Further Examination of Presumptive Rhizobium trifolii Mutants that Nodulate Glycine maxR. A. Ludwig, E. A. Raleigh, M. J. Duncan, E. R. Signer, A. H. Gibson, W. F. Dudman, E. A. Schwinghamer, D. C. Jordan, E. L. Schmidt and D. T. Tran · Proceedings of the National Academy of Sciences of the United States of America · 1979