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

Perennials are plants that live for multiple years. They come in all shapes and sizes and fill our gardens with colourful flowers and ornamental foliage. Many are hardy and can survive outdoors all year round, while less hardy types need protection over winter. The term herbaceous perennial is used to describe long-lived plants without a permanent woody structure (they die back to ground level each autumn), distinguishing them from trees, shrubs and sub-shrubs.

Honey fungus can attack many woody and herbaceous perennials. No plants are completely immune, but there are some that are only rarely recorded as being affected (see the 'Control' section below).

Aesculus, Betula (birch), Buddleja, Ceanothus, Cedrus, Cercidiphyllum, Cotoneaster, × Cuprocyparis leylandii (leyland cypress), Forsythia, Juglans, Laburnum, Ligustrum (privet), Liquidambar, Photinia, Quercus, Rhododendron (azalea), Salix (willow), Sorbus, Syringa (lilac), Thuja, Viburnum and Weigela are all particularly susceptible to honey fungus.

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 followed by an unusually heavy crop of fruit (usually just before death of the plant)
• Premature autumn colour
• Cracking and
  The flow of sap, usually from a wound or pruning cut on woody plants. Certain plants are more prone to bleeding, including grapevines, acers and birches, so these are best pruned while dormant.
  
  Sap (which may be discoloured, depending on the cause) bleeding from the bark of trees and shrubs can sometimes be a symptom of disease (e.g. bleeding cankers, acute oak decline), insect infestation (e.g. some wood-boring beetles), woodpecker damage, or adverse growing conditions (e.g. severe drought)

  bleeding of the
  Bark is the outermost layer of woody plants (trees, shrubs and woody climbers). It is several cells thick and provides protection against physical damage, disease and environmental stresses. Bark comes in a wide variety of colours and patterns, and these can help gardeners when identifying plants. The fissures and crevices of bark on older plants also creates valuable habitat for many garden creatures as well as lichens and small plants.

  bark at the base of the stem
• If suitable conditions permit, mushrooms are produced in autumn from infected plant material

Below ground

• Dead and decaying roots, with sheets of white fungus material (mycelium) between bark and wood, smelling strongly of mushrooms. This can often be detected at the collar region at ground level, and more 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 above) are often difficult to detect, especially for the most pathogenic species, and they are particularly difficult to find in the soil

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 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. It should protrude 2-3cm (about 1in) above soil level. Regular deep cultivation will also break up rhizomorphs and limit spread.

Avoid the most susceptible plants and instead use plants that are rarely recorded as being affected by honey fungus. Some less affected plants include: Arundinaria (and other bamboos), Buxus sempervirens, Callicarpa, Catalpa, Chaenomeles, Chimonanthus, Cordyline, Erica, Garrya, Ginkgo, Hypericum, Jasminum, Pittosporum, Rhamnus, Sarcococca, Tamarix, and Vaccinium.

See this link for a more complete list of susceptible and less affected woody plants.

There are very few records of honey fungus affecting any member of the grass family. This suggests that ornamental grasses could be a good option for growing in areas of a garden where honey fungus is present, although to our knowledge the susceptibility of different grass species and cultivars has yet to be evaluated experimentally.

Chemical control

There are no chemical controls available.

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. The remaining species A. cepistipes, 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 is considered to be less damaging although more research is needed to find out how destructive these species are.

The fungus spreads 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 (3¼ft) 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 mushrooms (see images 5 and 6 above) sometimes appear briefly on infected stumps in autumn. However, the absence of mushroooms is no indication that the fungus is not active in the soil and many plants may be killed before mushrooms appear.

Early indications from ongoing RHS research suggest that spores from the mushrooms of Armillaria mellea are more important in spreading the fungus than previously thought. The

Genes are small sections of DNA that code for particular traits. During sexual reproduction, genes of both parents are mixed to create a genetically different offspring. Plants grown from seed may not be 'true to type' and look like their parents, but genetic variation can be beneficial, creating new, interesting characteristics and sometimes improving resilience to environmental conditions or disease. On the other hand, plants grown from cuttings are genetically identical to their parents.

A. gallica produces large and easily visible rhizomorphs quite often found in

Can refer to either home-made garden compost or seed/potting compost: • Garden compost is a soil improver made from decomposed plant waste, usually in a compost bin or heap. It is added to soil to improve its fertility, structure and water-holding capacity. Seed or potting composts are used for growing seedlings or plants in containers - a wide range of commercially produced peat-free composts are available, made from a mix of various ingredients, such as loam, composted bark, coir and sand, although you can mix your own.

Read more by visiting our page on RHS research on honey fungus and other diseases.