Scientists solve riddle of cabbage tree deaths

By BRIDGET CARTER

Scientists have discovered the cause of the cabbage tree disease which has wiped out large numbers of the native tree.

Landcare Research and HortResearch scientists say a bacterium called phytoplasma, which lives in plant sap, causes the leaves to turn yellow and drop.

The breakthrough - the result of a decade of research - gives plant experts hope of devising a long-term plan to manage the disease.

Landcare Research plant pathologist and ecologist Dr Ross Beever said his research team used DNA techniques and electron microscopy to rule out tree ageing, funguses, viruses, and environmental factors such as enhanced UV as the cause of the mysterious disease.

Although typical plant disease-causing bacteria were not found, the phytoplasma was, he said.

Results of the Government-financed research have been accepted for publication by the international journal Plant Disease.

Dr Beever said that in some areas, particularly in the north, the disease had wiped out the cabbage trees, which grow throughout the North Island and the upper South Island.

The research indicated that the bacterium is native to New Zealand flax and caused epidemics of yellow leaf flax disease early last century, leading to the collapse of the once flourishing flax fibre industry.

The microscopic pest affects other native plants such as karamu, black tree fern, kohuhu and puriri.

The phytoplasma has also been found in strawberries in New Zealand, and in Australia, it is linked with diseases of grape and papaya.

Dr Beever said that although the cabbage tree pathogen had been identified, many questions remained.

It was critical to find the insect which carried the phytoplasma. He said suspected carriers included the passionvine hopper and the green plant hopper, both introduced from Australia.

"This knowledge is important for devising methods for managing the disease long-term," he said.

"We also need a clearer picture of what plant species are affected by the pathogen, and how."

Dr Peter Maddison, of the Royal Forest and Bird Protection Society, said the trees began dying in the early 1990s and no one had any idea why. Climate warming was suggested as one cause.

Dr Beever said rural landowners could increase cabbage trees' chances of survival trees by fencing off existing tree patches to keep them clear of grazing stock, allowing natural regeneration to occur.