Advice
RHS Help & Advice
Honey fungus
Symptoms
- Trees, shrubs, other woody plants and herbaceous perennials die back, leaves may be discoloured and wilt or fail to appear in spring.
- A plant may die quickly or over a period of years.
- Flowering or fruiting plants may excel themselves before they die.
- Gum may exude from the trunks of infected conifers.
- Beneath the bark, the roots and trunk may be covered with a sheet of white fungal growth (mycelium), which smells quite strongly of mushrooms. Roots that have been infected for some time show a soft, mushy, stringy decay.
- Black bootlace-like strands (rhizomorphs) also occur beneath the bark and on the roots. The 'bootlaces' vary in thickness and length, and resemble old roots (see pic below right). They can be difficult to find, and the white growth below the bark, as described above, is the most diagnostic feature of a honey fungus attack.
- Clumps of honey-coloured, pale-stemmed toadstools may grow round the base or roots of infected plants in the autumn. They possess a whitish, collar-like ring on the stem, just below the cap.
Cause
There are several species of honey fungus, or Armillaria, present in the UK, but only two of these (A. mellea and A. ostoyae) will infect and kill healthy plants. The fungus spreads from infected plants by direct contact with the roots of neighbouring plants and also through the soil by means of the bootlace structures (right), which can attack neighbouring healthy plants.
A third species, A. gallica, is generally saprophytic, although it can be pathogenic in certain circumstances. It is most noticeable and often most alarming 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. However, it is known to attack and kill plants which are under stress from other sources (e.g. drought).
Overall, plants under stress due to poor growing conditions tend to be more vulnerable, but certain genera are more susceptible than others and these are listed below.
Non-chemical control
Dig out and dispose of infected plants with as much of the root system as possible.
Keep plants growing strongly by feeding and mulching regularly.
Once you find honey fungus in your garden plant only species showing resistance (see below).
Chemical control
None available.
Susceptible plants
Betula (birch)
Cedars
Cotoneaster
X Cupressocyparis leylandii (Leyland cypress)
Forsythia
Hydrangea
Ligustrum (privet)
Malus (apples and crabs)
Peonies
Prunus (apricots, cherries, peaches
and plums)
Rhododendrons/azaleas
Ribes (currants)
Roses
Salix (willows)
Syringa (lilac)
Viburnum
Wisteria
More resistant plants
Acer negundo (but not other Acer
species)
Actinida
Abutilon
Bamboos
Carpenteria
Catalpa (Indian bean tree)
Celastrus
Ceratostigma
Cercis
Chaenomeles (Japanese quince)
Clematis
Cotinus (smoke bush)
Fothergilla
Hebe
Juglans nigra (but not Juglans
regia)
Kerria
Lavandula
Passiflora (passion flower)
Phlomis
Pieris
Pittosporum
Quercus (oak)
Rhus (sumach)
Romneya
Sarcococca
Tamarix
Taxus (yew)
Further information
Current research into honey fungus at RHS Garden Wisley