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The common bean (Phaseolus vulgaris L.) is cultivated widely in Central and South America and particularly in the Northwest of Argentina. In order to describe the diversity of the common bean nodulating rhizobial population from the bean producing area in Northwest Argentina (NWA), a collection of about 400 isolates of common beans recovered from nodules and soil samples from NWA were characterized by using nifH-PCR, analysis of genes coding for 16S rRNA and nodC, and REP-fingerprinting, respectively. It was found that species Rhizobium etli is predominant in common bean nodules although a high degree of diversity was found within the species. Other bean nodulating genotypes recovered from soils by using Leucaena sp. as the trapping host was found to have the 16S rDNA alleles of species such as Sinorhizobium fredii, Sinorhizobium saheli, Sinorhizobium teranga, Mesorhizobium loti, and Rhizobium tropici. Some of the bean genotypes that were found to be more efficient in green house experiments were selected and assayed in two successive bean-cropping seasons in the field environment in NWA, and an increase in yields with inoculation was found. The performance of strains isolated from the region indicates potential for exploiting the diversity.

A collection of rhizobial isolates from nodules of wild beans, Phaseolus vulgaris var. aborigineus, found growing in virgin lands in 17 geographically separate sites in northwest Argentina was characterized on the basis of host range, growth, hybridization to a nifH probe, analysis of genes coding for 16S rRNA (16S rDNA), DNA fingerprinting, and plasmid profiles. Nodules in field-collected wild bean plants were largely dominated by rhizobia carrying the 16S rDNA allele of Rhizobium etli. A similar prevalence of the R. etli allele was observed among rhizobia trapped from nearby soil. Intragroup diversity of wild bean isolates with either R. etli-like or Rhizobium leguminosarum bv. phaseoli-like alleles was generally found across northwest Argentina. The predominance of the R. etli allele suggests that in this center of origin of P. vulgaris the coevolution of Rhizobium spp. and primitive beans has resulted in this preferential symbiotic association.