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 Juglans nigra (black walnut) and Acer negundo (box elder).
Acer (except A. negundo), Aesculus, Betula (birch), Buddleja, Ceanothus, Cedrus, Cotoneaster, × Cuprocyparis leylandii (leyland cypress), Fagus (beech), Hydrangea, Juglans (except J. nigra), Ilex (holly), Ligustrum (privet), Magnolia, Malus (apple), Photinia, Prunus (except P. spinosus), Pyrus (pear), Quercus (except Q. cerris, Q. ilex, and Q. rubra), Rhododendron (azalea), Ribes (currant), Rosa, Salix (willow), Sorbus (except S. aria), Syringa (lilac) and Viburnum are all particularly susceptible to honey fungus.
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.
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.
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.
Rhizomorphs (bootlaces) 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 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.