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

Abstract

Aboveground fungal pathogens can substantially reduce biomass production in grasslands. However, we lack a mechanistic understanding of the drivers of fungal pathogen infection and impact. Using a grassland global change and biodiversity experiment we show that the trade-off between plant growth and defense is the main determinant of infection incidence. In contrast, nitrogen addition only indirectly increased incidence via shifting plant communities towards faster growing species. Plant diversity did not decrease incidence, likely because spillover of generalist pathogens or dominance of susceptible plants counteracted negative diversity effects. A fungicide treatment increased plant biomass production and high levels of infection incidence were associated with reduced biomass. However, pathogen impact was context dependent and infection incidence reduced biomass more strongly in diverse communities. Our results show that a growth-defense trade-off is the key driver of pathogen incidence, but pathogen impact is determined by several mechanisms and may depend on pathogen community composition.

More diverse tree communities promote foliar fungal pathogen diversity, but decrease infestation rates per tree species, in a subtropical biodiversity experiment

“…Secondly, we show that less conserved leaf traits like leaf magnesium content and stomata size are important in determining foliar fungal pathogen richness and infestation, besides other commonly-found phylogenetically conserved traits, that are linked to the growth-persistence tradeoff, such as leaf habit and specific leaf area (Cappelli et al, 2020;Kembel & Mueller, 2014). With this our results urge future studies to include a wide range of leaf traits, including phylogenetically conserved and less conserved leaf traits to better predict foliar fungal pathogen richness and infestation.…”

Biodiversity loss underlies the dilution effect of biodiversity

“…Our prediction that biodiversity loss underlies the dilution effect was grounded in host community competence, because host communities become more competent as biodiversity is lost (e.g., Johnson et al ., 2013; Liu et al ., 2017); however, biodiversity loss could also influence the dilution effect by other potential mechanisms. As an example, biodiversity loss does not necessarily alter nutrient availability in the same way that nutrient availability influences biodiversity, with implications for higher trophic levels (Grace et al ., 2016; Cappelli et al ., 2020). Gradients that are or are not associated biodiversity loss could also differ in host abundance or density, connectivity of hosts and parasites, or host temporal turnover (Keesing et al ., 2006, 2010; Young et al ., 2014; Johnson et al ., 2015a).…”

Nitrogen availability and plant functional composition modify biodiversity-multifunctionality relationships

“…Previous studies have shown that specialist soil pathogens can drive diversity-productivity relationships by reducing biomass particularly at low diversity 23,24 . In our experiment, foliar pathogen impact was not reduced in (untreated) high diversity communities, perhaps due to a shift towards generalist pathogens 51 or because increased spillovers of pathogens (between species) counteracted resource concentration effects in diverse mixtures 52 . The absence of an effect of foliar fungal removal on biodiversity-multifunctionality relationships suggests that resources are more important context drivers than foliar enemies, but changes in other enemy groups or in foliar fungal species composition still need to be investigated further.…”

“…The plots were sprayed with two fungicides to try to achieve broad suppression of the fungal community (“Score Profi”, 24.8 % Difenoconazol 250 g.L −1 and “Ortiva”, 32.8% Chlorothalonil 400 g.L −1 6.56% Azoxystrobin 80 g.L −1 ) four times during the growing season (beginning of April and June, late July and September) and the same amount of water was applied to the untreated plots. The fungicides were very effective in suppressing rusts and mildews but less effective in reducing infection by leaf spots, and had very few non target effects 51 . The experiment was weeded three times a year to maintain the diversity levels.…”