xanthomonas hortorum xanthomonas hortorum xanthomonas hortorum is a plant pathogen which causes bacterial leafspot and headrot in lettuce. xanthomonas hortorum leaf spot on english ivy plant, caused by xanthomonas hortorum scientific classification domain: bacteria phylum: pseudomonadota class: gammaproteobacteria order: xanthomonadales family: xanthomonadaceae genus: xanthomonas species: x. hortorum binomial name xanthomonas hortorumvauterin et al. 1995