Improvement of bean yield and Fusarium root rot biocontrol using mixtures of Bacillus, Pseudomonas and Rhizobium

During a screening of beneficial rhizobacteria strains with capability of suppressing Fusarium root rot and improving bean yield, 680 bacteria belonging to Bacillus, fluorescent and non-fluorescent Pseudomonas, and Rhizobium were isolated from commercial field soils and plant root nodules in Zanjan province, Iran. Of these, two hundred representative isolates were biochemically characterized and screened in vitro for inhibiting mycelia growth of Fusarium solani f. sp. phaseoli. Sixteen rhizobacteria isolates were able to strongly inhibit fungal growth. Five promising rhizobacteria were evaluated for suppressing the disease and promoting plant growth in greenhouse plants. Mixed inoculation of rhizobacteria caused greater increases in disease suppression, dry and fresh matter weight of plant (aerial part) and root, plant height, and pod number per plant compared to diseased control and absolute control. Treatments involving Bacillus subtilis produced higher number of pods compared to other single and dual treatments, except for Rhizobium-Pseudomonas treatment. Treating with rhizobacteria decreased disease severity ratings by 50 to 62% compared to untreated plants. Further examination of the rhizobacteria under field conditions is required to uncover their efficacy in biofertilization and viability as biocontrol agent in bean crops.