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Honey fungus or Armillaria is a genus of parasitic fungi that live on trees and woody shrubs. It includes about 10 species formerly lumped together as A. mellea.
Armillaria is long lived and form some of the largest living organisms in the world. The largest single organism (of the species Armillaria ostoyae) covers more than 3.4 square miles (8.9 km²) and is thousands of years old. Some species of Armillaria are bioluminescent and may be responsible for the phenomena known as foxfire and perhaps will o' the wisp.
As a forest pathogen, Armillaria can be very destructive. It is responsible for the "white rot" root disease (see below) of forests and is distinguished from Tricholoma (mycorrhizal) by this parasitic nature.
Honey fungus as a plant disease (white rot root disease)
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 m 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.
Honey fungus can be prevented by removing tree stumps or other dead woody material such as roots from the soil, for example by mechanical stump-grinding. Killing stumps chemically is often not sufficient. Healthy growth of woody plants in the garden should be encouraged by correcting any drainage problems and adequate feeding and mulching. There is often concern that honey fungus can live on woody mulches, especially when the rhizomorphs are seen under the mulch. It is in fact quite safe to use woody mulches where honey fungus is present.
If the presence of honey fungus is confirmed, all dead or dying woody plants should be dug up and any roots or stumps removed. If removal of a stump is impossible, the stump can be ground, or chipped, by a contractor. The resulting woodchips should be burned or disposed of outside the garden, not used as a mulch. As a last resort, a stump can be treated with ammonium sulphamate (a stump killer sold as Amicide or Root Out). However this is not an approved organic product, and should not be used by registered organic growers.
If plants in a hedge are infected, the plants on either side of those contaminated should also be removed. Areas affected by honey fungus should be replanted with non-woody species, or by species showing resistance such as European yew (Taxus baccata), dogwood, beech (fagus) and Hebe. Fruit trees and bushes should not be grown on areas known to be infected, and particularly susceptible species such as birch, cypress, lilac, pine, privet, walnut and willow (Salix) should also be avoided. Without host plants the fungus will eventually die out.
Honey Fungus information and photo from Wikipedia