Is provenance or phylogeny a better predictor of growth and survival of a soil pathogen in leaf litter?

Abstract

Exotic plants have the potential to increase pathogen inoculum that can affect native plants. New Zealand's iconic kauri tree (Agathis australis) is threatened by disease caused by Phytophthora agathidicida, which is most prevalent in fragmented forests that have been invaded by or are adjacent to populations of exotic species. Exotic plants have been introduced intentionally (i.e., plantations and pastures) and unintentionally along the margins of kauri forests, yet it is unclear if invasive species play a role in pathogen spread. To determine the extent to which native and exotic plant litter supports P. agathidicida inoculum, we performed a phylogenetically controlled detached leaf assay. We inoculated 60 native and 44 invasive species’ leaves with three isolates of P. agathidicida collected from two different geographical regions of New Zealand, measured disease symptoms and re-isolated the pathogen from infected leaves. Lesions grew larger and faster on exotic leaves across all three isolates tested. However, pathogen recovery was not necessarily more likely from exotic leaves. In contrast, one of the three isolates grew faster when recovered from native compared with exotic leaves. Phylogeny did not predict disease expression. This data suggests that native and exotic plant litter may be reservoirs for P. agathidicida, but reservoir potential varies among isolates. These results also support key management tools used in New Zealand aimed at reducing pathogen spread by foot traffic in fragmented kauri forests, such as hygiene stations for shoe cleaning at trailheads and boardwalks in sensitive forest areas. Further, these tools may benefit forest management worldwide, as pathogens and exotic, invasive species increase at a global scale.