The poplar pathogen Sphaerulina populicola is genetically diverse and ubiquitously present in symptom-free host trees

Understanding the genetic variability and dispersal capabilities of plant pathogens is important to better predict pathogen spread and virulence, especially under global change conditions. The outcome of host-pathogen interactions directly depends on the dispersal capacity of the pathogens, on environmental conditions, and disease resistance of the host. Here, we analysed the distribution of finely resolved phylogenetic lineages of a poplar-specific fungal pathogen (Sphaerulina populicola) in western North America, across a 3,200 km latitudinal gradient of the host tree’s (Populus balsamifera) distribution. We specifically tested whether dispersal limitations or environmental filters limit the spread and establishment of the pathogen into new areas. We assessed the genetic diversity of the pathogen with ITS1 oligotypes, recorded by metabarcoding leaf-associated fungal communities. The distribution of the recorded 16 S. populicola oligotypes showed no geographic patterns, indicating the lack of dispersal limitation of the pathogen throughout the investigated area. Climatic conditions also did not seem to restrict the occurrence and abundance of the pathogen oligotypes. Finally, we found strong variation in oligotype presence within single localities, which may suggest the importance of biotic factors in regulating the infection of this well-dispersing fungal pathogen. Our interpretation implies that the future prevalence of S. populicola-related disease will likely depend on the dynamics of host-pathogen interactions, since the pathogen does not seem dispersal-limited at a continental scale.