[2][3] Phytophthora (from Greek φυτόν (phytón), "plant" and φθορά (phthorá), "destruction"; "the plant-destroyer") is a genus of plant-damaging oomycetes (water moulds), whose member species are capable of causing enormous economic losses on crops worldwide, as well as environmental damage in natural ecosystems.

[4][3][5] Phytophthora agathidicida was first discovered on Great Barrier Island in 1972 by Peter Gadgil, and was initially identified from slides as a different organism, P.

[8] In March 2006, entomologist Peter Maddison noticed a distinctly different infection in mature kauri in the Waitākere Ranges.

[1] It is not known when the microorganism arrived in New Zealand nor the source, but the centre of diversity of Phytophthora Clade 5 is believed to be in the East Asia / Pacific region.

[1] A media report of an unpublished study based on mitochondrial genomes of 17 isolates suggested that it may have existed in New Zealand for centuries and has only recently become a danger to kauri.

[15] 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".

[17] Current pathogen distribution knowledge is based on soil sampling, ground-truthing, and aerial surveillance, but that has usually been limited to stands of kauri showing symptoms, at a coarse scale.

[39] 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.

[40][41] 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.

On 20 February 2018, Auckland Council announced that all forested areas of the Waitākere 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 preventive measure, even though kauri dieback had not yet been recorded there.

[51] 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 sulfate.