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( Armillaria mellea and A. ostoyae)

Honey Fungus is usually top of the list of diseases which concern gardeners as it can cause dramatic damage if it affects a much-loved tree, a hedge, or a shrub which may have been an important feature. There are seven species in Britain, four have been recorded in gardens and two cause serious damage; A. mellea attacks mainly broadleaved trees and shrubs; A. ostoyae affects conifers. The name derives from the honey colour of the mushroom-like fruiting bodies.
The foliage of an affected plant wilts, turns yellow and there is premature leaf fall. Dieback of individual branches occurs, leading to the death of the whole plant which can happen over a period of two weeks to several years, depending on the maturity of the plant. There may be cracks in the bark at the base of the trunk, with an exuding sticky substance. (This cracking can also be the symptom of the root-rotting disease, Phytophthora or a bleediing canker caused by bacteria). The stress may induce the plant to produce an excess of flowers or fruit.
A clump of large, yellow-brown to dark brown, mushroom-like growths covered in darker hairy scales, emerge from the base of the stem or trunk in the autumn. The gills are cream with a pinkish tinge at first, turning yellow-brown with scattered darker spots. These are the fruiting bodies which produce millions of spores, but are not the main means in infection. Not all honey-coloured fruiting bodies are honey fungus, look for a white collar below the cap and a yellow to honey-coloured downy stem. The flesh is white and can be eaten (bitter if uncooked), but is not a great delicacy.

Black rhizomorphs with white centres, which resemble boot laces, radiate underground from an infected tree or stump and are the main means of further infection. These are fragile with finer ones which are branched like fibrous plant roots, so can be difficult to find, the larger ones of less virulent species are more easily spotted. If pulled they have a rubbery feel and the black outer coating gives a crackling sound where it fractures to expose the white core. The usual spread is about 7 metres to a depth of 70cm but recent research using DNA analysis has found specimens covering an area about 70 hectares (173 acres) which makes it one of the largest living organisms.

The fine rhizomorphs of Honeyfungus above, and the thicker 'Bootlaces' below.

When a new host is reached, white sheets of the mycelium grow up the stem or trunk under the bark for up to several metres. It is this separation of the bark from the wood which leads to the death of the tree. The upper canopy can survive for some time as water can still pass upwards, but the death of the bark prevents the downward movement of sap. A method of confirming an infection is to peel off a little of the bark to reveal the white mycelium which has a 'mushroomy' smell.

picture of honey fungus mycelium
A section of root from an ash tree showing the white mycelium under the bark.

By digging out any affected stumps and roots the spreading rhizomorphs are deprived of their food source, so they should die back. Deeply digging over the surrounding soil will break them up and prevent spread.

There are no approved chemical treatments * for amateur use. It may be worthwhile employing professional services that are licensed to use chemical treatments not available to gardeners, such as agricultural contractors.
For the more energetic, a vertical barrier of heavy plastic or butyl rubber sheeting buried to a depth of about half a metre around the affected tree, should halt the spread of the rhizomorphs. If a hedge is infected remove a healthy plant on either side of the infected ones.
* Following the introduction of EU Directive 91/414/EEC on plant protection products in July 2003, the manufacturer of Armillatox did not seek approval as a pesticide so it is no longer recommended for use as a treatment or preventative - Jayes Fluid is a similar product derived from tar oil and is also listed in the Directive.

As with all diseases good garden hygiene is the best way to prevent infection, so remove stumps of trees and shrubs which could be open to the disease. When preparing ground for planting remove any old roots or pieces of wood.

Leaving the area fallow for 6 to 12 months after removing all fragments of wood will deprive any remaining rhizomorphs of a food source. It can be temporarily covered with weed fabric, grassed over or planted with herbaceous plants which should not be attractive to the honey fungus.

Some trees and shrubs are more resistant to Honey fungus, eg. yew, bay, bamboos, beech, pittosporum hebe and cornus, so they can be tried as replacements were an infection has occured.

The mycelia of fungi compete for space and will inhibit each others' growth, so the spores of a benign fungus can be used to inoculate an area where Honey Fungus could attack. Mycorrhizal fungi can prevent the rhizomorph from entering the roots, so including some of the spores when replanting should improve the resistance of any new planting. In commercial forestry the spores of an antagonistic fungus are added to the chain-saw oil which drips continuously onto the moving chain for lubrication and is dispersed during the cutting process. Another method of inoculation is to insert pellets of the antagonist into the stumps - this method has been used to fight off Silver Leaf disease when pruning stone fruit trees.

There is a small risk of infecting plants using mulch made from diseased material. So care must be taken not to include such material in home-made composts which tend not to reach temperatures high enough to kill spores or fungal fragments.

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