Honey fungus is one of the most dreaded diseases of trees and shrubs in landscape plantings. The disease can often infect and damage the roots of a plant without causing visible symptoms as well as produce tough rhizomorphs (fungal "roots") that can branch out through the soil in search of fresh hosts.
Honey fungus is the popular name for the genus Armillaria, of which seven species are present in the UK.
Only two of them, A. mellea and A.ostoyae, are believed to be aggressive pathogens of otherwise healthy trees. Some of the others, such as A. gallica, are believed to be only able to infect plants that are already suffering from some form of environmental stress or are already diseased. Control of honey fungus is difficult and labour intensive so accurate identification is important because it may help avoid unnecessary action.
It is not always possible to distinguish species by their fruiting bodies in the field but genetic-fingerprinting identification techniques have been developed.
Research has shown that there is little risk of introducing honey fungus on woodchip or bark mulches, or that these encourage its spread.
How to recognise it
Sheets or fans of white fungal mycelium beneath the bark of infected main roots and at the base of the stem or trunk are the most important distinguishing feature.
The mycelium has slight luminosity and a strong, sweet odour.
Fruiting bodies are yellow-brown or honey-coloured toadstools with a distinctive white ring on the stem and caps 10-15cm in diameter. They produce white spores and are usually found in small clusters around the base of dead or infected trees and shrubs. The toadstools can appear any time from late summer through to December but last only a few days.
It can be hard to distinguish the species by their toadstools. However, A. ostoyae has a pinkish cap. Presence of toadstools alone is not considered sufficient to prove a tree is diseased - they may have been produced by a less aggressive species of Armillaria growing saprophytically. Toadstools may be absent, even on a severely diseased tree.
Rhizomorphs are not considered to be a helpful diagnostic feature.
The largest and most conspicuous are produced by the least pathogenic species, while those of the most troublesome, A. mellea, are finer and are hard to find in soil.
Honey fungus is a root and lower stem rot caused by fungi in the genus Armillaria. Armillaria species vary in pathogenicity. A. mellea and A. ostoyae are the most capable of infecting healthy plants. A. gallica is most commonly found in compost or in soil or colonising already-dead plants.
However, there is some evidence that it can infect and kill plants already under stress.
Armillaria can live saprophytically on stumps, logs or pieces of root. Rhizomorphs - strands of hyphae in a protective sheath - radiate out from an established colony and are responsible for spreading the fungus. Tree and shrub roots can be infected by contact with rhizomorphs or by root to root contact.
The fruiting bodies also produce spores which can infect through wounds and cut surfaces. Symptoms and death are caused by the fungus rotting and digesting wood and inner bark tissue. Honey fungus is most often associated with woody plants but herbaceous perennials and bulbs may also be hosts.
- These include leaf yellowing, wilting, early leaf fall and/or premature autumn colour which is followed by dieback and eventual death of plant.
- In some cases, infection may lead to bark splitting and possibly bleeding or resin exudation, especially on conifers.
- An unusually heavy crop of flowers or fruits may precede death.
- Sudden death of plants in summer may be linked to drought stress where roots have already been weakened by long-standing infection.
Treatment: biological control
There are currently no approved biological controls although research has looked at the possible use of antagonistic fungi such as Trichoderma and Dactylium species.
These have controlled A. mellea through antibiotic production, competition for resources and direct parasitism.
Planting trees inoculated with appropriate mycorrhizae may aid natural resistance to stress and root infections.
Treatment: cultural control
- Use resistant species where possible or appropriate.
- Match species to site to avoid plant stress.
- Avoid waterlogging, shading, drought and high plant densities.
- Protect roots from damage by other pests and diseases or machinery.
- Avoid chipping wind-thrown trees unless you are sure they are not infected with honey fungus.
- Avoid replanting at an infected site for at least 12 months to starve the fungus.
- Avoid replanting susceptible species close to known infected sites. A list of susceptible and tolerant plant species is available at www.dovebugs.co.uk.
- Physical barriers such as heavy-duty plastic sheet buried vertically can protect important specimens from infection spread. Sink to at least 50cm deep.
- Pruning off infected roots may save an important specimen if infection has not spread too far.
- Ideally, infected stumps and roots should be removed but this is often unsuccessful as the soil disturbance stimulates rhizomorph growth.
- Any freshly cut stump should be removed if possible to prevent its acting as a future reservoir of infection.
- Stumps may be killed and decay hastened in the garden by applications of approved products containing glyphosate.
- Heat will control the disease but care with home composted materials is needed to ensure they reach an adequate temperature during composting.
Treatment: chemical control
In a commercial situation, soil sterilisation using approved products will control the disease. Systemic applications of fungicide into the woody tissues of trees and shrubs are difficult and costly. Chemical control in the soil can be hampered by the tough skin of the rhizomorphs. Some fungicides also bind to soil particles before reaching their target.
Fully updated by Dove Associates.
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