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.