Response of dry bean (Phaseolus vulgaris L.) to inoculation with indigenous and commercial Rhizobium strains under organic farming systems in Minnesota

Organic farmers recognize the importance of using Rhizobium to meet crop N fertility needs and to reduce use of chemical fertilizers. Field experiments were conducted during the 2015 and 2016 seasons at different organic fields to assess the effect of indigenous Rhizobium leguminosarum bv. phaseoli (OrgK9) and commercial R. tropici (CIAT899) strains on symbiotic performance and grain yield of bean varieties, Eclipse and Redhawk. Averaged for both varieties, nodule numbers on plants inoculated with CIAT899 (35.2 ± 3.7 and 26.3 ± 2.6 /plant) and OrgK9 (29.6 ± 2.5 and 26.8 ± 2.1 /plant) were significantly greater (P < 0.0001) than the control (18.9 ± 2.2 and 10.5 ± 3.1) in 2015 and 2016, respectively. Bean genotype Eclipse cultivated at Lamberton and inoculated with OrgK9 had (P < 0.0001) greater number of nodules compared with CIAT899-inoculated or non-inoculated plants. The N-content (3.3 ± 0.04%) of plants inoculated with CIAT899 was greater (P < 0.0001) compared with plants inoculated with OrgK9. Strain CIAT899 increased nodule occupancy by 29 and 62% and 4 and 26% on Red Hawk cultivated in the Lamberton and Farmington fields compared with the control in season 2015 and 2016, respectively. The majority of nodules formed on Red Hawk (76%) and Eclipse (64%) at Farmington were occupied by CIAT899 and OrgK9 in 2015 and 2016, respectively. Eclipse inoculated with OrgK9 had a significantly greater seed yields (2683 ± 402 and 2546 ± 247 kg/ha) compared with control (2265 ± 996 and 2058 ± 51 kg/ha) in both years, respectively. These results clearly indicate that symbiotic performance and grain yield of dry bean can be significantly increased by using Rhizobium inoculation under organic farming systems.