A generalist nematode destabilizes plant competition: no evidence for direct effects, but strong evidence for indirect effects on rhizobia abundance

1. Difficulties quantifying pathogen load and mutualist abundance limit our ability to connect disease dynamics to host community ecology. For example, specific predictions about how differential pathogen load is hypothesized to drive host competitive outcomes are rarely tested. Additionally, although infection is known to affect mutualists, we rarely measure the magnitude of pathogen effects on mutualist abundance across host competitive contexts. We tested for both mechanisms in a plant-rhizobia-nematode system.

2. We paired the legume Medicago lupulina with intra- and interspecific plant competitors, with and without a generalist nematode parasite Meloidogyne sp. Relative change in plant biomass was used to determine how nematode inoculation affected plant competitive outcomes. We counted nematode galls to test for direct effects of parasitism on plant competition, and rhizobia nodules to test for indirect effects of nematode presence on rhizobia abundance.

3. Parasites were destabilizing despite similar nematode load across competition treatments. During inter- compared to intraspecific competition, nematode inoculation decreased nodulation on M. lupulina, increased nodulation on Trifolium repens, and had no effect on nodulation on Chamaecrista fasciculata.

4. We found no support for hypothesized direct effects of nematode load on competitive outcomes, and strong but idiosyncratic indirect effects of nematode inoculation on rhizobia abundance.

Methods
Data was collected from a glasshouse experiment conducted at University of Pittsburgh in 2019.