Arbuscular mycorrhizal fungi (AMF) are widespread soil dwelling microorganisms that associate with plant hosts. AMF receive carbon from the host as a result of the mutualism, while the plant’s ability to acquire nutrients is enhanced by AMF. Additionally, AMF benefit their host in the form of pathogen protection. While it is known that increased AMF species richness positively correlates with aboveground plant productivity, the relationship between AMF diversity and pathogen protection is not well understood. In a growth chamber study, the plant host Leucanthemum vulgare, a non-native plant species in North America, was introduced to all combinations of three AMF species either in the presence or absence of the plant root pathogen Rhizoctonia solani. In the presence of the pathogen, the plant host increased its dependence on the AMF symbiosis. However, the richest AMF species assemblage did not provide the greatest pathogen protection. Understanding how diverse groups of AMF protect plants from pathogen attack provides insight into how plant communities are formed and structured. / NSERC