Kauri (Agathis australis), which is one of the world's largest and longest‐living conifer species, is under threat from a root and collar dieback disease caused by the oomycete pathogen Phytophthora agathidicida. The noted incidence of kauri dieback has increased in the past decade, and even trees >1000 years old are not immune. This disease has profound effects on both forest ecosystems and human society, particularly indigenous Māori, for whom kauri is a taonga or treasure of immense significance. This review brings together existing scientific knowledge about the pathogen and the devastating disease it causes, as well as highlighting important knowledge gaps and potential approaches for disease management. The life cycle of P. agathidicida is similar to those of other soilborne Phytophthora pathogens, with roles for vegetative hyphae, zoospores and oospores in the disease. However, there is comparatively little known about many aspects of the biology of P. agathidicida, such as its host range and disease latency, or about the impact on the disease of abiotic and biotic factors such as soil health and co‐occurring Phytophthora species. This review discusses current and emerging tools and strategies for surveillance, diagnostics and management, including a consideration of genomic resources, and the role these play in understanding the pathogen and how it causes this deadly disease. Key aspects of indigenous Māori knowledge, which include rich ecological and historical knowledge of kauri forests and a holistic approach to forest health, are highlighted.

Kauri dieback is a pest issue that is increasingly affecting kauri forests A water and soilborne pathogen Phytophthora taxon Agathis (PTA) has been identified as a causal agent of kauri dieback at multiple locations particularly within Auckland and Northland In 2008 a passive surveillance and adaptive management programme was initiated to manage the disease across the natural range of kauri Surveys were initially undertaken to determine the distribution and rate of spread of kauri dieback on private land in the Auckland region Methods to evaluate and monitor overall tree health disease symptoms and other potential contributing factors were developed Diagnostic sampling was undertaken to isolate and identify pathogens associated with kauri dieback Along with PTA other Phytophthora species and environmental stress were frequently associated with symptoms at over 400 properties inspected Further management is now required to develop control tools and mitigate further spread

In 2012, trials were established in four kauri forest sites severely affected by kauri dieback (Phytophthora agathidicida = P. taxon Agathis, PTA) to determine the potential of phosphorous acid (phosphite) as a control tool. Baseline assessments of 162 trial trees included canopy disease rating, trunk lesion dimensions and lesion activity (recent bleeding/ oozing). Phosphite (Agrifos®600) at concentrations of 7.5% or 20% was injected (20 ml) at 20-cm intervals around the trunk. Control trees were left untreated. After 1 year, half the previously injected trees were re-injected, in all cases with 7.5% phosphite. Phytotoxicity symptoms (leaf yellowing, browning or leaf/twig abscission) were noted in some phosphiteinjected trees, particularly where the 20% concentration was used. After 3 years, many more trunk lesions remained active (expressing ooze, continued expansion) in untreated trees (58.5%) than in phosphite-treated trees (0.8%). Average lesion expansion after 3 years was 12.7 cm in untreated and 0.4 cm in phosphite-treated trees.

In kauri forest soils surveys Phytophthora taxon Agathis (PTA) P cinnamomi P multivora and P cryptogea were detected frequently In vitro and glasshouse studies determined that all four Phytophthora species produced lesions on excised kauri leaves and stems Lesion advance was significantly slower with P cinnamomi P multivora and P cryptogea than with PTA When 2yearold kauri seedlings were trunkinoculated lesion spread was rapid with PTA trunks were girdled and all trees died within 46 weeks Phytophthora cinnamomi P multivora and P cryptogea produced substantially smaller lesions than PTA no trees died and plant growth was only slightly suppressed Following soil inoculation with PTA all kauri seedlings died within 10 weeks There were no deaths following soil inoculation with P cinnamomi P multivora or P cryptogea although feeder root damage was observed and the respective pathogens were reisolated Results suggest that PTA is an aggressive pathogen and the other three species are weaker pathogens of kauri

Phytophthora taxon Agathis (PTA) is a serious problem in Auckland and Northland kauri forests Phosphorous acid (phosphite) is a potential treatment for infected or threatened trees In vitro tests on phosphiteamended agar showed that PTA was more sensitive to phosphite than other Phytophthora species commonly controlled by this chemical Before progressing to forest trials phosphite efficacy was tested on PTAinoculated kauri seedlings in the glasshouse Twoyearold kauri seedlings were inoculated with PTA applied directly to trunk wounds or by soil application Phosphite was applied as a foliar spray as a trunk injection or as a soil drench either 5 days before or 5 days after inoculation All untreated control trees died whether trunk or soilinoculated With phosphite injection survival was 100 following PTA soil inoculation and 67 following trunk inoculation Foliar spray and soil drenchapplied phosphite treatments were less effective than trunk injection although some trees survived