Mātauranga-guided screening of New Zealand native plants reveals flavonoids from kānuka (Kunzea robusta) with anti-Phytophthora activity

Kauri is an ecologically important and culturally treasured tree species in Aotearoa New Zealand. It is under threat from the pathogenic oomycete Phytophthora agathidicida, which causes kauri dieback disease. We hypothesised that mātauranga Māori (Māori knowledge) of kauri forest health could be used to identify native plants that produce anti-Phytophthora compounds. We tested this hypothesis by using knowledge descended from Te Whare Wananga o Ngāpuhi to select and screen four native plants for activity against P. agathidicida and also P. cinnamomi (a broad host-range pathogen). Extracts of kānuka (Kunzea robusta) were active against various life cycle stages. Bioassay-directed isolation led to three flavanones, previously unreported from New Zealand Kunzea, as the main bioactives. These compounds have not previously been reported as having anti-Phytophthora activities. They inhibited P. agathidicida zoospore germination with IC₅₀ values of 1.4–6.5 µg/mL, making them the most potent inhibitors reported against this stage of the life cycle. The three flavanones also inhibited zoospore motility at 2.5–5.0 µg/mL, and showed some inhibition of mycelial growth at 100 µg/mL. They were generally less active against P. cinnamomi. Overall, the results from this study emphasise the value of using mātauranga Māori in the response to kauri dieback.