Co-inoculation of selected nodule endophytic rhizobacterial strains with Rhizobium tropici promotes plant growth and controls damping off in common bean

Abstract

Production of common bean (Phaseolus vulgaris) is limited by the occurrence of damping off (rhizoctoniosis), which is caused by the fungus Rhizoctonia solani. However, the co-inoculation of plant growth-promoting rhizobacteria (PGPR) involved in biological control along with diatomic nitrogen (N2)-fixing rhizobia can enhance N nutrition and increase production. In this context, finding microorganisms with synergistic effects that perform these two roles is of fundamental importance to ensure adequate yield levels. The aim of this study was to evaluate the effects of co-inoculation of nodule endophytic strains of the genera Bacillus, Paenibacillus, Burkholderia, and Pseudomonas with Rhizobium tropici CIAT 899, an N2-fixing rhizobial strain, on the biocontrol of damping off and growth promotion in common bean plants. Greenhouse experiments were conducted under axenic conditions using the common bean cultivar Pérola. The first experiment evaluated the potential of the 14 rhizobacterial strains, which were inoculated alone or in combination with CIAT 899, for the control of R. solani. The second experiment evaluated the ability of these 14 rhizobacterial strains to promote plant growth with three manners of N supply: co-inoculation with CIAT 899 at low mineral N supply (5.25 mg N mL−1), low mineral N supply (5.25 mg N mL−1), and high mineral N supply (52.5 mg N mL−1). The use of rhizobacteria combined with rhizobia contributed in a synergistic manner to the promotion of growth and the control of damping off in the common bean. Co-inoculation of the strains UFLA 02-281/03-18 (Pseudomonas sp.), UFLA 02-286 (Bacillus sp.), and UFLA 04-227 (Burkholderia fungorum) together with CIAT 899 effectively controlled damping off. For the common bean, mineral N supply can be replaced by the co-inoculation of CIAT 899 with plant growth-promoting strains UFLA 02-281/02-286/02-290/02-293. Nodule endophytes UFLA 02-281/02-286 are promising for co-inoculation with CIAT 899 in the common bean, promoting synergy with rhizobial inoculation and protection against disease.