From lichens to crops: Pathogenic potential of Pseudomonas syringae from Peltigera lichens is similar to world-wide epidemic strains

The presence of bacteria belonging to the Pseudomonas syringae complex (P. syringae) in the natural vegetation of several Icelandic habitat types has been recently reported, raising questions about the risk to Icelandic crops, particularly given the expected increase in agricultural activity due to climate warming. This study takes advantage of Icelands unique characteristics and the discovery of P. syringae in Peltigera lichens to gain a better understanding of the potential risk posed by this newly discovered ecological niche. The main objective is to evaluate the pathogenic potential and fitness in crops of P. syringae strains isolated from Peltigera lichen sampled in Iceland, focusing on strains that belong to phylogroups 1 and 2, which commonly contain epidemic strains. The results indicate that P. syringae isolated from Icelandic Peltigera lichen have a comparable fitness to epidemic strains in eight out of ten tested plant species. Furthermore, pathogenicity assessment on three plant species highlighted that certain strains also caused similar symptoms and disease severity compared to epidemic strains. These findings provide valuable insights into the potential risks posed by P. syringae from Icelandic natural habitats and illustrate how strains from these habitats have a wide pathogenic potential to crops without having encountered these crops in the last several thousand years of their presence in Iceland.