The unstoppable kauri-killing disease that's forced iwi to declare a rahui across the Waitakere Ranges may have been lurking in our wilderness for centuries, scientists say.
A new study led by Bio-Protection Research Centre researchers based at Massey University suggests Phytophthora agathidicida (PTA), the fungus-like organism that causes kauri dieback, has been in New Zealand for much longer than previously thought.
The disease has become prominent over the past decade, spreading throughout the Auckland region, the Coromandel, and to Waipoua Forest in Northland, the home of our most iconic kauri - Tane Mahuta.
A kauri dieback programme was launched in 2009 to halt the disease's spread, but has been largely in vain.
The Massey study comes as the Government says it will undertake a comprehensive review of the dieback programme, with Minister of Forestry Shane Jones calling it "grossly substandard to date".
Jones will be speaking with officials on Monday morning about the best course of action and will brief the Prime Minister on the review this week. Jones wants the disease to be treated in future as a major biosecurity threat.
"If it's possible for us to move swiftly and cull diseased cows and stop the transport of potentially diseased cows off private farms, we need a similar level of vigour in safeguarding areas where our kauri are still strong," he said.
This month, Auckland councillors voted to beef up measures in the Waitakere Ranges Regional Park, where infection rates have jumped from eight per cent to 19 per cent in just five years.
But the council stopped short of shutting the 16,000ha park, despite calls from environment groups and West Auckland iwi Te Kawerau a Maki, which has declared their own.
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PTA, formally identified less than a decade ago, could sense a kauri tree's roots and swim toward them using a tail-like flagella.
These could move through water-logged soil at incredible speeds of up to 70cm an hour, before forming a cyst in the roots and starting an infection that eventually starves the tree to death.
There is no cure and the disease kills most, if not all, the kauri it infects.
Plant geneticist Dr Richard Winkworth and collaborators have been using genome sequencing to investigate when PTA arrived and how it has changed since arriving.
"It had been suggested that PTA arrived in New Zealand not long before the first diseased trees were found in the early 1970s," Winkworth said.
"However, our results suggest PTA was diversifying in New Zealand kauri forests around 300 years before that."
It must have arrived even earlier, he said.
"Humans may have brought it here – perhaps the pathogen was carried to New Zealand by Polynesian settlers or the earliest European explorers – or it may even have been here before humans arrived."
The researchers have sequenced and analysed complete mitochondrial genomes of 17 PTA samples collected from sites across the geographical range of the disease.
"The samples we have collected suggest several genetic subgroups within PTA," Winkworth said.
"To better understand the history of spread through the kauri forests we need to increase our sample size.
"However, we do see, for example, that several genetic subgroups are present in the Waitakere Ranges, perhaps as the result of human activity."
The results threw up a crucial question: if PTA has been in New Zealand for at least the last 300 years, why has it only recently become a significant problem?
"The results suggest that the relationship between PTA and its host may have changed."
There are several ways this might have happened.
One was that genetic changes to PTA had made it more virulent.
"It is a possibility, but our results suggest it is not as simple as a single pathogenic form evolving and spreading through the forest."
An alternative is that environmental changes have resulted in the disease emerging.
"Our results are consistent with this possibility," Winkworth said.
"Since humans arrived, we have been altering New Zealand environments."
The combination of heavily fragmenting the kauri forests with ongoing human-mediated disturbance and climate change may have led to emergence of the disease.
"Perhaps we introduced another pathogen that, in combination with PTA, results in disease.
"If we are to fight back effectively we need to better understand the relationship between when PTA arrived, its pattern of spread, and the emergence of kauri dieback disease.
"Identifying why kauri dieback disease emerged might help us to move beyond containment to managing and controlling it."
The research team has also been developing a cheap, effective DNA test that is simple enough for community groups to use in the field, but that is as accurate as laboratory-based testing.
"We are hoping to evaluate the test in field trials in the next few months," he said.
"We hope that this will make it easier to monitor where PTA is, both for the purposes of management but also to enable further research."
The Massey-led research has been largely funded by the Bio-Protection Research Centre, with research assistance from Manaaki Whenua Landcare Research, Toi Ohomai Institute of Technology, Scion, and the University of Auckland.