Abstract
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.
Highlights
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 survival of remnant kauri is threatened by kauri dieback: a lethal root rot disease caused by the oomycete pathogen Phytophthora agathidicida (Weir et al, 2015), for which there is no known cure
Phytophthora agathidicida is probably exotic to New Zealand (NZ), but its origins, and how long it has been co-evolving with kauri forest, are unknown (Weir et al, 2015)
Summary
Phytophthora species are Oomycota (Cavalier-Smith, 2018), with many renowned for their impact as primary. The symptoms of kauri dieback disease, which are only observed during the chronic phase of (b) disease, are crown decline (a, left), as well as resin production (‘gummosis’) at the collar and lower trunk region (b). The causal agent of kauri dieback disease was first identified as Phytophthora heveae (Gadgil, 1974). When the pathogen was isolated again in 2006, the advent of a DNA-based phylogeny for the genus (Cooke et al, 2000) enabled identification of a mismatch between DNA sequences that led to a reassessment of the pathogen’s morphology, identifying it as a new species temporarily designated as Phytophthora ‘taxon Agathis’ (PTA) (Beever et al, 2009). Phytophthora agathidicida is probably exotic to NZ, but its origins, and how long it has been co-evolving with kauri forest, are unknown (Weir et al, 2015)
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