Plant pathogens are often classified into groups (pathovars, formae speciales, races) that purport to assemble strains of a same phylogenetic cluster that have distinct host ranges. However, there are many examples where host ranges cannot be clearly delimited, such as the overlapping continuum of host range in Pseudomonas syringae. The “co-evolutionary arms race” between plants and pathogens is supposed to structure plant and pathogen populations. Yet, the evolution of many plant pathogens at the scale of life on land (that begun ca. 450 million years ago) does not necessarily mirror that in their modern crop hosts. We are determining how the fitness of strains of P. syringae is delimited within and beyond angiosperms and if the fitness of bacteria varies relative to taxonomic groupings (order, family, genus) or the evolutionary proximity of plants. Plants inoculated for bacterial fitness measurements include >50 species of Polypodiales, Ginkgoales and five orders of angiosperms. Strains representing the most ancient and environmentally ubiquitous lineages of P. syringae were able to multiply in plants across all orders tested, with no apparent effect of the taxonomic groups of plants. Tests with more recent and less widespread lineages are in progress. The overall variability in fitness within plant species suggests a role for stochastic processes in microbial multiplication in plants.