By SIMON COLLINS science reporter
New Zealand's unique cabbage tree is fighting off the mystery disease that has killed up to a million trees in the past two decades.
An eight-year trial by Landcare Research of cabbage trees taken from 29 sites nationwide has found that trees from North Island
sites that were most affected by the disease appear to have developed resistance to it.
Ten per cent of the 400 trees planted at Landcare's Mt Albert Research Centre in 1994 have died of the disease, known as Sudden Decline.
But cabbage tree expert Dr Ross Beever said almost all the victims came from South Island sites where the disease had not spread. This was the first sign that the North Island natives were developing resistance.
"This means the natural populations probably will recover from this rebuff, as long as we keep sufficient protected areas to allow them to regenerate," he said.
The cabbage tree, known to the Maori as ti kouka, is a New Zealand icon, found from North Cape to Stewart Island. There is nothing quite like it anywhere else.
Takaka botanist Philip Simpson, whose book on the tree was published last year, says there are between 3 and 4 million cabbage trees - "one for each one of us".
He guesses that about a quarter have been killed by Sudden Decline.
"In some regions it is just about 100 per cent," he said.
"It's amazing. Driving through Auckland now, you only see young ones ... "
But he said he had begun to hear stories of the resistance that Dr Beever's research was now finding.
Dr Beever and other researchers have established that the disease is caused by a bacterium called phytoplasma australiense, which has been found inside cells of the main root of infected cabbage trees.
The same bacterium has killed grapes and strawberries in Australia and flax, strawberries and karamu trees in New Zealand.
Ironically, Dr Beever believes it may have been spread by people planting karamu to restore native forests. The bacterium is transmitted by an insect that sucks the sap from karamu leaves and then infects the next plant it bites.
So far scientists have not been able to prove which insect is spreading the disease in karamu, strawberries and cabbage trees, but Dr Beever said the prime suspect was the passion vine hopper.
* Dr Simpson is researching a book about pohutukawa and rata and wants to hear "personal stories" about these trees.
He is at Faulkner Rd, Pohara, RD1, Takaka
email philip@goldenbay.net.nz
phone/fax (03) 525-6223.
Further reading:
nzherald.co.nz/environment
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