Xanthomonas arboricola pv. fragariae: unraveling a complexity

The species Xanthomonas arboricola is divided into 6 pathovars, including celebensis, corylina, fragariae, juglandis, populi, and pruni. X. arboricola pv. fragariae (Xaf), was reported as the causal agent of bacterial leaf blight on strawberry and considered a potential threat to European strawberry production (Janse et al., 2001).
The poor criteria that are currently used for classification of strains as Xaf, and its ambiguous pathogenicity on strawberry, were the main motives for the current study. We made a polyphasic analysis of 12 Xaf-catalogued strains from different culture collections. First, their phylogenetic background was analysed by partial 16S rDNA and gyrB sequencing and MALDI-TOF MS. Then, the strains were screened for the presence of two type three secretion system (TTSS) related genes, and tested for symptom production on strawberry plants. Phylogenetically,
the 12 strains formed a heterogeneous complex showing different affiliations to
the pathotype strains of other X. arboricola pathovars. Homologues of the TTSS related genes hrcQ and avrBs2 were detected in only 3 of the 12 strains. Moreover, none of the strains reproduced the leaf blight symptoms upon inoculation in strawberry. Also, the comparison of drafted whole genome sequences of two Xaf strains, the pathotype strain LMG 19145 from Italy and strain LMG 19146 from France, indicated enough sequence divergence in their phylogenetic marker genes for considering the two strains as different genomovars of X. arboricola.
These results indicate that the name Xaf is currently used for a genetically diverse
assortment of strains with unclear pathogenic potential, which questions the
validity of Xafas a separatepathovarof X. arboricola. Inconclusion, thespecies X arboricola accommodatesmanyunspecifiedplantassociatedbacteriawithanunclearphytopathologicalsignificance.



reference: JanseJD, RossiMD, GorkinkRFJ, DerksJHJ, Swings J, Janssens D, Scortichini M, 2001. Bacterialleafblightofstrawberry (Fragariae (x) ananassa) causedby a pathovarof Xanthomonas arboricola, notsimilarto Xanthomonas fragariae Kennedy & King. Descriptionofthecausalorganismas Xanthomonas arboricola pv. fragariae (pv. nov., comb. nov.). PlantPathology50, 653-65.