Diversifying planted forests to reduce the risks associated with large scale disturbances, such as pathogens, is a major aim of sustainable forest management. Previous meta-analyses have shown that insect pest damage is lower in mixed forest stands compared to monocultures, but the same has not been shown for pathogens. Here, we provide the first systematic review and meta-analysis specifically testing the effects of tree species diversity on pathogen damage. Relevant studies were retrieved using bibliographic databases and internet searches, as well as previously unpublished data sets contributed by stakeholders. We found that more diverse forest stands overall had significantly lower pathogen damage, and that this result was most pronounced in temperate forests for which the most studies were available. Although in some cases tree diversity had a strong effect, this was not universal and was influenced neither by pathogen specialism, nor by the presence of alternative hosts in stands. Several studies reported specific effects of tree neighbours in mixed stands, including impacts on the microclimate of stands. Future work should focus on mechanistic explanations that could underpin neighbour identity effects in mixed forests. We suggest the use of the disease triangle as a tool for considering the multiple factors that can influence pathogen damage in mixed forest stands. Keywords: Mixed stands; Tree diversity; Forest pathogens; Sustainable forest management
