Arborists and foresters are faced with a wide range of wood decay fungi that can infect living trees. Most belong to the large group Basidiomycetes. Apart from honey fungus (Armillaria spp.), these fungi tend to be unable to infect living tissue. Instead, they rely on infection through wounded or damaged tissue and may need to become established on dead wood before attacking living tissue. Once established, they can digest cellulose and lignin, key components of timber, thus severely weakening the tree's structural integrity.
Giant polypore (Meripilus giganteus) is of particular concern because it can cause extensive internal damage before any external evidence of its presence is noticed. It infects and grows in the roots of mature broadleaf trees - it has also been found on araucaria - and rarely extends into the trunk. The first evidence of its presence may be crown dieback or defoliation because the bracket-like fruiting bodies are not always present.
Research work suggests that there is no clear relationship between the appearance of fruiting bodies and disease severity. In some cases, fruiting bodies can be present for a number of years with no apparent detriment to the tree but there can be a greater risk of branch snapping and tree instability.
Remember that as decay progresses and the infected tree weakens, more than one species of wood decay fungus may be present, some of which may produce more obvious or longer-lived fruiting bodies than Meripilus giganteus.
How to recognise it
Meripilus giganteus produces bracket-type fruiting bodies in late summer or autumn at the base of the trunk of an infected tree, or from the ground immediately above the infected roots.
Fruiting bodies often appear in dense clusters, up to 1m across, with individual fruiting bodies 20-30cm across. They are yellowish brown with concentric zonal markings on the upper surface - the lower, spore-producing surface is creamy white. The flesh stains black if bruised or cut and disintegrates into a sticky mass if exposed to frost.
The fruiting bodies of Grifola frondosa appear similar but this species is usually only found on oak and the fruiting body flesh does not stain black when bruised or cut. Decay caused by this species can extend several metres up the trunk.
Giant polypore can cause extensive internal decay, rendering mature trees liable to collapse or toppling. This can occur before external symptoms are evident. Initial visible symptoms are likely to be foliage dieback and crown thinning associated with damage to the roots caused by the fungus. Fruiting bodies are annual, appearing between late summer and the end of autumn.
The full extent of the root damage may not be evident unless excavated for inspection. Upper roots may appear sound despite extensive decay 50cm or more below the soil surface. Decayed roots have the consistency of a sponge.
Giant polypore is the common name of the wood decay fungus Meripilus giganteus. It is a member of the polypore family, which produce wind-dispersed spores from thousands of vertical pores on the lower surface of their fruiting bodies.
The large numbers of spores that are produced are effectively dispersed by wind currents and they are therefore likely to be present in most areas where trees are or have been growing. Wood decay fungi secrete enzymes that enable them to digest cellulose or lignin, the two main structural components of wood, thus severely weakening the tree's strength.
Meripilus giganteus attacks trees' roots and trunk base. The fungus is especially damaging on beech - although oak, London plane, sycamore and lime trees can also become infected. Infection normally takes place through wounds or roots damaged by drought, waterlogging or wind rocking.
Treatment: biological control
Mycorrhizal soil treatments can be applied at planting alongside remedial maintenance treatments - such as aeration and nutrition - for mature trees. These may help the roots survive sub-optimum growing conditions or to resist infection, but will not control existing infection.
Treatment: cultural control
- Felling for safety reasons is usually the only option for infected trees in public access areas.
- Regular inspections/surveys of trees in your care should include assessment of crown growth.
- Immediately investigate the causes of any crown thinning or dieback. Although often caused by drought or waterlogging, these symptoms could indicate root pathogens including Meripilus giganteus.
- Maintain optimum soil conditions for healthy roots through the use of aeration, mulching or irrigation as necessary to reduce the risk of root damage leading to infection and to give the tree's natural defences the best chance to isolate any infections that do occur.
- Contractors can provide a thermal imaging service that can help to identify whether plants have any disease issues internally.
Fully updated by Dove Associates.
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