Competition studies with fast-growing Rhizobium japonicum strains

The ability of eight fast-growing strains of Rhizobium japonicum to compete for nodule sites against two slow-growing strains of R. japonicum was measured using different input ratios (1:1, 1:10, and 10:1) on Glycine max cv. Peking and cv. Jacques 130 in growth pouches. The slow growers formed >60% of the nodules on cv. Peking even when the fast growers were added at a 10:1 ratio in their favor. We also measured the competitive ability of 10 fast-growing strains of R. japonicum, including these 8 strains, and ANU240 (a fast-growing broad host range strain) at two inoculum levels, 106 and 109 cells/seed on cv. Peking and cv. Jacques 130 in pots containing two midwestern soils which contained high numbers of indigenous rhizobia. In one soil, 3 of the 10 fast-growing strains occupied >60% of the nodules on cv. Peking at 109 inoculum level. No nodules were formed by the fast-growing strains on cv. Jacques 130 in any of the experiments.