Characterization of Diverse Rhizobium trifolii Isolates

Forty cultures of Rhizobium trifolii isolated from diverse soils in western Oregon were characterized on the basis of host‐plant specificity and serological and cultural reactions. There was large variation in effectiveness when the rhizobia were examined on 12 cultivars of subclover ( Trifolium subterraneum L.), although most of the strains demonstrated fair to average effectiveness on the majority of the subclover cultivars. The rhizobia were of generally poor effectiveness with other Trifolium species. Antibiotic resistant mutants were selected for two of the strains, and these were tested on the subclover cultivars as a mixed inoculum. Six cultivars were preferentially nodulated by one of the two strains, and individual plants that contained one of the strains in the majority of its nodules were identified. Thirty‐four of the isolates could be categorized into six serological groups based on whole‐cell agglutination reactions, whereas three cultures demonstrated very specific antibody reactions and could be identified on this basis. All 40 strains grew in acid broth at pH 4.5, but only 12 out of 40 grew at pH 3.0. At neutral pH, the mean generation time was 1½ to 3 hours, although there were a few that grew faster. At pH 4.5 the generation times were increased for most of the cultures but usually by only 1 or 2 hours. The serological groupings and growth characteristics were not found to be related to the effectiveness categories of the strains in any apparent way. The possibilities for selecting clover lines that are preferentially nodulated by a specific Rhizobium strain and identifying isolates that have research potential based on effectiveness patterns among different host‐plant cultivars are described.