The interaction between Septoria stem canker and the mycobiome of Populus trichocarpa stems

Sphaerulina musiva , a fungal pathogen causing leaf spot and canker disease of poplar trees ( Populus spp.), was recently introduced to the Pacific Northwest, where it threatens commercially valuable plantations and native riparian ecosystems. Vascular tissue was collected from the stems of 410 symptomatic and asymptomatic trees in an S. musiva -infested Populus trichocarpa plantation. Resident fungal endophyte communities were characterized with ITS amplicon sequencing. Canker expression and S. musiva presence corresponded with reduced fungal diversity across multiple indices. Fungal endophyte communities of symptomatic tissues were frequently dominated by S. musiva and thus compositionally distinct from those of asymptomatic tissues. Asymptomatic-tissue communities from healthy and diseased stems did not differ in composition or diversity, indicating disease-associated low-diversity mycobiome states are local to the cankered site. The relative abundance of S. musiva was positively correlated with stem cankering. S. musiva was negatively correlated with key non-pathogenic fungal endophytes, which were themselves strongly inter-correlated. Our results illustrate S. musiva ’s ability to exploit the vascular microhabitat of susceptible Populus trichocarpa stems and dominate resident fungal assemblies at the site of infection. IMPORTANCE Plant-associated microbial communities can both mediate and be modified by pathogen infection. Thus, understanding disease outcomes of complex plant pathosystems requires characterization of pathogen-phytobiome interactions. Efforts to characterize these interactions have yielded microecological insights with applied relevance for disease management in herbaceous leaf- and root-associated pathosystems. However, pathogen-phytobiome interactions in the vascular tissues of hardwood stems remain largely unexplored. Our findings illuminate the ecological organization of the Populus trichocarpa stem mycobiome under S. musiva disease pressure and advance understanding of microfungal community dynamics in the Septoria stem canker pathosystem. Additionally, we identify potentially interactive fungal taxa that may disproportionately shape mycobiome structure and disease dynamics in Populus trichocarpa stems.