Phage sensitivity and virulence relationships in Xanthomonas pruni

Abstract

Variants of Xanthomonas pruni selected after exposure to an excess of phage were less virulent on “Sunhigh” peach seedlings than the parent isolates. The variants were further characterized by markedly reduced capacity to adsorb phage irreversibly. None of the X. pruni variants were lysed by any of eight phage selections from morphologically distinct plaques at routine test dilution, even though some of the variants adsorbed up to 50% of the plaque-forming units in a mixture of the phage selections. In log-dose/probit-response analyses of infectivity titrations, the slope values for all the X. pruni variants ranged from b = 1·06 to b = 2·70. These values are consistent with plant responses produced in compatible host-pathogen combinations by the cells acting independently. Generally, colonies of the X. pruni variants were smaller than those of the parent isolates and low convex or slightly concave in the center. Selection for and survival of phage-resistant, virulent X. pruni variants under natural conditions remains to be determined.