RhizoVital®, were selected based on elicitation of growth promotion in tomato and pepper in previous tests. The inoculants Equity (multiple strains), BioYield (two strains), and FZB42 induced significant reductions in nematode eggs per gram root, juvenile nematodes per ml of soil, and galls per plant on tomato. AgBlend, containing microbial metabolites, reduced number of galls. Treatment with each of the inoculants also increased root weight. Rhizosphere populations of total bacteria and aerobic endospore-forming bacteria (AEFB) were increased following treatment with AgBlend, BioYield and FZB42. Strain FZB42 had an unique colony morphology, allowing its detection in the rhizosphere where it became the dominant strain. Soil microbial activity, as assessed by fluorescein diacetate hydrolysis, was not affected by inoculants. These results indicate that the selected microbial inoculants increase rhizosphere bacterial populations, and in the case of FZB42, actively colonize the rhizosphere, thereby inducing suppressiveness to nematodes, without necessarily enhancing soil microbial activity. Further, induction of soil suppressiveness against M. incognita was related to bacterial population size in the rhizosphere, when inoculants that contained two PGPR strains and also microbial metabolites were used.