Pathogenic and genetic relatedness among Xanthomonas axonopodis pv. allii and other pathovars of X. axonopodis

Xanthomonas axonopodis pv. allii is phenotypically and genetically diverse and its relationship to other X. axonopodis pathovars within DNA homology group 9.2 is unknown. In growth chamber experiments, disease symptoms were produced on onion only by inoculation with X. axonopodis pv. allii. Citrus bacterial spot symptoms were induced by X. axonopodis pvs. alfalfae, citrumelo, and allii on Duncan grapefruit and key lime. X. axonopodis pv. allii multiplication and persistence in Duncan grapefruit were equal to those of an aggressive strain of X. axonopodis pv. citrumelo, but populations of X. axonopodis pvs. alfalfae, betlicola, citrumelo, phaseoli, and vesicatoria were 1.3 to 4.0 log units less than X. axonopodis pv. allii in onion. Genomic fingerprinting by repetitive sequence-based polymerase chain reaction demonstrated that X. axonopodis pvs. allii, alfalfae, and citrumelo are distinct from other Xanthomonas species and X. axonopodis pathovars, but these pathovars were indistinguishable from each other. Three genotype groups were apparent among DNA homology group 9.2 strains, and generally correspond to the aggressiveness and genotype groups previously described for X. axonopodis pv. citrumelo. X. axonopodis pvs. allii, alfalfae, and citrumelo appear to have recently diverged from a common ancestral strain.