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PESTS AND DISEASES OF FORESTRY IN NEW ZEALAND

Armillaria and Trichoderma

Scion is the leading provider of forest-related knowledge in New Zealand
Formerly known as the Forest Research Institute, Scion has been a leader in research relating to forest health for over 50 years. The Rotorua-based Crown Research Institute continues to provide science that will protect all forests from damage caused by insect pests, pathogens and weeds. The information presented below arises from these research activities.

From Forest Health News 85, May 1999.

Recent coverage in the Christchurch Press (8 April) and on television has brought the problem of Armillaria root disease in the Christchurch Botanical Gardens and Hagley Park to national attention. The portrayal of Armillaria as the bad fungus being valiantly destroyed by the injection of the good fungus, Trichoderma , needs some elaboration. There are three native species of Armillaria (in a genus of about 30 species around the world) which occur throughout the native forests and shrublands of New Zealand. Their principal role in the forest is the decay of dead wood although under some circumstances they are capable of killing trees and then rotting their dead wood. This probably does not occur often as Armillaria co-evolved with New Zealand's native trees and they are able to resist this pathogenic attack.

Since the arrival of Europeans, New Zealand's forest cover has been significantly reduced and where pasture has succeeded forest Armillaria has been eliminated from the landscape. However where exotic trees have been planted straight after native forest clearance there has been an ongoing problem of tree death. These deaths may be the result of a lack of resistance of exotics to our native Armillaria or because the exotics find the places they have been planted unsuitable and are subject to stress. Armillaria is able to take advantage of this stress to overcome the trees' resistance to disease.

Where exotic trees are planted on what has been pasture for up to eighty years Armillaria simply does not occur. But as parks and gardens are developed our towns and cities are turned into woodlands and Armillaria spores blow in on the wind from native forest and colonise these new woodlands. It is inevitable that wherever trees and woody shrubs grow Armillaria will eventually move in. This is a natural part of any ecosystem and may account for the event occurring in Christchurch.

Armillaria is a soil fungus which rots woody roots and fallen wood that is in contact with the ground. It is long lived and reproduces usually only in the autumn. By comparison Trichoderma is extremely short lived, its spores geminating on material with a high sugar content where it reproduces rapidly and in great numbers and then moves on to the next sugary food source. To maximise its food source Trichoderma releases chemicals that inhibit the growth of other fungi including Armillaria . This has been shown time and again in laboratory experiments and led to the belief that Trichoderma might be used to control Armillaria in the real world. This is illusory as Armillaria rots wood which has a very low, if not non-existent, sugar content and spends many years doing so whereas Trichoderma rapidly uses any available sugars, reproduces and is gone. So, other than a possible brief inhibiting of Armillaria growth there is no likely long-term cure using Trichoderma injected into the trunk of a diseased tree. Despite thirty years of trying to develop a technique to utilise Trichoderma, no effective control has ever been achieved, or is ever likely to be developed. It is also notable that the developers of these products provide little, if any, scientific data to back up their claims for control.

The only effective proven control is the long-term management of our urban woodlands. This may involve the removal of diseased trees, roots and all, and rotating plantings. Areas presently planted with trees might in the next rotation be planted as lawns and flower beds. Conversely, areas in lawns and playing field now may be the wooded walks in a century. Trees are long lived but mortal and we should bear this in mind and plan not just for the next financial year, or the next council term, but for 150 to 300 years into the future. Many of the trees we plant today will be still around then.

(Geoff Ridley, Forest Research)

 

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(Scion is the trading name of the New Zealand Forest Research Institute Limited.)

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