Emergence and host range expansion of an epidemic lineage of Ralstonia solanacearum

Members of the Ralstonia solanacearum species complex are soil-borne pathogens of important crop plants. Strain-level host specialisation is considered typical, yet outbreak monitoring in Martinique revealed the emergence of a new lineage with dramatically expanded host range. We show the outbreak was caused by the parallel expansion of lineages from two different Ralstonia species. One lineage of Asian origin retains a signature of host specialisation, while the emergent broad host range lineage originated in the Americas. The presence of similar integrative and conjugative elements in both lineages suggests they are exchanging genetic material. The emergent lineage acquired genes encoding novel catalytic and metabolic functions, but shows little variation among virulence genes considered host specificity determinants. The host range expansion event was not caused by the evolution of a virulence strategy targeting host immunity, but is more likely due to the acquisition of novel substrate utilisation pathways enhancing pathogen fitness in soil or plant tissues.