Royal Horticultural Society

What is honey fungus?

Honey fungus is the common name of several species of fungi within the genus Armillaria. Honey fungus spreads underground, attacking and killing the roots of perennial plants and then decaying the dead wood. It is the most destructive fungal disease in UK gardens.

Honey fungus can attack many woody and herbaceous perennials. No plants are completely immune, but some have very good resistance, such as Taxus (yew), Juglans nigra (black walnut) and Acer negundo (box elder).

Betula (birch), Buddleja, Ceanothus, x Cupressocyparis leylandii (leyland cypres), Ligustrum, Magnolia, Malus, Prunus (except P. spinosus), Rhododendron, Rosa, Syringa and Viburnum are all particularly susceptible to honey fungus.

Symptoms

Some of the symptoms you may see:

Above ground

• Upper parts of the plant die. Sometimes suddenly during periods of hot dry weather, indicating failure of the root system; sometimes more gradually with branches dying back over several years.
• Smaller, paler-than-average leaves.
• Failure to flower or unusually heavy flowering (usually just before death).
• Premature autumn colour.
• Cracking and bleeding of the bark at the base of the stem.

Below ground

• Dead and decaying roots, with sheets of white fungus material between bark and wood, smelling strongly of mushrooms. This can often be detected at the collar region at ground level, and rarely spreads up the trunk under the bark for about 1m (3¼ft). This is the most characteristic symptom to confirm diagnosis.
• Rhizomorphs (see images 2, 3 and 4 below) are often difficult to detect especially for the most pathogenic species and they are particularly difficult to find in the soil.

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To check whether a plant is infected with honey fungus, peel away the bark at the base. Look for a white or creamy white, paper thin layer of fungal tissue (mycelium), the consistency of the skin of a mushroom, as shown here.

Rhizomorphs (boot laces) can be seen protruding from this root, as well as the white fungal mycelium found underneath the bark that has been peeled away.

This fungus spreads from an infected plant through the soil using black or brown root-like cords called rhizomorphs (seen here), which are the origin of the name ‘bootlace fungus’. These develop mostly 2.5–20cm (1–8in) below the soil surface in moist soil, but may be found deeper in dry soils. When the growing tips come into contact with the roots of susceptible living plants, they are able to penetrate the tissues and grow through to the inner layers of the bark.

In advanced stages, flattened rhizomorphs may develop beneath the bark. These are often pale yellow or red in colour, but soon become black or brown.

The toadstools are produced in dense clumps and are very variable in size and colour even within each species. The cap or stalk is often honey coloured, hence the common name of the fungus; otherwise the toadstools are some shade of brown. A whitish collar-like ring is always present on the stem of pathogenic species, just below the gills.

In some seasons the fungus may produce toadstools in the late summer or autumn. They may emerge around the bases of dead tree stumps or on the ground over dead roots. They can, however, arise from rhizomorphs in the soil, well away from any apparent source of infection.

Control

There are no chemicals available for control of honey fungus. If honey fungus is confirmed, the only effective remedy is to excavate and destroy, by burning or landfill, all of the infected root and stump material. This will destroy the food base on which the rhizomorphs feed and they are unable to live and grow in the soil when detached from infected material.

Non-chemical control

To prevent honey fungus spreading to unaffected areas, a physical barrier such as a 45cm (18in) deep vertical strip of butyl rubber (pond lining) or heavy duty plastic sheet buried in the soil will block the rhizomorphs. Regular deep cultivation will also break up rhizomorphs and limit spread.

Avoid the most susceptible plants and instead plant more resistant ones. Some more resistant plants include: Taxus, Quercus ilex, Buxus sempervirens, Carpinus betulus, Acer negundo, Ginkgo, Juglans nigra, Nyssa, Laurus nobilis, Berberis, Erica, Garrya, Pittosporum, Chaenomeles, Fremontodendron, Hypericum, Tamarix, Vaccinium, Clematis, Arundinaria (and other bamboos).

See the download for a more complete list of susceptible and resistant plants.

Download

Honey fungus - resistant and susceptible plants (Adobe Acrobat pdf)

Biology

There are seven species of Armillaria in the UK. The most common species in gardens are A. mellea and A. gallica. There is a rarer occurrence of A. ostoyae and A. cepistipes. The remaining species A. tabescens, A. borealis and A. ectypa have not been found in gardens according to a survey done by RHS scientists. A. mellea and A. ostoyae are the most damaging species. A. gallica and A. cepistipes are considered to be less damaging although more research is needed to find out how destructive these species are.

The fungi spread underground by direct contact between the roots of infected and healthy plants and also by means of black, root-like structures called rhizomorphs (often known to gardeners as ‘bootlaces’), which can spread from infected roots through soil, usually in the top 15cm (6in) but as deep as at least 45cm (18in), at up to 1m (3 1/4ft) per year. It is this ability to spread long distances through soil that makes honey fungus such a destructive pathogen, often attacking plants up to 30m (100ft) away from the source of infection.

Clumps of honey coloured toadstools sometimes appear briefly on infected stumps in autumn, but can be safely ignored because the spores are unimportant in the life cycle of the fungus in gardens.

A. gallica produces large and easily visible rhizomorphs quite often found in compost heaps. As a precaution, do not use infested compost around woody plants.

Links

RHS research on honey fungus