|Van Santen, Edzard|
Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/18/2008
Publication Date: 12/1/2008
Citation: Burkett-Cadena, M., Burelle, N.K., Lawrence, K.S., Van Santen, E., Kloepper, J.W. 2008. Suppressiveness to root-knot nematodes mediated by rhizobacteria. Biological Control. 47(1):55-59. Interpretive Summary: Management alternatives for control of to root-knot (Meloidogyne spp.) nematodes are currently being investigated due to environmental concerns and increased regulations on use of chemical fumigants. Biological control using microbial antagonists has potential to supplement or replace chemical nematicides. Among the biological control agents studied are plant growth promoting rhizobacteria. These antagonists can limit nematode infection and reproduction. Because sustainable agricultural practices typically function through the activity of soil microorganisms, obtaining beneficial communities that promote soil health can contribute to reducing root-knot nematode damage. We hypothesized that PGPR-based inoculants can be used to induce suppressiveness to soil-borne pathogens by enhancing soil microbial activity and by sustaining stable populations of PGPR in the rhizosphere. Induction of soil suppressiveness against Meloidogyne incognita using rhizobacterial inoculants and its relation to soil microbial activity and bacterial population size were studied using commercially-available rhizobacterial inoculants (Equity®, BioYield®, and, AgBlend®) and FZB42 wild-type strain. Treatments with the inoculants Equity, BioYield, and FZB42 induced significant reductions in numbers of nematode eggs per gram of tomato root, numbers of juveniles per ml, and numbers of galls per plant. Additionally, increases in total bacteria and heat-tolerant bacteria in the tomato rhizosphere were detected in plants treated with BioYield and FZB42. The inoculants did not induce enhanced soil microbial activity as assessed by fluorescein diacetate hydrolysis. Hence, there was no correlation between microbial activity and bacterial population density in the rhizosphere. These results indicated that selected microbial inoculants colonize the root system, establish stable populations, and thereby induce suppressiveness to soilborne pathogens, without necessarily enhancing soil microbial activity.
Technical Abstract: 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 a 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 Meloidogyne incognita was related to bacterial population size in the rhizosphere, when inoculants that contained two PGPR strains and also microbial metabolites were used.