Phylodynamic Insights into Global Emergence and Diversification of the Tomato Pathogen <i>Xanthomonas hortorum</i> pv. <i>gardneri</i>

The emergence of plant pathogens is often associated with waves of unique evolutionary and epidemiological events. <i>Xanthomonas hortorum</i> pv. <i>gardneri</i> is one of the major pathogens causing bacterial spot disease of tomatoes. After its first report in the 1950s, there were no formal reports on this pathogen until the 1990s, despite active global research on the pathogens that cause tomato and pepper bacterial spot disease. Given the recently documented global distribution of <i>X. hortorum</i> pv. <i>gardneri</i>, our objective was to examine genomic diversification associated with its emergence. We sequenced the genomes of <i>X. hortorum</i> pv. <i>gardneri</i> strains collected in eight countries to examine global population structure and pathways of emergence using phylodynamic analysis. We found that strains isolated post-1990 group by region of collection and show minimal impact of recombination on genetic variation. A period of rapid geographic expansion in <i>X. hortorum</i> pv. <i>gardneri</i> is associated with acquisition of a large plasmid conferring copper tolerance by horizontal transfer and coincides with the burgeoning hybrid tomato seed industry through the 1980s. The ancestry of <i>X. hortorum</i> pv. <i>gardneri</i> is consistent with introduction to hybrid tomato seed production and dissemination during the rapid increase in trade of hybrid seeds. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.