Plant diseases are often rarer in species-rich ecosystems than in agricultural monocultures. However, it is unclear how plant species composition contributes to this pattern. In this thesis we looked at pathogenic fungi that infect plant roots. These fungi play an important role in the dynamics of natural ecosystems, but also cause substantial losses in agriculture. We experimentally investigated how belowground plant-fungal interactions lead to disease patterns in mixed plant communities. It appears that specific neighboring plants can regulate the belowground disease risk of host plants. These effects were not simply a result of whether the neighboring plants were host plants, but also depended on the age and root density of the neighboring plants. These findings provide inspiration for the targeted design of mixed cropping systems that are intrinsically more robust against soil-borne diseases.