ABSTRACT
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.
ABSTRACT
The genomic, biological, and serological characterization of tomato necrotic spot virus (ToNSV), a virus first described infecting tomato in California, was completed. The complete genomic sequence identified ToNSV as a new subgroup 1 ilarvirus distinct from the previously described tomato-infecting ilarviruses. We identified ToNSV in Indiana in 2017 and 2018 and in Ohio in 2018. The coat protein coding region of the isolates from California, Indiana, and Ohio have 94 to 98% identity, while the same isolates had 99% amino acid identity. ToNSV is serologically related to TSV, a subgroup 1 ilarvirus, and shows no serological relationship to ilarviruses in the other subgroups. In tomato, ToNSV caused symptoms of necrotic spots and flecks on leaves, necrotic streaking on stems, and necrotic spots and circular patterns on fruit resulting in a yield loss of 1 to 13%. These results indicate that ToNSV is a proposed new subgroup 1 ilarvirus causing a necrotic spotting disease of tomato observed in California, Indiana, and Ohio.
