Building Bridges Belowground: Pythium oligandrum En-hances Rhizosphere Network Connectivity
Year: 2026
Type: preprint
Abstract: Biological control agents (BCAs) offer a potential sustainable alternative to fungicides for crop disease management, but their effects on resident plant microbiomes and microbial interactions remain poorly understood. Here, we investigated the effects of the BCA Pythium oligandrum on disease control and the rhizosphere microbiome of starch potato cv. Kuras in a field trial. We compared a laboratory strain and a commercial formulation (Polygandron) with commonly used fungicides during early blight development. The laboratory strain suppressed early blight at plant senescence, but this effect did not persist until harvest, whereas fungicide treatment provided near-complete disease suppression throughout the growing season. P. oligandrum caused transient changes in rhizosphere fungal community composition at flowering, while bacterial community richness varied among sampling times in untreated controls. However, bacterial co-occurrence network analysis revealed pronounced treatment-specific changes in microbial connectivity. Application of the laboratory strain increased network connectivity, resulting in almost a threefold increase in mean degree and greater interconnectedness among bacterial taxa. In contrast, Polygandron and fungicide treatments reduced mean degree, indicating reduced connectivity within the bacterial network. Fungicide treatment additionally altered network organisation, including increased betweenness centrality and changes in modularity. Together, our results show that P. oligandrum can modify microbial interactions in the potato rhizosphere without substantially altering overall bacterial community composition, highlighting the value of integrating microbial network structure with community-level analyses when evaluating the ecological effects of BCAs. Lay summary Biological control agents offer a more sustainable alternative to fungicides for managing crop diseases, but their effects on beneficial and resident microbes are not fully understood. We investigated how Pythium oligandrum , a potential biological control agent, affected early blight and the microbial communities around the roots of starch potato plants grown in a field trial. We compared a laboratory strain and a commercial formulation (Polygandron) with conventional fungicide treatment. The laboratory strain reduced early blight symptoms towards the end of the growing season, but this effect did not persist until harvest. Fungicides provided more consistent disease control throughout the season. While treatments had limited effects on the overall composition of bacterial communities, they changed the patterns of associations between bacterial groups. The laboratory strain almost tripled the average number of associations per bacterial group, whereas Polygandron and fungicide treatments reduced this connectivity. Fungicides also changed how these associations were organised. These findings show that biological control agents can influence relationships among root-associated microbes even when the overall bacterial community changes little. Understanding these microbial networks alongside community composition could help improve the evaluation of biological control strategies and support the development of more sustainable crop protection practices. Teaser text Biological control with Pythium oligandrum reshapes bacterial network connectivity in the potato rhizosphere, revealing microbial changes that remain hidden when community composition is considered alone.
Institution: Swedish University of Agricultural Sciences
Language: English
Subfield: Plant Science
Domain: Life Sciences
SDGs: Zero hunger, Life on land
Open Access status: green
Awards: The expression and evolution of Phytophthora infestans effectors in potato fields and implications for disease control., Pathogen-informed Resistance to Oomycete diseases in Ecosystems, Agriculture and Aquaculture, Plant Health in relation to microbial influence, Plant immunity in the field in relation to microbial influence
OpenAlex ID: W7223551501