Cabbage Tree Mystery Better Understood
Over the last few years, a serious (and up to now, mysterious) disease has affected the cabbage tree (Cordyline australis), also known as ti kouka. Many trees have died as a result of the disease, which causes the leaves to yellow and fall off. Usually the tree dies in 3 to 12 months. In some areas, particularly in the north, no big trees are left.
Landcare research (a Crown Research Institute - CRI) has now identified the disease. It is caused by an elusive type of bacterium, called a phytoplasma, that lives in plant sap. The phytoplasma is native to NZ flax, and has spread to other species. The pathogen caused massive epidemics of yellow leaf disease in flax early last century, destroying the once extensive flax swamps of Manawatu and contributed to the eventual collapse of the once flourishing flax fibre industry.
As well as spreading to the cabbage tree, this microscopic pest 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, mamaku (Cyathea medullaris); kohuhu (Pittosporum tenufolium) and puriri (Vitex lucens). The phytoplasma has also been found in strawberries in NZ, and in Australia it is linked with diseases of grape and papaya.
Although the cause of the disease has now been identified much work still needs to be done, for example:
- identifying the plant-hopping insect or insects that spread the phytoplasma (important in devising methods for managing the disease long-term); and
- clarifying what plant species are affected by the pathogen, and how.
There are things that people can do to increase the survival of cabbage trees in the short term. For example, fencing off existing cabbage tree patches, to keep them clear of grazing stock, allows natural regeneration to occur. Planting cabbage trees in gardens and land restoration projects will also play an important role in maintaining cabbage trees.
Contact for Enquiries
The Ministry of Agriculture and Forestry
Pastoral House
25 The Terrace
PO Box 2526, Wellington
Tel: 0800 00 83 33
Fax: +64 4 894 0720
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