Honey fungus is a potentially fatal pathogenic organism that affects trees, shrubs, woody climbers and, rarely, woody herbaceous perennials. Honey fungus grows on living trees as well as on dead and decaying woody material.
Honey fungus spreads both from living trees, dead and live roots and stumps by means of reddish-brown to black root-like rhizomorphs ('bootlaces') at the rate of around 1 metre a year, although infection by root contact is also possible. Infection by spores is rare.
Rhizomorphs grow relatively close to the soil surface (in the top 20 cm) and invade new roots, or the root collar (where the roots meet the stem) of woody plants. An infected tree will die once the fungus has girdled it, or when extensive root death has occurred. This can happen rapidly, or may take several years. Infected plants will deteriorate, although may exhibit prolific flower or fruit production shortly before death.
Initial symptoms of honey fungus infection include the dying back of leafy branches or failure of leaves to appear in spring. Black bootlace-like strands appear under the bark and around the tree, and fruiting bodies grow in clusters from the infected plant in autumn and die back after the first frost. However these signs do not necessarily mean that the pathogenic (disease-causing) strains of honey fungus are a cause of plant decline or death, so other identification methods are advised before a diagnosis is made.
The presence of thin sheets of cream-coloured mycelium, giving off a strong smell of mushrooms, beneath the bark at the base of the trunk or stem, sometimes extending upwards, or a gum or resin exuding from cracks in the bark of conifers, indicates that honey fungus is a likely cause of problems. If further confirmation is required, it is advisable to seek the advice of a qualified tree surgeon.