Evaluating Rhizobium and Bradyrhizobium species as potential biocontrol agents for root rot fungi in soybean seedlings

Soybean (Glycine max) is among the legumes that are highly prone to soil-borne pathogens that causes root-rot diseases limiting growth and development and resulting in low yields of plants. This study was conducted to test the ability of three rhizobia strains, Rhizobium sp. TZSR12C, Rhizobium sp. TZSR25B and Bradyrhizobium sp. TZSR41A, in comparison with the commercial biocontrol (Trichoderma harzianum in suppressing the growth of root rot fungal pathogens (Fusarium solani, Rhizoctonia solani, Fusarium oxyporum, and Macrophominaphaseolina) in vitro and greenhouse conditions. The rhizobium cell filtrates were used in testing their activities against fungal pathogens under in vitro while the solid biofertilizer formulations containing the respective rhizobia inoculants were used to inoculate the soybean seeds sown in pathogen-contaminated soil under greenhouse conditions. Results showed that all rhizobia isolates and T. harzianum were capable of suppressing the fungal pathogens both under in vitro and greenhouse conditions with the highest inhibition zone (8.3 mm) and colony diameter (25.0 mm) being in Rhizobium sp. TZSR25B against F. oxyporum under in vitro conditions. In greenhouse experiments, Rhizobium sp. TZSR12C had the highest performance in inhibiting the infection of plant up to 27.78% with severity of 5.56% in roots and 0.00% infection in foliage against the combination of F. solani, R. solani, F. oxyporum, and M. phaseolina. It was found that, on their performance, the tested rhizobia strains can potentially be utilized as biocontrol agents against the fungal pathogens in the rhizosphere of soybean plants.