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.

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.

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).