Aims <p>Root pathogens are known to structure plant communities. Yet, our understanding of the belowground transmission of pathogens remains limited. We propose that root systems act as a contact structure in soil, structuring transmission pathways between plants for fungal root pathogens.</p> Methods <p>Here, we investigated how three characteristics of contact structures affect disease transmission of a fungal root pathogen (<i>Rhizoctonia solani</i>) in a focal host plant species (<i>Leucanthemum vulgare</i>) in communities with host or non-host species. We experimentally manipulated 1) possibility of direct root contact between focal plants, 2) neighbour root identity, and 3) neighbour plant density. We used two neighbouring species: an alternative host (<i>Plantago lanceolata</i>) and a non-host (<i>Anthoxanthum odoratum</i>).</p> Results <p>We found that without neighbours, direct root contact between focal host plants had little effect on disease transmission. However, with neighbouring hosts, disease transmission was most enhanced when their roots were aggregated at intermediate plant densities. This likely occurred because host neighbour roots provided additional nutrient ‘patches’ for the fungal pathogen. Conversely, with neighbouring non-hosts, transmission was most reduced when their roots were aggregated at the highest plant density. This suggests that a ‘root wall’ formed by non-host neighbours can block pathogen transmission.</p> Conclusions <p>Our work reveals that roots can function as contact structures for fungal root pathogens. Species-specific spatial variation in root density and root contact can thus steer belowground pathogen transmission in diverse plant communities. These insights may contribute to designing diverse plant communities in agricultural systems that minimize the risk of belowground disease transmission.</p>