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Over the past ten years, a pathogen called Phytophthora agathidicida (fi-tof-thor-ah agatha-dis-id-ah) (PA) has spread through New Zealand’s kauri forests. It causes a disease called kauri dieback, which is unique to kauri, currently incurable, and almost always fatal.

“If the kauri die out, everything in our forest will be affected,” says Will Ngakuru of Te Roroa iwi. Te Roroa are the kaitiaki of Waipoua Forest, where Tāne Mahuta stands. “In our iwi, we believe that we belong to the forest and the forest belongs to us. We need to do whatever it takes to protect the Waipoua Forest from PA.”

Te Roroa are not alone. Scientists, government �agencies, schools, iwi, and citizen volunteers throughout the country are developing measures to control and stop this “biological bulldozer” in its tracks.

by André Ngāpō

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Kauri Dieback

Tāne Mahuta, New Zealand’s largest and most well-known native tree, could soon be under attack from a microscopic assassin.

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The spread of a killer

Phytophthora literally means “plant destroyer”. It refers to a category of tiny water moulds that live in the soil and enter plants through their roots. From there, they damage the tissues that carry nutrients within the plant, causing it to slowly starve to death.

Phytophthora was detected in kauri on Great Barrier Island in the 1970s and has since spread to Auckland, Northland, and the Coromandel. In 2008, scientists identified the unique type of Phytophthora targeting kauri as Phytophthora agathidicida and labelled the disease kauri dieback.

Since then, kauri dieback has spread with alarming speed. Scientists Dr Nick Waipara and Dr Stan Bellgard have been tracking the disease in the Waitakere Ranges in Auckland. Rates of kauri dieback have almost tripled since 2011, making the Waitakeres the worst hit area in the country. In 2016, over 22,500 trees were checked and close to 20 percent of them (4,275) were infected.

Kauri dieback disease cycle

PA oospores (spores that don't move) living in the soil are introduced into a kauri forest. �This is normally done through �soil arriving on footwear, animals, equipment, and vehicles.

In moist soil, oospores germinate, or begin growing, and form sporangia – structures that produce zoospores (mobile spores).

Sporangia release zoospores into the soil after heavy rain.

Zoospores move through wet soil, and sometimes through streams, to infect nearby kauri roots.

Zoospores spread throughout the roots and up the tree by infecting tissues that transport nutrients and water to the canopy.

Oospores form within infected kauri tissues.

Oospores are released into the soil as root tissues in the tree decay.

Key

oospore

zoospore

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Masking the scent

Scientists throughout New Zealand are researching ways to limit the spread of

kauri dieback. As part of New Zealand’s Biological Heritage National Science Challenge, Dr Monica Gerth is leading an investigation into why PA is attracted to �kauri in the first place.

One way that PA spreads is through zoospores – the mobile spores that are released from PA cells when it rains. Zoospores can “swim” through wet soil to reach kauri roots, navigating by “smelling” �or chemically sensing kauri, like mosquitoes drawn towards human bodies.

Dr Gerth wants to find out why �zoospores are attracted to some plants and not others. To do this, she’ll test samples �of native plants to see which ones the �zoospores are drawn to or repelled by. “Then we can try to isolate the chemicals that the spores are sensing,” says Dr Gerth. Once the chemicals have been identified and isolated, they could be added to the soil to distract or repel zoospores from reaching kauri.

Dr Gerth and her team will draw on �the help of Māori experts in ngāhere kauri (kauri forests) to decide which native plants to test. “We believe that mātauranga Māori offers important insights for this research, as native plants are almost certainly important sources of naturally occurring attractants and repellents,” says Dr Gerth. These native plants have been used in rongoā for treating fungal infections, raising the hope that they could also prevent the spread of kauri dieback.

While the research opens exciting possibilities, Dr Gerth knows it’ll be a �slow and difficult process. “It’s similar to the battle for a cure for cancer,” she adds. “Scientists have been working on this for decades. We don’t have a cure yet, but people haven’t given up, and we’ve made tremendous progress.”

“It’s similar to the battle for a cure for cancer.”

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PA sporangia under a microscope. The dark spots are zoospores ready to be released.

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Dr Monica Gerth

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Photographs: (top left, inset “dead branches”, inset “thinning canopy”) Copyright © Nick Waipara, used with permission; (bottom left and gum bleeding) Copyright © The New Zealand Institute for Plant & Food Research Limited, used with permission; (inset “yellow leaves”) “Kauri leaves and female cones on a young tree” by Tatters from https://goo.gl/jQNrBA is licensed under CC BY 2.0. Background: “Birch Texture” by William Warby from https://goo.gl/a5zU5Z is licensed under CC BY 2.0.

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Treating the disease

While the search for a cure continues, a project �in Auckland is focusing on treating kauri that are already infected. Project leader Dr Ian Horner has found that injecting phosphite directly into trees can boost their immune system, helping them resist the symptoms of kauri dieback. Phosphite �is a chemical used by fruit growers to protect their crops against Phytophthora root rot.

When treating the trees, Dr Horner first sizes them up to decide how much phosphite is needed. He then drills holes into the trees’ trunks and screws in several syringes of phosphite solution.

“The phosphite solution moves throughout �the tree, right up into the leaves and then back down into the root system,” says Dr Horner. “There it should do its job and fight the Phytophthora, possibly for up to five years before another injection is needed.”

Citizen scientists are volunteering to be part �of the investigation. Once they’ve signed up, they’re given everything they need to inject the trees themselves and collect data to assess the treatment’s progress.

