Abstract
Polyphasic comparisons were carried out to characterize the pathogen of bacterial spot disease of grapevine (Xanthomonas arboricola) (Sawada et al., 2011) and that of bacterial spot diseases of stone fruits (X. arboricola pv. pruni) in detail. For that purpose, we isolated both pathogens in the area selected as a model, where the two diseases respectively occur annually in neighboring orchards, in Minami-Alps City, Yamanashi Prefecture in September 2010 to characterize them genotypically and phenotypically, to perform cross inoculation experiments, and to compare these results with those obtained from reference strains of X. arboricola pathovars. All strains of pv. pruni (1) are genetically highly uniform, (2) naturally infect peach, plum and apricot, (3) are highly virulent and stably pathogenic to peach, (4) do not naturally infect grapevine, and (5) are negative in the potato soft rot test. On the contrary, isolates causing bacterial spot disease of grapevine (1) are not genetically uniform and are scattered within the X. arboricola clade, interspersed with existing pathovars, in a gyrB phylogenetic analysis, (2) are weak, opportunistic pathogens, and only if the conditions are ideal, can they infect grapevine naturally, (3) do not naturally infect peach, plum or apricot, and (4) cause severe, rapid rot in the potato soft rot test. These results show that the latter pathogen should be regarded as a heterogeneous complex of genetically diverse, indigenous bacteria, although some phenotypes can be considered as common attributes. We concluded that it is inappropriate to propose a novel pathovar for the latter pathogen, until the pathovar of the genus Xanthomonas is further clarified and defined phenotypically and genotypically.
