Kauri Dieback Disease
Kauri Dieback disease is killing our kauri trees.
No matter how well we clean our shoes, the microscopic spores that cause Kauri Dieback Disease are being moved around tracks in the Waitākere Ranges. Te Kawerau ā Maki have placed a Rāhui over the entire Waitākere Forest (Te Wao Nui o Tiriwa). For the health of the forest, they are asking people to stay away from the bush to give scientists time to develop a solution, and time for our forest to heal.
What is Kauri Dieback Disease?
Kauri Dieback is caused by a microscopic water mould called Phytophthora agathidicida. This mould is new to science and takes two forms.
The soil form of this mould are known as oospores. They can survive in dried soil, on boots, and on equipment for up to 8 years (and counting) and it only takes a pinhead of soil to transfer them to a new site. They will germinate and form a structure (called the sporangia), which produces the mould’s water form, the zoospore.
Zoospores have a tail that can swim and are often released during and immediately after rain. Although this form is short-lived and can be killed by seawater, the zoospores can still move through the water film in soil. An individual zoospore will only move about 6cm in its lifetime, but overall this natural spread of the disease has been estimated as up to 3 metres per year.
How does kauri dieback spread?
Kauri Dieback is spread almost exclusively by soil disturbance. The greater amount of disturbance, the greater risk of spread.
People are the biggest cause of soil disturbance in the Waitākere Ranges and can transport the disease around the forest on their feet and on outdoor equipment. Collectively, people can transport multiple kilograms of soil around the forest. In the Waitākere Ranges, we see a high correlation with infection and the track networks, which is further proof that people are spreading the disease at a rapid rate.
Pigs also cause soil disturbance while rooting around for food on the forest floor when there is nothing else to eat. When pig numbers in the Waitakere Ranges were reduced by intensive hunting, the vegetation began to regenerate and the pigs then fed on that rather than digging up the forest floor. This reduced the soil disturbance impact (and therefore the risk of kauri dieback spread) of pigs by 90-95%.
Small animals like birds, rats, possums move miniscule amounts of soil and are less of a risk. Dogs pose a moderate risk and should be kept on leads at all times in forested areas.
Do all trees that get kauri dieback die?
Yes. All trees, no matter their age, size or state of health, will eventually die if they are infected with Kauri Dieback disease trees.
Is there a cure?
No, although scientists have been trialling a treatment that involves injecting trees with a chemical called phosphite. This can keep trees alive, but is not a cure and does not get rid of the infection, immunise the tree, or treat the disease in the soil.
What about natural resistance?
We have not yet found any resistant trees, although there is ongoing research by Scion to investigate this further.
It is unlikely that kauri will have natural resistance to this disease because their population has been reduced significantly, lowering the diversity in their gene pool.
Since logging cleared 99.9% of kauri from their natural range we now only have 0.1% of the gene pool for kauri remaining. Kauri live for 800-5000 years, so it is impossible for a species with that lifecycle to evolve resistance to a pathogen which is killing them within a few years.
When existing trees die, other forest species that rely on kauri will die with them. In the unlikely event of resistant kauri being found, we would need to replant them everywhere and wait 200 years or more for a forest to be created. We simply cannot rely on any of this.
Can we get a kauri seed bank so we can replant kauri if we find a cure?
Kauri seeds don’t survive long and must germinate within about 4 months. In the forest, kauri seedlings germinate quickly and remain small for decades waiting for a gap in the canopy to appear. When a part of the canopy is cleared, they all shoot up towards the light. This is why you find groves of kauri rickers that are all the same age, they are brothers and sisters.
Where Phytophthora agathidicida is in the soil the seedlings don’t survive. If we don’t stop the spread of Kauri Dieback around our forests, we may see the last generation of kauri.
Once an area is infected with Kauri Dieback, it can not be replanted as seedlings will die.
Why do we need to worry about an area or close tracks if the trees are already dead?
Phytophthora agathidicida remains in the soil well after kauri die and can still be moved to other areas to infect new trees. It may also infect other species so containing and preventing spread from infected areas is a priority.
Does kauri dieback kill other trees?
A 2016 Master’s thesis by Jessica Ryder found that that Phytophthora agathidicida can infect other native plant species such as tanekaha and rewarewa. No other research work has yet been done on the host range for this disease. Evidence from other similar Phytophthora in Australia shows that a single host is highly unlikely and that when levels of the pathogen reach higher levels in the soil, other species will start to show symptoms and may die.
How widespread is kauri dieback in New Zealand?
Many other forests are known to be infected but the extent of infection within those forests is not known. This is because comprehensive surveillance and monitoring has not yet been done.
Only forests in the Auckland Region have been systematically monitored, both from the air and on the ground. This is how we know that Kauri Dieback in the Waitākere Ranges has more than doubled in 5 years from 8% to 19%.
Over 22,400 individual trees have been surveyed in the Waitakeres. The only forests in the country known to be free of kauri dieback are the Hunua Ranges, most of the Coromandel and most offshore islands. Keeping the infection out of these forests is a top priority.
What about Glyphosate, 1080 or other toxins, is that causing kauri dieback?
No. The water mould Phytophthora agathidicida is causing this disease, nothing else. 1080 has never been used in the Waitākere Ranges and perfectly healthy trees are killed by the disease.
