Research
Honey fungus
What is honey fungus?
Honey fungus (Armillaria) is a common fungus that exists frequently on dead plant material, but some species have the ability to become parasitic on living roots of many plants. Most often afflicting woody plants, it may also attack herbaceous perennials and even bulbs.
There are seven species of Armillaria present in the UK, but only A. mellea and A. ostoyae will infect and kill healthy plants. A third species, A. gallica, is generally saprophytic, although it is known to attack and kill plants which are under stress from other conditions (e.g. drought). It is the most noticeable of the species, causing alarm to gardeners, as it produces numerous, long, thickened rhizomorphs, often in and around compost heaps. Here it is breaking down the plant matter, releasing nutrients back into the soil. The other four species are purely saprophytic and do not feed off living plants.
How does it affect the garden?
Although the active part of the fungus is beneath the ground, it above-ground effects can be dramatic, as in the sudden death of a shrub. Premature autumn colour and leaf fall, or an unusually heavy crop of flowers or fruits may precede death. It is important to examine any dead perennial plants for signs of honey fungus. cutting back the outer bark at ground level is likely to reveal a thin creamy-white layer of fungal mycelium, with a strong mushroomy smell, covering the underlying wood. Sometimes the fungus infects and kills the roots, and the plant, without reaching the base of the trunk, and it is necessary to excavate the root system to detect it. Affected roots may be wet and rotten. Sometimes, the black 'bootlaces' or rhizomorphs by which it spreads may be found in the soil around affected plants, although these can be hard to spot for A. mellea and A. ostoyae.
Control of honey fungus
When honey fungus has been confirmed, all dead plants should be dug up and destroyed. Tree stumps, which may act as reservoirs of infection, should be removed if practicable. Alternatively, their decay can be hastened by use of a proprietary formulation of ammonium sulphamate. Where the source of infection is precisely located, its spread may be inhibited by inserting a vertical barrier of heavy polythene or butyl rubber pond liner material to a depth of about 30cm (12in). Improving vigour of trees and shrubs, by attention to soil drainage and nutrition, should render them less likely to succumb to honey fungus.
More information on diagnosis and control of honey fungus
Research at Wisley
There is an active research program in the Pathology section at RHS Garden WIsley on the fungus Armillaria.