|A kauri tree with a bleeding trunk lesion|
Kauri dieback describes observed symptoms of ill health in Kauri trees, which include root rot and associated rot in a collar around the base of the tree, bleeding resin, yellowing and chlorosis of the leaves followed by extensive defoliation, and finally, death. Phytophthora agathidicida is an oomycete plant pathogen which affects the New Zealand kauri tree (Agathis australis) and has been the focus of the initial response to observed Kauri dieback. So far significantly less effort has been made to understand the other elements of the disease notably the role of other biological agents, the role of abiotic environmental changes in kauri health, or the role of stand development (Stewart 1989). As such it is somewhat unclear if P. agathidicida is preciptating or only hastening kauri death. 
Phytophthora (from Greek φυτόν (phytón), "plant" and φθορά (phthorá), "destruction"; "the plant-destroyer") is a genus of plant-damaging oomycetes (water molds), whose member species are capable of causing enormous economic losses on crops worldwide, as well as environmental damage in natural ecosystems.
The species name agathidicida means "kauri killer", from the genitive noun agathid- (meaning "of the kauri genus Agathis") and the Latin suffix -cide (from the verb cadere, to kill).
Symptoms of kauri dieback include root rot of both fine-feeder and larger structural roots; a collar rot lesion causing resin production (“gummosis”) at the collar and lower trunk region; severe chlorosis and defoliation of the canopy; and overall crown decline. Infection by kauri dieback can rapidly kill seedlings and trees of all ages. Trees of all size classes are killed in natural forest remnants, amenity garden and park trees, and kauri plantations.
Phytophthora agathidicida was first discovered on Great Barrier Island in 1972, but was initially identified from slides as a different organism, P. hevaea. Until 2006, all Phytophthora in mainland New Zealand kauri trees was considered to be Phytophthora cinnamomi; this species is extremely destructive in Western Australia, and can also infect kauri trees and produce a dieback syndrome. However, it is not considered to be as serious an issue for kauri as P. agathidicida.
In March 2006, entomologist Peter Maddison noticed a distinctly different infection in mature kauri in the Waitakere Ranges. Plant pathologists Ross Beever and Nick Waipara recognised this as a distinct Phytophthora species and it was named Phytophthora 'taxon Agathis' (abbreviated PTA). It was formally named Phytophthora agathidicida in 2015.
Phytophthora agathidicida is a species in the group of Phytophthora called ‘Clade 5’ which is defined by ITS DNA sequences. Within Clade 5 P. agathidicida can be distinguished from the other species by DNA sequence differences and oospores that have a moderately bumpy surface. In pure agar culture the optimum growth temperature is 21.5 C.
History of spread
Whether P. agathidicida is native or exotic remains unresolved. (Bellgard et. al 2013) . A recent study has suggested that it may have existed in New Zealand for centuries, possibly predating human arrival, and has only recently become a danger to kauri. However, the 100% mortality rate and speed with which the disease has spread suggests that it is a more recent arrival. Another study linked infected forests with plantings from the Sweetwater Nursery in Waipoua Forest by the New Zealand Forest Service in the 1950s. Testing, via observation and soil, has been done in the Waitakere Ranges and reported on, but the peer review section of the report is left blank.  An independent review of the epidemiology of the Kauri study, found that "the Auckland data is of limited use if we want to conduct an analysis to identify factors associated with PTA being present". 
|March 1972||First isolated from Great Barrier Island|
|1974||Publication describing this isolation, initially identified as P. heveae |
|March 2006||First found in the Waitākere Ranges|
|June 2006||First documented as a possible new species with the tag name "Phytophthora taxon Agathis"|
|2008||Declared as an "unwanted organism" under the Biosecurity Act 1993|
|2015||Formal publication of the name "Phytophthora agathidicida" |
|2 December 2017||Te Kawerau ā Maki place rāhui over Waitākere Ranges forest|
|5 December 2017||Auckland Council decided not to close Waitākere Ranges|
|15 February 2018||NZ Parliament Environment Select Committee calls for briefing on kauri dieback |
|20 February 2018||Auckland Council votes to close entire Waitākere Ranges forests|
|18 March 2018||DOC closed Goldie Bush|
|1 May 2018||MPI issues "Controlled Area Notices" for the Waitākere Ranges and parts of the Hunua Ranges|
|12 May 2018||Rahui closes Okura Bush|
|June 2018||Conservation status of kauri announced as "threatened species"|
|25 June 2018||Government announces an independent panel to review kauri dieback|
|July 2018||Found in North Shore reserves|
|August 2018||Closure of all Forest and Bird reserves that have kauri |
Phytophthora agathidicida has two forms, one lives in the soil and the other is waterborne. The soil form consists of tough spores called oospores that it is spread in infected mud tracked from tree to tree. This form can survive in dried soil on boots and equipment for up to 10 years or more. The waterborne form is a spore called zoospore that can sense a kauri tree’s roots, and swim towards them using a tail-like flagellum. It can move up to 3 m per year through waterlogged soil towards kauri roots. The “natural” spread of the disease via the waterborne zoospore occurs downhill in the soil's water film and by being carried down in watercourses. 