What other factors increase the spread of Kauri Dieback?
Anything that compromises tree health and stresses trees, such as climate change, drought, excess water, pests, driveways and buildings on roots, soil compaction, and herbicides sprayed on roots will make individual trees more susceptible to a pathogen, but these factors are not the cause of the disease.
They may be contributing to why kauri are succumbing so quickly to this pathogen, but for each individual tree it is a complex picture of a combination of factors. The majority (70%) of infected trees in the Waitākere Ranges are within 50 metres of a walking track which clearly indicates that people are the main vectors for spreading this disease.
Why does it matter if we lose kauri, it’s just one species?
Kauri is a keystone species and an ecosystem engineer. Kauri create their own soil type called a podsol in which only certain other species can survive. At least 17 other plant species depend entirely on kauri, so if we lose kauri we will lose them too. The assemblage of plants in a New Zealand kauri forest is unique, there is nothing else like it on earth. If kauri become extinct we will lose these forests forever. Kauri is a taonga to Māori, a scenic treasure and an historic resource used to build waka, yachts, houses, and furniture.
What about the other forests that don’t have kauri, won’t they be ok?
We don’t know because little research has been done on the host range for Phytophthora agathidicida so we don’t know what other species it affects. It is highly likely that it will affect other species and kill them, so if we continue to spread it around all our forests we may find in the future that it threatens the survival of other species and other forests all over the country.
How can we stop the spread?
By asking people to stay away from areas where there are infected and healthy kauri. This will remove at least 70% of the problem in the Waitākere Ranges.
At the same time pigs, the other main vector, need to be eradicated. Fencing off areas of healthy kauri ecosystem to prevent pigs and people from bringing the disease into them is necessary and a priority.
Upgrading the track infrastructure to make the tracks dry and get them off kauri roots will minimise the risk of moving soil around and spreading the disease. Boardwalks and engineered tracks that are convex and allow water to run off them instead of pooling and creating a mud slurry is what is required urgently. Tracks should be rerouted away from kauri if possible.
How effective is the sterigene spray and is it the right thing to use?
We don’t know because the research to investigate what kills Phytophthora agathidicida has not been completed, despite being a priority for almost 10 years.
However, we do know that the oospores can survive in just a pinhead of soil, so if you don’t clean your boots and equipment (bikes, walking sticks etc) first to completely remove all soil particles then spraying on top of soil will not kill the disease.
Proper cleaning is far more effective than spraying. A quick spray without cleaning achieves nothing.
You should scrub and clean your shoes and equipment thoroughly under an inside tap (not in the garden) before and after visiting any forest.
How effective are the cleaning stations?
The basic ones (crate / brush / spray bottle) are not at all effective because they are difficult to use and the diseased soil is being brushed onto the track surface for others to stand in. It is recognised that these need to be upgraded urgently. Most cleaning stations that retain the removed material are more effective, but only if they are used properly to remove every tiny particle of soil.
How long must we stay out of the forest?
Until the required track upgrades have been completed to make them dry and improved cleaning stations are put in place. The amount of money invested in these upgrades will dictate how long that will take. You can lobby your Councillors and MPs to provide the money for this work.
How do we deal with dead kauri trees, can we chop them up and remove them?
No. Dead trees with kauri dieback must remain on your property and not be chopped up. They will still be infectious (as will your soil) and cutting them up will release millions of spores of the pathogen that can easily be spread to other sites. There are Standard Operating Practices (SOP) from Auckland Council & the Kauri Dieback Programme that define how you should work around kauri. All kauri should be assumed to be infected and the relevant SOP followed.
What about predator or weed control work, can this continue?
Yes. Under the rāhui, a warrant can be issued to a group undertaking essential work to look after the forest. The warrant requires the group to be trained in the cultural understanding of the rāhui and compliance with the strict phyto-sanitary requirements for working among kauri.
Compliance can then be monitored and audited. If you want to help look after the forest please join one of the many groups undertaking this work that have been warranted to do so by Te Kawerau ā Maki or apply for your group to get a warrant.
The Council has closed some tracks, can I still use the ones that are open?
The rāhui closed all the tracks in the forest. Since 2018 Auckland Council has been working with Te Kawerau ā Maki to agree when upgraded tracks have reached the required standard and can be reopened. Tracks cannot be reopened without the agreement of Te Kawerau ā Maki. Therefore all open tracks are now able to be used without breaching the rāhui. The rāhui continues to apply to all closed tracks and all off track areas of the forest. Link to the list of open tracks is here.
It is very important to keep people on the upgraded tracks because if they go off track they will continue to spread the disease and thereby increase the scale of the problem. Please do not leave the track for any reason.
Please respect the rāhui and stay out of the closed areas of the Waitākere Ranges and any other forests which have kauri. It is not safe for people to walk near kauri on inadequate track infrastructure. It will just result in the death of more trees. Thank you for your understanding and support.
The Last Kauri
The Nukes (a local Titirangi ukelele trio) have dedicated their song Last Kauri to the Waitākere Rāhui campaign & told us we can use it however we’d like to.
They performed recently in Titirangi and did a shout out about the rāhui, asking a packed house to respect it.