Sick plants in grassland communities: a growth-defense trade-off is the main driver of fungal pathogen abundance and impact

Aboveground fungal pathogens can substantially reduce biomass production in grasslands. However, we lack a mechanistic understanding of the drivers of fungal infection and impact. Using a global change biodiversity experiment we show that the trade-off between plant growth and defense is the main determinant of fungal infection in grasslands. Nitrogen addition only indirectly increased infection via shifting plant communities towards more fast growing species. Plant diversity did not decrease infection, likely because the spillover of generalist pathogens or dominance of susceptible species counteracted dilution effects. There was also evidence that fungal pathogens reduced biomass more strongly in diverse communities. Further, fungicide altered plant-pathogen interactions beyond just removing pathogens, probably by removing certain fungi more efficiently than others. Our results show that fungal pathogens have large effects on plant functional composition and biomass production and highlight the importance of considering changes in pathogen community composition to understand their effects