Anticipating the emergence of plant diseases requires understanding the demographic and evolutionary processes shaping pathogen populations. Most reconstructions rely on genomic analyses of contemporary samples, but these approaches lack temporal depth and can limit the accuracy of inferred scenarios. The analysis of DNA from herbarium specimens provides a complementary framework by directly capturing past pathogen diversity.
Here, we combined genomic data from 172 contemporary strains of Xanthomonas citri pv. citri with sequences obtained from 13 herbarium specimens showing citrus canker symptoms, collected between 1845 and 1975. Despite the challenges associated with degraded DNA, this dataset enabled the calibration of a robust molecular clock and the reconstruction of the pathogen’s spatiotemporal history.
Our results indicate that X. citri pv. citri originated in the Himalayan piedmont region at the end of the last glacial period. From this center of origin, it diversified and adapted to a range of environmental conditions and citrus hosts while progressively expanding its geographic range in a continuous manner across neighboring regions, likely along major land-based trade routes such as the Silk and Incense roads. From the 17th century onward, dispersal dynamics shifted toward long-distance intercontinental “jump” events associated with global trade. Successive introductions across continents ultimately led to a pandemic distribution by the late 19th century.
