A soil-borne generalist pathogen regulates complex plant interactions

Aims Plant responses to soil-borne pathogens may be modified by neighbouring plants of other species that suppress or support the pathogen without suffering any strong effects. We assessed how the effects of the soil-borne pathogen Phytophthora cinnamomi on three susceptible species (Xanthorrhoea semiplana, Eucalyptus sieberi and Allocasuarina muelleriana) were modified by two species of acacia: Acacia pycnantha (reportedly resistant) and Acacia myrtifolia (reportedly tolerant). We hypothesised that the presence of other species in the neighbourhood triggers indirect effects of the pathogen on the highly susceptible hosts that alter the outcome of the species interactions. Methods We grew plants of the susceptible species singly or paired with either acacia species, in pots inoculated or not with P. cinnamomi and monitored survivorship over 3-8 months. Results The highest mortality of X. semiplana occurred when A. myrtifolia was the neighbour in inoculated pots. In non-inoculated pots, A. myrtifolia did not affect survival of X. semiplana. The pathogen had a strong detrimental direct effect on the performance of the susceptible host and contributed indirectly to the advantage of the tolerant neighbour. Overall, acacias were asymptomatic but served as hosts of the pathogen. However, A. myrtifolia was highly susceptible when paired with A. muelleriana. Conclusions Phytophthora cinnamomi may influence the processes that determine plant community structure, but the direction and magnitude of the effects depend on the differential impacts on the plants and their interactions.