RA3 PAPER: Phytophthora associated with kauri

Phytophthora Communities Associated with Agathis australis (kauri) in Te Wao Nui o Tiriwa/Waitākere Ranges, New Zealand

April 2024

Hunter S, Horner I, Hosking J, Carroll E, Newland J, Arnet M, Waipara N, Burns B, Scott P, Williams N. 2024. Phytophthora Communities Associated with Agathis australis (kauri) in Te Wao Nui o Tiriwa/Waitākere Ranges, New Zealand. Forests 15(5) 735

ABSTRACT

Studies of Phytophthora impacts in forests generally focus on individual species without recognition that Phytophthora occur in multispecies communities. We 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 health monitoring was conducted on 767 randomly selected mature kauri trees. Phytophthora species were detected using both soil baiting and DNA metabarcoding of eDNA. Four species were detected with soil baiting (P. agathidicida, P. cinnamomi, P. multivora, and P. pseudocryptogea) 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 arrivals. P. agathidicida presence was strongly correlated with declining canopy health, confirming its role as the main river 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 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.

KEYWORDS

Phytophthora; Agathis australis; kauri dieback; soil baiting; metabarcoding; New Zealand; epidemiology; invasive pathogen; forest management