Microbial Diversity Limits Belowground Colonization of <i>Sphaerulina musiva</i>

Sphaerulina musiva is a fungal pathogen responsible for defoliating leaf spots and fatal stem cankers on Populus trichocarpa and hybrid Populus trees. Although its impact on tree health is well-documented, the influence of the plant-associated microbiome on S. musiva colonization remains unknown. Despite most of the disease cycle occurring aboveground, previous surveys of belowground plant-associated microbial communities have detected S. musiva in rhizosphere soils and roots. However, its ecological role in belowground habitats is still not well understood. Thus, we conducted a growth chamber experiment in which field soils that differed in microbial diversity were inoculated with two genetically distinct S. musiva isolates. Soils and root samples were destructively sampled at 10 and 25 days postinoculation. The persistence of S. musiva in belowground niches and microbial community responses were assessed through targeted amplicon sequencing of the internal transcribed spacer region and the 16S rRNA gene. S. musiva relative abundance was the highest at day 10 postinoculation in soils with reduced resident microbial diversity. By day 25, however, the pathogen was nearly undetectable in both roots and soils. Additionally, we observed shifts in soil fungal community composition, but they were dependent on resident soil microbial diversity, plant host presence, and sampling time. These findings highlight the intricate relationship between soil microbial communities and pathogen dynamics. Future research should aim to identify the factors that enable S. musiva to thrive belowground in natural environments and to elucidate the mechanisms by which the soil microbiome either inhibits or promotes the pathogen's establishment. [Formula: see text] Copyright © 2026 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .