RESUMO
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.
RESUMO
While links between soil and plant health are implied, there are few opportunities to empirically evaluate this due to inherent differences among sites. An exception is a long-term experiment established in 2007 (repeated in 2008) in Ridgetown, ON, where improved soil health scores and changes in soil microbial communities were observed in the medium-term with annual cover crops (CC). This led us to hypothesize that CC-induced changes in soil health might affect bacterial spot (Xanthomonas hordorum pv. gardneri) and anthracnose (Colletotrichum coccodes) development in processing tomato. Five CC treatments (no CC control, winter cereal rye, oat, radish, and mix of radish + rye) planted after winter wheat harvest were evaluated in 2019 and 2020 (CC grown nine times over 12 years). Fruit yields and net revenue were similar or greater with CC than without. In 2019, there was greater defoliation (area under the disease progress stairs = 4,370 ± 204), percent red fruit (71.0% ± 5.38), and rots (1.91% ± 0.5) in no CC than with radish (3,410, 39.1%, and 0.62%, respectively, P ≤ 0.0366), indicating earlier fruit maturity in no CC plots. Similarly, no CC had a greater incidence of red fruits with anthracnose (25.8% ± 2.89) compared with all CCs but rye (7.4 to 12.1% ± 2.89; P = 0.0029). Environmental conditions in 2020 were less favourable for disease development. Defoliation was not affected by CC treatment (P = 0.1254), and anthracnose incidence was low (≥90.3 ± 1.22% healthy fruit), which may have limited the ability to detect treatment effects (P = 0.2922). Long-term cover crops have the potential to produce greater or equivalent tomato yield with decreased defoliation and anthracnose fruit rot.