Assembly processes and network structure of rhizosphere fungal communities in healthy and Ralstonia solanacearum-infected tomato plants.

INTRODUCTION: Bacterial wilt caused by Ralstonia solanacearum is a major constraint to tomato production worldwide. Rhizosphere microbial community is closely linked to plant health. However, how R. solanacearum invasion alters the assembly processes and co-occurrence network in tomato rhizosphere fungal communities remains largely unclear.

METHODS: Here, we collected rhizosphere soil samples from three major tomato-producing regions in China to investigate how R. solanacearum invasion affects fungal community composition, network topology, and assembly processes in tomato rhizosphere.

RESULTS AND DISCUSSION: In this study, fungal community structure differed significantly between healthy and diseased rhizosphere soils, driven significantly by soil pH (P < 0.05). The density of pathogen R. solanacearum in the diseased rhizosphere soil was significantly higher than that in the healthy tomato rhizosphere (P < 0.05). Genera Trichoderma, Talaromyces, Mortierella, and Penicillium were significantly enriched in healthy soils (P < 0.05). Network analysis revealed diseased soils had simpler and less stable fungal networks, with decreased node and edge numbers, reduced natural connectivity, and a smaller proportion of connectors. The neutral community model and null model revealed that stochastic processes explained 33.10% of the community variation in healthy tomato rhizosphere fungal communities, whereas they explained 47.40% in diseased soils, with a higher migration rate (m) in diseased soils (0.0050) than in healthy soils (0.0027). These results indicate that fungal assembly was governed by deterministic processes in healthy soils, while stochastic processes dominated in diseased soils. The fungal communities in diseased soils exhibited narrower niche breadth, and lower proportions of generalists and specialists than those in healthy soils. Collectively, these findings indicate that R. solanacearum invasion leads to niche contraction, reduced generalists and specialists, and increased migration rates in fungal communities. These changes render species coexistence more dependent on dispersal and drift, shifting community assembly from deterministic to stochastic processes and potentially reducing network stability. This study provides deeper insights into soil fungal community assembly and structure following R. solanacearum infection in tomato.