Building Bridges Belowground: Pythium oligandrum En-hances Rhizosphere Network Connectivity

Biological control agents (BCAs) offer a potential sustainable alternative to fungicides for crop dis-ease 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 con-trol and the rhizosphere microbiome of starch potato cv. Kuras in a field trial. We compared a labora-tory 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 com-munity composition at flowering, while bacterial community richness varied among sampling times in untreated controls. However, bacterial co-occurrence network analysis revealed pronounced treat-ment-specific changes in microbial connectivity. Application of the laboratory strain increased net-work connectivity, resulting in almost a threefold increase in mean degree and greater interconnected-ness 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. Togeth-er, 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 in-tegrating microbial network structure with community-level analyses when evaluating the ecological effects of BCAs.