Effect of a bacteriophage on colonisation and nodulation of clover roots by paired strains of Rhizobium trifolii.

The effect of a virulent bacteriophage on the competitive behaviour of paired strains of Rhizobium trifolii in the root zone of clover plants was examined. The presence of bacteriophage reduced the population density of susceptible strains on the root surface and, in nodulation, favoured resistant or even partially resistant strains which were otherwise less able to form nodules. These effects occurred with different growth temperatures, host species, and ratios of strains in the inoculum.