Disease Affecting Native And Exotic Plants

  • John Luxton
Food, Fibre, Biosecurity and Border Control

The Minister for Food, Fibre, Biosecurity and Border Control, John Luxton today announced the identification of a disease organism that is causing the die-off of native and exotic plants.

"Recent DNA work has identified an emerging threat to both our native forests and some crops. Researchers have discovered that a disease very similar, if not identical to, that which has been killing cabbage trees is also affecting native pittosporums and coprosmas. Researchers also suspect other natives such as puriri and mamaku (black tree ferns) are affected. The disease is thought to be spread between plant species by insects commonly referred to as planthoppers."

The disease organism implicated is the phytoplasma that causes the yellow leaf disease of flax. Yellow leaf disease, first reported in 1908, was a major cause of the eventual demise of the Manawatu flax industry early this century. The same phytoplasma, or one very similar, is causing concern in Australia where it has been associated with diseases of strawberry, papaya and grape.

Mr Luxton said the Government was taking this emerging threat very seriously.

"I have called Government agencies to work together on this issue and a workshop is being held by the Department of Conservation in early December to discuss the problem with specialists from Landcare Research, HortResearch and Ministry of Agriculture and Forestry."

"Close contact with Australia will also be needed as the disease affects the plants in both countries. This issue may be on the agenda for the upcoming Australia New Zealand Environment and Conservation Council meeting in Hobart in December."

"We need more information before any management options can be considered," said Mr Luxton.

The research team led by Dr Ross Beever of Landcare Research and Dr Richard Forster of HortResearch suspects that one or more introduced planthoppers are involved in spreading the disease between different species of plants.

However, more work is needed to confirm exactly which planthoppers are the culprits. There are some 20 native planthoppers that may also be involved in the spread of the disease.

Since the mid 1970s, plants such as cabbage trees and coprosmas in New Zealand, and papaya and grape in Australia have been declining and dying in unusually high numbers. Recent DNA work has identified a phytoplasma very similar, if not identical to, the flax yellow leaf phytoplasma, as the main cause of these die-offs.

Disease symptoms vary between the species, but usually the leaves show unusual yellowing or reddening and fall off prematurely leaving bare branches. The branches die back from the tip, leading to eventual plant

death. Once initial symptoms appear, plants seldom recover. Symptoms of the disease have been seen in most of the North Island and the top half of the South Island. This distribution matches that of passionvine planthopper, an introduced species that has been known in New Zealand since the 1970s.

Phytoplasmas are specialised bacteria lacking cell walls. They live in the vascular system of plants and are spread by sap-feeding insects, such as planthoppers.