Down Garden Services

HONEY FUNGUS

( Armillaria mellea and A. ostoyae)

There are six species in Britain, two of which cause serious damage; A. mellea attacks mainly broadleaved trees and shrubs; A. ostoyae affects conifers. The name derives from the colour of the mushroom-like fruiting bodies. The foliage of affected plants 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). 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 cooked (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. They can spread for about 7 metres to a depth of 70cm. 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 'fungal' smell.

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

Remove any infected trees or stumps and their roots with as much soil as possible, dig a trench about 70cm deep around the area and pour in some diluted formalin or Armillatox^* as a sterilant. A vertical barrier of plastic burried to a depth of one 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 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.
Some trees and shrubs are more resistant to Honey fungus, eg. yew, beech, 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 Honeyfungus could attack. This technique has been used in commercial forestry where the spores of an antagonistic fungus are added to the chain-saw oil, and are 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.

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