While phosphite injections aren’t a cure, the early signs are promising. Lesions in some young trees are healing. There’s hope that the injection will prolong the lives of infected kauri.

Kauri dieback symptoms

Dead branches

Thinning canopy

Yellow leaves

Gum bleeding

and lesions

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Dr Ian Horner injecting phosphite into a kauri trunk

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Fishing for Phytophthora

While PA mostly spreads through soil, recent evidence has shown that it can also move through waterways. Year 5 and 6 students from Dawson Primary, Rongomai School, and Kauri Flats School in Auckland have been working on a tool to help scientists detect how widespread this problem could be.

To find out if Phytophthora exists in rivers and streams, some scientists use small bags filled with leaves to “fish” for evidence. The bags are like small nets that are attached to a rope and cast across a river. The leaves inside the bags act as bait, attracting any Phytophthora spores in the water. After leaving the bags in a waterway for two weeks, the scientists then test the leaves for signs of infection.

However, the design of the bags makes it difficult to remove the leaves, creating a high risk of contaminating the sample. This often leads to inaccurate results. Teacher Nick Pattison challenged his students at Dawson Primary to come up with a device that solved this problem. The device needed to meet three key criteria:

The students sketched out ideas and teamed up with engineers from Fisher & Paykel to learn about materials they could use. Their final design was called a bait cassette. It was a small rectangular box made from plastic that could be filled with kauri leaves.

They created samples with a 3D printer at the Auckland University of Technology and then tested the device with Dr Stan Bellgard. The testing process revealed a few flaws in the design. The lid that allowed the cassette to be filled with leaves would often open in the water, causing leaves to fall out. The cassettes were also too light and weren’t staying submerged.

Could hold leaves as bait
Could float in water but remain partly submerged
Could be emptied without contaminating the leaves

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Original bag device

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Student innovation

Nick took these problems to his new class at Rongomai School, who refined the design even further. They adapted the lid so it could be fastened with string to keep it shut. They also added a divider so stones could be used to weigh the cassette down.

Scientists throughout New Zealand have been quick to

adopt the device, and it’s constantly being tested and improved

– currently by students at Kauri Flats School. The students

know that as long as the disease exists, there is always room

for further innovation.

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Students preparing the bait cassettes

Hole fitted on top to allow the cassette to be fastened and hung from a piece of string, stopping it from floating down river

Slots down the sides to allow water to flow through while keeping the bait from escaping

Stones to weigh the cassette down

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Signs and icons: By George Frost, copyright © Crown 2017�Photographs: (left) “New Zealand: Kauri National Park” by Eli Duke from https://goo.gl/Vmuwiu is licensed under CC BY 2.0.�(right) “Four sisters” by itravelNZ® - New Zealand in your pocket™ from https://goo.gl/b8gBqo is licensed under CC BY 2.0.

Guardians of the forest

Back at Waipoua Forest, Will Ngakuru sees the effect Tāne Mahuta has on people every day. “People love Tāne when they see him. He is impressive.” But this popularity also puts Tāne at greater risk of exposure.

Human activities are by far the biggest reason for the spread of kauri dieback. PA spores can spread from an infected area through the soil on boots, tools, and mountain-bike tyres. To limit the spread, Te Roroa and the Department of Conservation have developed two easy rules for visitors to follow:

Stick to boardwalks, platforms, and clearly marked tracks.

Use cleaning stations to brush and spray shoes

to remove PA spores.

“Most people want to help �protect kauri as soon as they hear about dieback. They’ll stick to the boardwalks and tracks; they’ll use �the cleaning stations,” says Will. “But some don’t clean their boots. Others ignore signage and go off the tracks. Some even climb the fences to go right up to the trees. We’ve got infected trees just over 100 metres away from Tāne. This is a serious problem.

We don’t want to stop people from coming here, but we care about the trees.”

Will knows that the best �international practice for stopping �the spread of Phytophthora is to �place a closure on the infected �area. This is something that Māori �have traditionally done in certain circumstances and is known as a �rāhui. “This may have to happen

one day soon,” he says.

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Kauri dieback cleaning station

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Photograph: “Kauri leaves and female cones on a young tree” by Tatters from https://goo.gl/jQNrBA is licensed under CC BY 2.0.�Background: “Birch Texture” by William Warby from https://goo.gl/a5zU5Z is licensed under CC BY 2.0.

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It’s been nearly fifty years since the discovery of kauri dieback, and the spread of PA shows little sign of slowing. However, the engagement of scientists, schools, communities, and iwi has given our kauri a fighting chance. As the search for a cure continues, it’s going to take a sustained, unified effort to keep it from claiming our giants.

Tū mai Tāne Mahuta, tū mai koe

nō neherā anō koe

Te uri whakahirahira

nō tua whakarere iho koe

Stand tall, Tāne Mahuta, stand tall as you have done for aeons of time

You the impressive descendant from the beginning of Aotearoa

Glossary

aeons – a very long period of time

canopy – highest branches of a tree forming a continuous layer of leaves

gum – a sticky substance produced by some trees that hardens when it dries.

In kauri, gum can fill in damaged areas of the tree’s trunk

immune system – biological processes that an organism uses to defend itself against disease

kaitiaki – guardians

lesions – the damaged parts of a plant or animal body, caused by injury or disease

mātauranga Māori – the knowledge base from te ao Māori

nutrients – substances, such as minerals and vitamins, that plants and animals need to live and grow

pathogen – harmful micro-organism

rāhui – ban

rongoā – traditional Māori medicine

spores – cells released from an organism (for example, fungi) that are able to grow into new organisms in the right conditions