Deciphering the interactions between plant species and their main fungal root pathogens in mixed grassland communities

1. Plant diversity can reduce the risk of plant disease, but positive, and
neutral effects have also been reported. These contrasting relationships
suggest that plant community composition, rather than diversity per se,
affects disease risk. Here, we investigated how diversity and composition
of plant communities drive root-associated pathogen accumulation
belowground. 2. In a temperate grassland biodiversity experiment,
containing 16 plant species (forbs and grasses), we determined the
abundance of root-associated fungal pathogens in individual plant species
growing in monocultures and in 4-species mixtures through Illumina MiSeq
amplicon sequencing. 3. In the plant monocultures, we identified three
major fungal pathogens that differed in host range: Paraphoma
chrysanthemicola, associated with roots of forb species of the Asteraceae
family, Slopeiomyces cylindrosporus, associated with grass species, and
Rhizoctonia solani, associated with multiple forb and grass species. In
mixtures, there was no significant reduction in relative abundance of
these pathogens in their host species as compared to monocultures.
However, in mixtures, there was a significant increase in relative
abundance of each pathogen in several non-host and host plant species.
Across mixtures, plant community composition affected pathogen relative
abundance in individual plant species. This effect was driven by the
presence of a particular neighbouring plant species (depending on the
pathogen), rather than functional group composition (i.e. grass/forb
ratio) or averaged pathogen pressure (based on monocultures) of all
neighbours. Specifically, the presence of neighbour host species Achillea
millefolium significantly increased P. chrysanthemicola, but decreased R.
solani relative abundance in several host and non-host plant species in
mixtures. 4. Synthesis: Our results indicate that interactions between
different plant species – both host and non-hosts – and fungal pathogens
underlie effects of plant diversity on root pathogen abundance. Non-host
species may act as pathogen reservoirs in diverse plant communities, as
they harboured certain pathogens in mixtures, but not in monocultures.
Additionally, particular host species can strongly affect pathogen
abundance in other (host and non-host) plant species in plant mixtures,
suggesting clear effects of species identity in the diversity-disease
relationship. Belowground disease risk thus depends on plant community
composition rather than diversity per se, via specific interactions
between plant species and their root-associated pathogens.