The potential of undersown species identity vs. diversity to manage disease in crops

1. In the absence of chemical control with its negative side effects, fungal pathogens can cause large yield losses, requiring us to develop agroecosystems that are inherently disease resistant. Grassland biodiversity experiments often find plant species diversity to reduce pathogen pressure, but whether incorporating high biodiversity levels in agricultural fields have similar effects remains largely unknown. 2. We tested if undersown plant species diversity could reduce barley disease, and whether the effect was mediated through above- or belowground mechanisms, by combining an agricultural field trial with a soil transplant experiment. 3. As predicted, barley disease decreased in the presence of undersown plants. Undersown species richness had no effect, but their abundance led to early season disease reduction. Aboveground mechanisms underpinned this disease reduction. Barley yield slightly decreased with increasing undersown species richness, and undersown species varied in their impact on yield. 4. We identified two undersown species with similar functional traits that contributed most to disease reduction and had the potential to increase barley yield. Furthermore, our results indicate that aboveground mechanisms caused this. We show that agroecosystem functioning can be improved without trade-offs on yield by targeted selection of undersown species. ### Competing Interest Statement The authors have declared no competing interest.