Phylogenetic Analyses of Xanthomonads Causing Bacterial Leaf Spot of Tomato and Pepper: Xanthomonas euvesicatoria Revealed Homologous Populations Despite Distant Geographical Distribution

Abstract

Bacterial leaf spot of tomato and pepper (BLS), an economically important bacterial disease caused by four species of <i>Xanthomonas</i> (<i>X. euvesicatoria</i> (<i>Xe</i>), <i>X. vesicatoria</i> (<i>Xv</i>), <i>X. gardneri</i> (<i>Xg</i>), and <i>X. perforans</i> (<i>Xp</i>)), is a global problem and can cause over 50% crop loss under unfavorable conditions. Among the four species, <i>Xe</i> and <i>Xv</i> are prevalent worldwide. Characterization of the pathogens is crucial for disease management and regulatory purposes. In this study, we performed a multilocus sequence analysis (MLSA) with six genes (<i>hrcN</i>, <i>dnaA gyrB</i>, <i>gapA</i>, <i>pdg</i>, and <i>hmbs</i>) on BLS strains. Other <i>Xanthomonas</i> species were included to determine phylogenetic relationships within and among the tested strains. Four BLS species comprising 76 strains from different serological groups and diverse geographical locations were resolved into three major clades. BLS xanthomonads formed distinct clusters in the phylogenetic analyses. Three other xanthomonads, including <i>X. albilineans</i>, <i>X. sacchari</i>, and <i>X. translucens</i> pv<i>. undolusa</i> revealed less than 85%, 88%, and 89% average nucleotide identity (ANI), respectively, with the other species of <i>Xanthomonas</i> included in this study. Both antibody and MLSA data showed that <i>Xv</i> was clearly separated from <i>Xe</i> and that the latter strains were remarkably clonal, even though they originated from distant geographical locations. The <i>Xe</i> strains formed two separate phylogenetic groups; <i>Xe</i> group A1 consisted only of tomato strains, whereas <i>Xe</i> group A2 included strains from pepper and tomato<i>.</i> In contrast, the <i>Xv</i> group showed greater heterogeneity. Some <i>Xv</i> strains from South America were closely related to strains from California, while others grouped closer to a strain from Indiana and more distantly to a strain from Hawaii. Using this information molecular tests can now be devised to track distribution of clonal populations that may be introduced into new geographic areas through seeds and other infected plant materials.