Soybean genotype restricting nodulation of a previously unrestricted serocluster 123 bradyrhizobia

The displacement of indigenous Bradyrhizobium japonicum strains of serocluster 123 in soybean [Glycine max (L.) Merr.] nodules in favor of a more effective inoculant strain has been suggested as an approach to enhance N₂ fixation and thereby increase soybean productivity. We previously identified soybean genotypes that restricted the nodulation and competitiveness of the serotype strain USDA 123, but did not affect the nodulation or competitiveness of many other serocluster 123 isolates. It was the objective of this work to identify a soybean genotype that reduced the nodulation and competitiveness of serocluster 123 isolate MNl-lc, a strain that nodulated the previously identified USDA 123-restricting genotypes very well. A total of 850 plant introductions and cultivars were screened in the greenhouse with isolate MNl-lc. Five soybean genotypes had reduced nodulation when compared with the control cultivars. In two subsequent tests in soil and vermiculite, two of the genotypes, PI 283326 and PI 417S66, restricted the nodulation of strains MNllc, USDA 110, and USDA 138. PI 417566 nodulated and fixed N₂ effectively with strains USDA 123 and CB1809, while PI 283326 did not. In a test of competition for nodulation, 'Williams' soybean and PI 417566 were inoculated with both MNl-lc and the inoculantquality strain CB1809. Over 90% of the nodules formed on Williams contained MNl-lc, while less than 10% were occupied with CB1809. In contrast, 70% of the nodules of PI 417566 contained CB1809. Our results demonstrated the ability of PI 417566 to reduce the nodulation and competitiveness of MNl-lc in favor of the inoculantquality strain CB1809, and indicated that soybean genotypes may be used to exclude specific isolates or subpopulations of serocluster 123.