Nari Williams

Studies of Phytophthora impact in forests generally focus on individual species without recognition that Phytophthora occur in multispecies communities. This study investigated community structure of Phytophthora species in the rhizosphere of Agathis australis (kauri) in Te Wao Nui o Tiriwa/Waitākere Ranges, New Zealand, in the context of kauri dieback disease expression. Soil sampling and tree monitoring were conducted on 767 randomly selected mature kauri trees. Phytophthora species were detected using both soil baiting and DNA metabarcoding of environmental DNA (eDNA). Four species were detected with soil baiting (P. agathidicida, P. cinnamomi, P. multivora, and P. pseudocryptogea/P. cryptogea) and an additional three species with metabarcoding (P. kernoviae, P. cactorum/P. aleatoria and an unknown clade 7 species). Phytophthora cinnamomi was the most abundant species and was distributed throughout the forest. Both P. multivora and P. agathidicida were limited to forest edges, suggesting more recent introductions. P. agathidicida presence was strongly correlated with declining canopy health, confirming its role as the main driver of kauri dieback. The limited distribution of P. agathidicida and infrequent detections (11.0% samples) suggests that that this species is spreading as an introduced invasive pathogen and provide hope that with strategic management (including track upgrades and closures, restricting access to uninfected areas, and continual monitoring) uninfected areas of the forest can be protected. The frequent detections of P. cinnamomi and P. multivora from symptomatic trees in the absence of P. agathidicida suggest more research is needed to understand their roles in kauri forest health.