Kauri are a tuakana species in Aotearoa – they are like the older sibling, towering above the ngahere (forests), giving protection for the younger organisms.
Kauri forests once covered over a million hectares of Aotearoa, from the Far North to Te Kauri, near Kawhia. The number of kauri trees was reduced substantially by intensive felling during early European colonisation. Today, only about 142 hectares remain, and although it’s a protected species, the kauri faces a new and even greater threat.
Kua uru te mate kauri dieback ki ngā ngahere maha o te raki o Tāmaki-makau-rau. Pēra i te Waipoua te tūrangawaewae o te kauri rongonui o te motu – o Tāne-mahuta. (Kauri dieback disease has infiltrated many forests north of Auckland including the Waipoua Forest in Northland, home to our most iconic kauri – Tāne Mahuta.)Dr Ocean Mercier
Threat of disease
Kauri dieback (Phytophthora agathidicida) is a microscopic plant disease that spreads through soil. The disease infects kauri through the roots growing near the surface of the soil. Symptoms include gum loss from damage to the trunk, yellowing of the foliage, leaf loss and canopy thinning. Nearly every kauri that contracts the disease eventually dies.
Scientists are increasingly concerned that kauri dieback could actually lead to the extinction of the whole species. This threat has brought together the Ministry for Primary Industries, the Department of Conservation, Landcare Research and four regional councils, all working alongside North Island iwi to save the kauri.
For those of us that are working with this challenge, it affects us at a spiritual level. From a Māori point of view, these kauri are our brothers, we whakapapa back to the same place.Ian Mitchell
Biology of the microorganism
At Manaaki Whenua – Landcare Research in (Auckland), Dr Stanley Bellgard is part of a team looking for ways to stop the disease spreading. They’ve discovered that P. agathidicida produce oospores (resting spores). These have a tough outer surface and can lie dormant in the soil for more than 3 years. They germinate to form sporangia – structures that produce zoospores. Zoospores are mobile spores that travel through the water in soil and are normally released during and after heavy rain. Zoospores attach to root surfaces and germinate to produce mycelia (long threads), which infect kauri root systems. More sporangia are produced in the infected root system, and the infection worsens and spreads.
In the laboratory, P. agathidicida is detected by a bioassay. Pieces of plant material are used as bait to ‘fish’ the P. agathidicida out of the soil. The spores of P. agathidicida travel through the water and infect the plant tissue. Scientists then grow the microorganisms on an agar dish. Dr Bellgard says, “The most challenging aspect of this organism is that it is microscopic and remains hidden in the soil. It can only be seen under a compound light microscope.”
Investigating methods to fight kauri dieback
Dr Bellgard and his team are testing the efficacy of a chemical called TriGene (now called SteriGENE). TriGene has been used successfully to manage Phytophthora cinnamomi, a dieback disease in Western Australia. Scientists have discovered that a 2% solution of TriGene will kill the mycelium and zoospores of P. agathidicida. However, P. agathidicida also form long-lived, thick-walled oospores, and these are more resistant to TriGene .
Using TriGene as a spray to control P. agathidicida would also kill beneficial microorganisms and so is not an option. There are many other organisms in the soil that live with and benefit kauri, so the team have to be very selective about how they go about managing this disease.
Dr Ian Horner from Scion is investigating other ways to fight P. agathidicida. He is trialling the use of phosphorous acid (phosphite). Phosphite has been used in the horticultural industry for treating trees with similar Phytophthora diseases. First, the scientists calculate the volume of phosphite that the tree might require based on its size. They estimate one injector for every 200 mm of circumference. Holes are drilled into the tree, and the phosphite is injected into the tree.
Dr Horner and his team started with glasshouse trials using kauri seedlings, which were deliberately infected with the P. agathidicida pathogen. Next, some trees were treated with phosphite and some were not. The untreated trees all died within a few weeks of inoculation. The majority of the trees injected with phosphite survived. However, a kauri forest is a totally different system. To scale the trial up to treat larger kauri will have challenges.
A third area of research is looking at overseas kauri species that may be immune to kauri dieback. There has been testing on the Queensland kauri – Agathis robusta. Plants that were inoculated three years ago are still living, which could mean that the Queensland kauri is resistant to P. agathidicida. The challenge is how to utilise this research further to benefit the New Zealand kauri species.
Kia ora tonu ai te kauri i tōna manawakioretanga (With the survival of kauri in the balance), scientists, councils and iwi have joined forces to try to save the iconic tree.Dr Ocean Mercier
Scientists need time to find a way to fight kauri dieback. Preventing the spread of the disease is crucial, so the challenge to the kauri dieback team is how to control it. P. agathidicida are in the soil and can cause infection at any time. The spores are spread by people transferring soil from one forest to another, most commonly on their shoes. It can also travel in water in the soil, so it spreads easily and infects other kauri once it is established in an area.
Scientific findings combined with the knowledge of mātauranga Māori are vital to informing decisions on how to combat the spread of the disease. At present, there is no cure to eradicate the disease from New Zealand. The only strategy available at present is to stop it from spreading to areas that are free of the disease.
Nature of science
When faced with a challenge
Individually, and as communities, we all have a part to play in not helping to spread the disease.
Our tūpuna were very careful about how you went into and came out of the forest. You made sure that you were clean when you went in, you made sure you were clean when you came out, and you did that not only physically but in terms of cleaning your wairua and all of those things as wellHori Parata
Some simple actions we can all take:
- Make sure shoes, tyres and equipment are cleaned to remove soil and plant material before and after visiting any kauri forest.
- Use cleaning stations installed on major tracks, scrub all footwear to remove soil and spray with the disinfectant supplied.
- Stay on the tracks and avoid walking over kauri roots.
- If you see any signs of infected trees, alert the national kauri dieback team on 0800 NZ KAURI.
Side by side, western science and mātauranga will give us some really exciting ways to protect our kauri.Nick Waipara
Watch Series 2/Episode 10: Kauri Dieback: Death in the Ngahere
Project Mātauranga is a television series that investigates Māori
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