Role of rhizobia in suppressing the root diseases of soybean under soil amendment

Abstract

Rhizobia are soil bacteria, characterized by their unique ability to colonize the roots of leguminous crops, where they form nitrogen fixing nodules. Considerable evidence has been accumulated to identify the benefits associated with use of rhizobia as biocontrol agents against soil-borne pathogens, in addition to biological nitrogen fixation. In this study, out of four rhizobial isolates tested, Bradyrhizobium sp. inhibited the radial growth of all the test fungi viz: Macrophomina phaseolina, Rhizoctonia solani, Fusarium solani and F. oxysporum, while other 3 rhizobial isolates inhibited the growth of at least three fungi. Cell free culture filtrates of rhizobia also showed nematicidal activity by killing second-stage juveniles of Meloidogyne javanica at varying degrees. Application of rhizobial isolates alone or in mustard cake amended soil significantly (p<0.05) by suppression of root rotting fungi and root knot nematode on soybean roots. Rhizobia-treated plants showed less penetration of nematodes in roots than untreated control plants. Efficacy of rhizobia was found to increase against nematodes in mustard cake at 1% amended soil. Rhizobia also improved soybean growth by producing taller plants although plants showed poor nodulation. The tallest plant was found in some nitrogen fixing bacteria (NFB) + mustard cake (1%) treatment, but in most cases, the combined application did not offer any added advantage.