Classification and Identification of Xanthomonas translucens Isolates, Including Those Pathogenic to Ornamental Asparagus

In order to confirm and refine the current classification scheme of Xanthomonas translucens and to identify novel strains from ornamental asparagus, a collection of field and reference strains was analyzed. Reppolymerase chain reaction (PCR) genomic fingerprint profiles were generated from 33 isolates pathogenic to asparagus as well as 61 X. translucens reference strains pathogenic to cereals and grasses. Amplified ribosomal gene restriction analysis profiles were obtained from most of these and 29 additional Xanthomonas reference strains. Rep-PCR genomic fingerprint profiles of all strains were compared with those in a large Xanthomonas database using computer-assisted analysis. Rep-PCR genomic fingerprinting facilitated the characterization and discrimination of X. translucens, including the pathovars arrhenatheri, graminis, phlei, phleipratensis, and poae, as well as a number of strains received as X. translucens pv. cerealis. Strains received as pathovars hordei, secalis, translucens, undulosa, and other cerealis strains were grouped in two subclusters that correspond to the recently redefined pathovars X. translucens pvs. undulosa and translucens. All 33 novel isolates from ornamental asparagus (tree fern; Asparagus virgatus) were identified as X. translucens pv. undulosa. Moreover, a unique amplified small subunit ribosomal gene MspI/AluI restriction profile specific for all X. translucens strains tested, including those pathogenic to asparagus, allowed discrimination from all other Xanthomonas spp. Although phage tests were inconclusive, the classification of the asparagus strains within the X. translucens complex was supported by pathogenicity assays in which all the isolates from ornamental asparagus induced watersoaking on wheat. Surprisingly, several X. translucens reference strains affected asparagus tree fern as well. That the novel asparagus isolates belong to X. translucens pv. undulosa is extraordinary because all hosts of X. translucens pathovars described to date belong only to the families Gramineae and Poaceae, whereas asparagus belongs to the phylogenetically distant family Liliaceae.