Development of Host Range Mutants of Xanthomonas campestris pv. translucens.

Xanthomonas campestris pv. translucens is the causal agent of bacterial leaf streak of cereal grains and grasses, and individual strains within the pathovar differ in their host range among the cereals. Coinoculation of a wide-host-range and a narrow-host-range strain resulted in the wide-host-range reaction. Transposon and chemical mutagenesis of the wide-host-range strain Xct4, pathogenic on barley, wheat, rye, and triticale, resulted in variants with reduced host range. When pathogenicity was inactivated independently for barley, wheat, triticale, and rye, wild-type symptoms were retained on the other members in the host range. Testing of some host range mutants on additional varieties of the cereals indicated some cultivar specificity. In addition, mutants nonpathogenic on combinations of the hosts or on all hosts were isolated. This suggests that there are independent positive factors determining host range in this species, rather than an avirulence gene system such as those determining race specificity in other plant pathogens.