T.E.R:R.A.I.N - Taranaki Educational Resource: Research, Analysis and Information Network

Mould (Kauri dieback) Phytophthora agathidicida

Domain: Eukaryota
(unranked): SAR
Superphylum: Heterokonta
Class: Oomycetes
Order: Peronosporales
Family: Pythiaceae
Genus: Phytophthora
Species: P. agathidicida
Binominal name: Phytophthora agathidicida
Synonym: Phytophthora 'taxon Agathis'
Common name: Kauri dieback, PTA, Kauri disease, New Zealand kauri dieback. New Zealand kauri disease.

Phytophthora agathidicida is a microscopic water mould that is new to science. The effects of kauri dieback disease were first noticed in the 1970s on a tree on Great Barrier Island. It was formally described in 2015 (previously it was known as Phytophthora taxon Agathis). This microorganism causes an incurable, fatal disease which affects the New Zealand kauri tree (Agathis australis). Symptoms include root rot and associated rot in a collar around the base of the tree with cankers seeping gum, yellowing and chlorosis of the leaves followed by extensive defoliation, and finally death.

The mould Phytophthora agathidicida is related to the organism that caused the Great Potato Famine that killed nearly a million people in Ireland in the1800s. As at 2018 the only forests in the country known to be free of kauri dieback are the Hunua Ranges (west of Papakura, Auckland), most of the Coromandel and most offshore islands.

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.

The consensus among experts is that the predominant vector for the spread of the disease is human activity. 71% of the infected trees in the Waitakere Ranges are within 50 metres of public walking tracks. Feral pigs also can spread it due to their tendency to gnaw on the roots of Kauri trees but their effect is nowhere near as significant as humans.

While only Kauri trees develop the characteristic dieback disease following infection, it appears that several other native New Zealand forest plants can act as hosts for the pathogen not showing symptoms themselves, they include Leucopogon fasciculatus (Soft Mingimingi, Knightia excelsa (Rewarewa), Phyllocladus trichomanoides (Tanekaha) and Astelia trinervia ((Kauri grass).
At present, there is no cure for kauri dieback, and the disease kills most if not all the kauri it infects.

New Zealand’s Kauri Dieback Programme is a collaboration between the Ministry of Primary Industries, the Department of Conservation, Auckland Council, and the Regional Councils of Northland, Waikato, and Bay of Plenty and was set up in 2009 to determine the spread of the disease, create prevention strategies, and research possible cures.

An old mature Kauri killed by Kauri dieback

A photograph of a sign found on tracks in the Waitakere Ranges informing visitors about precautions against the spread of kauri dieback disease.

A video on Kauri dieback.

Some websites on Kauri dieback with details on how to stop its spread.

Thanks to Wikipedia for text and information: https://creativecommons.org/licenses/by-sa/3.0