Jamie Morton is the NZ Herald's science reporter.

Why is killer disease drawn to our kauri?

This kauri in the Huia Forest is one of nearly 20 per cent of kauri in the Waitakere Ranges now suffering from dieback disease. Photo / File
This kauri in the Huia Forest is one of nearly 20 per cent of kauri in the Waitakere Ranges now suffering from dieback disease. Photo / File

Scientists are seeking to solve one of the most intriguing mysteries of the disease that's killing our kauri - why it's attracted to the native tree's scent like we're drawn to coffee.

In one of two newly-funded studies targeted at the still-Incurable kauri dieback disease, Otago University biochemist Dr Monica Gerth will investigate what makes kauri so alluring, when other tree species in the forest are left untouched.

Answering this question could lead to ways to repel the fungus-like disease or effectively mask kauri from it.

In the decade since kauri dieback was first detected, it's spread throughout kauri forests in the upper North Island; in the worst-hit Waitakere Ranges Regional Park, infection rates had doubled over five years from 7 per cent to 19 per cent of kauri.

A major way it spread was through zoospores that swam through the soil.

With their long flagella and rudders, they could move through water-logged soil at incredible speeds of up to 0.7m an hour.

Once they find their target kauri roots, they form a cyst and initiate an infection that eventually starves the tree to death.

In her study, Gerth and her team will identify the chemical signals from kauri and other native plants that attract, repel or generally disrupt the "homing" ability of zoospores.

"We're basically looking at what the spores are smelling - so we're going to test lots of different plants that are found with kauri trees in the forest to figure out what chemical compounds in the plant the spores are using as signals to navigate," Gerth said.

Next, they'll join Lincoln University researcher Dr Amanda Black in testing the effect of these compounds on the movement of zoospores through soils.

Their ultimate goal is to create new agents to protect kauri forests, Gerth said.

Zoospore repellents could potentially be applied around kauri trees to deter the spores and in the long-term, the work could also be applied to the battle against other plant pathogens.

These are problematic around the world: another host-specific form caused the Irish potato famine, and one with a more general appetite, which Gerth has been studying, is attacking many Australian natives and avocado trees.

"We think with kauri dieback disease, it's probably something similar," Gerth said.

"Because it's so host-specific with its infection, we think it will have a host-specific chemotactic response - so we think there are some chemicals kauri trees are making that the spores are swimming towards."

Another unique part of the study would be its use of Matauranga Maori (Maori knowledge), which Gerth said would prove valuable as native plants were almost certainly important sources of naturally occurring chemo-attractants or repellents.

"As the first part of our project, we're hoping to build research collaborations with the Kauri Dieback Programme Tangata Whenua Ropu, kaitiaki and other experts in ngahere kauri, who can help guide our choice of native plants besides kauri to study."

Meanwhile, in another study also supported by one of the Government's National Science Challenges, focused on biological heritage, Plant and Food Research scientist Dr Ian Horner and colleagues will develop a new programme aimed at remedial treatment of kauri otherwise doomed to die.

Of the control tools investigated to date, phosphite injection treatment is the most thoroughly studied and promising, but it may still be some time before research is completed, with widespread release of the treatment tools.

In the meantime, researchers must apply treatments under strict protocols so scientists can properly measure its efficacy and possible side effects of the treatment, helping the wider research programme and the eventual roll-out of treatment tools.

Research was already underway with a pre-programme survey to find out community attitudes and responses to the trees, the disease and treatment options.

Kauri dieback disease

• Found in the Waitakere Ranges Regional Park, along with private land throughout the Auckland region, forest plantations at Omahuta, Glenbervie and Russell in Northland, Department of Conservation reserves at Okura, Albany, Pakiri, Great Barrier, the Coromandel Peninsula, Trounson Kauri Park and Waipoua Forest in Northland, home of our most iconic kauri - Tane Mahuta.

• A new survey of the Waitakere Ranges found more than half the substantial kauri areas contained symptoms of infection, and infestation rates were worst in those areas with the highest foot traffic, including areas where intensive pest control is carried out. Rates had doubled over five years.

• People walking in areas where kauri grow could help prevent dieback spread by ensuring shoes, tyres and equipment were cleaned to remove all visible soil and plant material before and after visiting forests. They should also use cleaning stations installed on major tracks, keep to paths and away from kauri roots, and keep dogs to leashes at all times.

- NZ Herald

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