Phytophthora agathidicida (PTA), the fungus-like organism that causes kauri dieback, has been in New Zealand much longer than previously thought, a study led by Bio-Protection Research Centre researchers based at Massey University suggests.
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,” Dr Winkworth says.
“However, our results suggest PTA was diversifying in New Zealand kauri forests around 300 years before that. It must have arrived even earlier. 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,” Dr Winkworth says.
“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 Waitākere Ranges, perhaps as the result of human activity.”
These results raise an important 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,” Dr Winkworth says.
There are several ways this might have happened. One is that genetic changes to PTA have 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,” Dr Winkworth says.
An alternative is that environmental changes have resulted in the disease emerging. The research results are consistent with this possibility.
“Since humans arrived, we have been altering New Zealand environments. Perhaps the combination of heavily fragmenting the kauri forests together with ongoing human-mediated disturbance and climate change has 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,” Dr Winkworth says. “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, robust 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.” Dr Winkworth says. “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.”
This Massey University-led research has been largely funded by the Bio-Protection Research Centre, and has involved researcher contributions from Manaaki Whenua Landcare Research, Toi Ohomai Institute of Technology, Scion, and the University of Auckland.