Data from: The effect of diversity on disease reverses from dilution to amplification in a 22-year biodiversity × N × CO2 experiment

Plant disease often increases with N, decreases with CO2, and increases as
biodiversity is lost (i.e., the dilution effect). Additionally, all these
factors can indirectly alter disease by changing host biomass and hence
density-dependent disease transmission. Yet over long periods of time as
communities undergo compositional changes, these biomass-mediated pathways
might fade, intensify, or even reverse in direction. Using a field
experiment that has manipulated N, CO2, and species richness for over 20
years, we compared the severity of a specialist rust fungus (Puccinia
andropogonis) on its grass host (Andropogon gerardii) shortly after the
experiment began (1999) and twenty years later (2019). Between these two
sampling periods, two decades apart, we found that disease severity
consistently increased with N and decreased with CO2. However, the
relationship between diversity and disease reversed from a dilution effect
in 1999 (more severe disease in monocultures) to an amplification effect
in 2019 (more severe disease in mixtures). The best explanation for this
reversal centered on host density (i.e., aboveground biomass), which was
initially highest in monoculture, but became highest in mixtures two
decades later. Thus, the diversity-disease pattern reversed, but disease
consistently increased with host biomass. These results highlight the
consistency of N and CO2 as drivers of plant disease in the Anthropocene
and emphasize the critical role of host biomass, despite potentially
variable effects of diversity, for relationships between biodiversity and
disease.