Honey Fungus
Honey Fungus
A disease of trees, shrubs, woody climbers and, rarely, woody herbaceous perennials, caused by species of the Armillaria fungus. There are six Armillaria species found in the UK that live on dead and decaying woody material; only two of these are also able to attack living plants.
Introducing the fungus
Honey fungus lives in dead and decaying roots, tree stumps and other woody items in the soil. It grows out from these in the form of black root-like rhizomorphs ('bootlaces') at the rate of around 1m a year. The rhizomorphs grow relatively close to the soil surface (in the top 20cm) and invade new roots, or the root collar (where the roots meet the stem) of woody plants. An infected tree will die once the fungus has girdled it, or when extensive root death has occurred. The fungus also spreads from root to root where they are in close contact. The honey fungus 'fruiting body' or toadstool appears in the autumn, if at all. Airborne infection by spores from the toadstools is rare.
Typical symptoms and indications of honey fungus
- A tree, shrub or section of hedge may die suddenly, or it may gradually decline in health over period of weeks, months or years. The plant may also have shown prolific flowering or fruiting previously, or early leaf fall or autumn colour.
- Other trees or shrubs in the area may also have died over a period of years.
- A resin, gum, or a watery liquid may ooze from the bark at the base of the stem.
- Bark around the base of the tree dies and comes away easily.
- Clumps of honey-brown toadstools may appear around the base of the tree, or nearby stumps, in late September and early October.
- Black root-like structures, like bootlaces in appearance and known as rhizomorphs, may be seen in the soil, in compost heaps and piles of leaves, and under mulches. They vary in thickness and length. In most cases, the rhizomorphs that are the most obvious are those of the Armillaria species that only live on dead wood.
Identifying the problem
The presence of Armillaria toadstools, or black rhizomorphs, does not necessarily mean that the pathogenic (disease causing) strains of honey fungus are present in the garden. Nor do they necessarily indicate that honey fungus is the cause of the death or decline of particular plants. Unfortunately identification of the different Armillaria species is not easy, so other methods should be used to confirm honey fungus as the cause of disease or death.
Remove outer bark from the stem or trunk at ground level. If you see a sheet of creamy-white fungal growth, with a strong smell of mushrooms, under the bark, this confirms the presence of honey fungus.
If further confirmation is required, contact a reputable tree surgeon.
Honey fungus and mulches
There is often concern that honey fungus can live on woody mulches, especially when the rhizomorphs are seen under the mulch. It is in fact quite safe to use woody mulches where honey fungus is present as it will not significantly add to its spread or growth. However, do not introduce woody mulches that have been infected. Commercial mulches will carry little or no risk.
Prevention and control
- Where honey fungus is known to be a local problem, always remove stumps and roots when digging out woody plants.
- Encourage healthy growth of woody plants in the garden by correcting any drainage problems, and feeding and mulching as necessary.
- Grow less susceptible plants species (see list below).
Once honey fungus has been confirmed:
- Dig up and destroy all dead/ dying woody plants, removing the stump and as much of the root system as possible.
If removal of a stump is impossible, the stump can ground, or chipped, up by a contractor. The resulting woodchips should be burned or disposed of outside the garden, not used as a mulch.
- In a hedge, dig up a plant or two either side of the infected one.
- Replant with species showing resistance, or non woody species.
- You can prevent honey fungus spreading from a diseased plant to other plants by creating a vertical barrier, 50-60cm deep, in the soil. Thick plastic or butyl, as would be used for a pond liner, is ideal.
Their tree helpline 0906 5161147 (a pay by the minute service, £1.50 ) is an excellent source of advice on all matters relating to trees. Their website is: www.trees.org.uk
| Woody plants with some resistance to honey fungus | Woody plants to avoid (very susceptible) |
|---|---|
Abutilon |
Araucaria
- Monkey puzzle |
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