Breakthrough in Cabbage Tree Mystery: Cause Revealed

Forest and Bird, Issue 300, 1 May 2001, Page 10

Breakthrough in Cabbage Tree Mystery: Cause Revealed

Landcare

Research

— Diana Leufkens,

andcare Research has made major advances in understanding the ‘sudden decline’ disease which is killing the native cabbage tree Cordyline australis, also known as ti kouka. Cabbage trees have succumbed in large numbers to the disease, which causes the leaves to yellow and fall off, usually followed by the death of the tree three to 12 months later. In some areas, particularly in the north, no big trees are left. Landcare Research and HortResearch scientists used highly sensitive DNA techniques and electron microscopy to identify the cause of the disease as a phytoplasma, Phytoplasma australiense, an elusive type of bacterium that lives in plant sap. ‘Seeing the phytoplasma under the electron microscope was like finding a needle in a haystack; according to Dr Ross Beever of Landcare Research. DNA techniques indicate that the phytoplasma is native to New Zealand flax, and has spread to other species, Dr Beever says. The pathogen caused massive epidemics of yellow leaf disease in flax early last century, destroying the once extensive

flax swamps of Manawatu and was a major factor leading to the eventual collapse of the onceflourishing flax fibre industry. Dr Beever says as well as spreading to the cabbage tree, this microscopic pest also affects the health of other native species, particularly karamu Coprosma robusta. It is also suspected to cause dieback and death of the black tree fern or mamaku, kohuhu Pittosporum tenufolium 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. Altough the cabbage tree pathogen has been identified, Dr Beever says many questions still need to be answered. "The next major aim of the research is to identify the planthopping insect or insects that spread the phytoplasma. This knowledge is important for devising methods for managing the disease long-term. "We also need a clearer picture of what plant species are affected by the pathogen, and how. Do the insect vectors move the phytoplasma from the cabbage tree to other species? Does the pathogen affect commercial crops other than strawberry? Are there symptomless carriers of the phytoplasma? Are there natural sources of resistance? And is there is some way of repelling the insect vectors?’ While the research is on-going, Dr Beever says it has identified things that people can do to increase the survival of cabbage trees in the short term. ‘I would encourage rural landowners to fence off existing cabbage tree patches, to keep them clear of grazing stock, and thus allow more natural regeneration to occur. Planting cabbage trees in gardens and land restoration projects will also play an important role in maintaining cabbage trees as a special feature of our landscape, he says.