Bacterial Leaf Blight of Polyscias guilfoylei Caused by a Novel Pathovar of Xanthomonas euvesicatoria

Polyscias guilfoylei is a popular ornamental belonging to the Araliaceae family. The present study identified and characterized bacterial strains causing leaf lesions on P. guilfoylei in a nursery in Taiwan. Strains Pgu1 to Pgu5 were isolated from infected leaf tissues and Koch's postulates were fulfilled. Observation of Pgu1 under a transmission electron microscope revealed that its cells were single flagellated and rod shaped. Sequencing of Pgu1 to Pgu5's 16S ribosomal DNA showed that they belong to the genus Xanthomonas. The biochemical and physiological traits of these bacteria were determined, and many of them also resemble those of other xanthomonads. However, the strains were unable to produce yellow pigments typically found in most members of the Xanthomonas genus, even when grown on yeast dextrose calcium carbonate (YDC) agar. Physiological assays and phylogenetic analyses based on multiple loci showed that the isolates were closely associated with members of the species Xanthomonas euvesicatoria and phylogenetically distant from X. hortorum pv. hederae, the currently only known xanthomonad capable of inducing diseases on Polyscias spp. Artificial inoculation into different host plants revealed that a representative strain, Pgu1, is specialized to P. guilfoylei and perhaps other members of the Araliaceae family. Based on the results from the phylogenetic and phenotypic analyses, the present work concludes that these strains belong to a novel pathovar of X. euvesicatoria. The pathovar epithet polysciadis is proposed.