RESUMEN
The emergence of plant pathogens is often associated with waves of unique evolutionary and epidemiological events. Xanthomonas hortorum pv. gardneri 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 X. hortorum pv. gardneri, our objective was to examine genomic diversification associated with its emergence. We sequenced the genomes of X. hortorum pv. gardneri 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 X. hortorum pv. gardneri 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 X. hortorum pv. gardneri is consistent with introduction to hybrid tomato seed production and dissemination during the rapid increase in trade of hybrid seeds.
RESUMEN
Severe epidemics of bacterial spot have been observed in central-west Brazil in fields of processing tomato. Several xanthomonads, Xanthomonas axonopodis pv. vesicatoria, X. vesicatoria, or X. gardneri, can cause the disease; therefore, attempts were made to identify the pathogen species present in this region. A total of 215 strains were obtained from 10 commercial areas in 1997, 1998, and 2000. The strains were characterized using pulsed-field gel electrophoresis (PFGE) and by their amylolytic and pectolytic activities. Representative strains from each PFGE haplotype then were tested for pathogenicity on tomato and pepper, carbon source utilization, and whole protein sodium dodecyl sulfate-polyacrylamide gel electrophoresis. rRNA sequence comparisons also were performed. All strains recovered from six fields were classified as X. gardneri, whereas X. vesicatoria and X. axonopodis pv. vesicatoria also were detected in the remaining four fields. Strains of X. gardneri, which could be grouped into two PFGE haplotypes, were unable to hydrolyze starch and pectate and to utilize gentiobiose and maltose. They expressed the ß protein of 27 kDa and were pathogenic on tomato but variable on pepper. This is the first report of outbreaks of bacterial spot on tomato caused by X. gardneri.