P. agathidicida spores can be carried in soil the size of a pinhead. One key vector for spread of the disease is believed to be human activity. This can be seen as 71% of the infected trees in the Waitakere Ranges are within 50 metres of public walking tracks.
Phytophthora zoospores require water to germinate from the oospore, and can migrate by themselves through waterlogged soil. P. cinnamomi zoospores remain mobile for up to 10 hours, and can swim at speeds of up to 58 centimetres per hour. However, their direction of travel frequently changes, so they disperse no more than 6 cm by swimming. Consequently, long-distance dispersal of Phytophthora depends on other forms of transport.
Feral pigs have been blamed for the spread of kauri dieback due to their tendency to gnaw on the roots of kauri trees, and to transport infected soil on their snouts and trotters. Research in 2017 suggests the transport of infected roots via the gut of pigs is a relatively minor vector for the spread of the disease.
While only kauri trees develop the characteristic dieback disease following infection, it appears that seven other native New Zealand forest plants can act as hosts for the pathogen without showing symptoms themselves. These include Dracophyllum, tanekaha (Phyllocladus trichomanoides), tall mingimingi (Leucopogon fasciculatus), rewarewa (Knightia excelsa) and Astelia trinervia. It is thought that the community of symbiotic mycorrhizal fungi living on the roots of healthy kauri trees may help protect them from Phytophthora infection.
Auckland Regional Council (ARC) began disease surveys and information workshops in October 2006. A Joint Agency was formed in November 2008 comprising the ARC, Northland Regional Council (NRC), the Department of Conservation (DOC) and Ministry of Agriculture and Fisheries Biosecurity New Zealand (MAFBNZ), to develop joint communications and share information.
This was replaced in November 2009 by the National Kauri Dieback Management Programme, sponsored and funded by MAFBNZ, DOC, ARC, NRC, Environment Waikato (EW), Environment Bay of Plenty (EBoP), and tangata whenua. This began a five-year national programme of research and science oversight, surveillance, education and outreach. In 2014 this was renewed for a further 10 years.
Footwear cleaning stations
To prevent the spread of the disease, footwear-washing stations have been set up at the entrances and exits of walking tracks. These cleaning stations provide Trigene detergent spray (also known as Sterigene) and scrubbing brushes or grates. However, research indicates that the effectiveness and compliance is poor. A 2016 report estimated that 83% of track users were failing to clean their footwear at the cleaning stations provided.
DOC has since conducted research to improve the design of these cleaning stations to find out whether other designs are more likely to be used. Some improved designs were trialled during the Hilary running event in the Waitakeres and have been installed in the Waipoua forest.
Closing of forests
On 2 December 2017 the local iwi Te Kawerau ā Maki placed an unofficial rāhui (traditional prohibition) over the kauri forest that covers large areas of the Waitakere Ranges to try to slow kauri dieback's spread, but initially at least the rāhui was frequently breached by the general public. The Waitakere Ranges were consequently the only large area of forest close to West Auckland that was still open to the public. On 20 February 2018, Auckland Council announced that all forested areas of the Waitakere Ranges would be closed to the public, as the rāhui had not been effective. At-risk parts of the Hunua Ranges to the south-east of Auckland were also to be closed as a preventative measure, even though kauri dieback had not yet been recorded there.
A number of compounds have been identified which are active in vitro against the pathogen responsible for kauri dieback, including common broad-spectrum antifungal agents such as copper sulphate. However, there is as yet no established treatment for infected trees, with the large size of mature kauri trees and the remote location of many infected areas making any treatment challenging. Trials are underway using injections of phosphite into infected kauri trees, as this is an established treatment against Phytophthora cinnamomi in fruit trees such as avocados. Ian Horner at Plant & Food Research began studying the effectiveness of phosphite in 2010. Phosphite injected directly into the trunk of affected trees has been shown to stop the progress of the disease, halting sap bleeding, enabling new bark to grow and prolonging the tree's life. This treatment is effective on trees of a consistent size, but research continues on effective dosages for trees of different sizes. Phosphite treatment can be effective against individual kauri trees, but will not kill the Phytophthora pathogen in the soil, potentially allowing it to continue to spread to healthy trees nearby.
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