Meripilus Giganteus (Meripilus)
Arborists are faced with a wide range of wood decay fungi, such as Meripilus that can infect living trees which, apart from honey fungus, tend to be unable to infect living tissue.
Meripilus is a polypore fungus found throughout the Northern Hemisphere and widely distributed in Europe. It causes a white rot in various types of broadleaved trees, particularly beech trees but also fir, spruce, pine, oak and elm species.
This bracket fungus, so called because it has a bracket or shelf like shape to it, is commonly known as the giant polypore and can be found in large clumps at the base of trees or some, some distance away from the trunk indicating infected roots.
Meripilus (also commonly known as giant polypore) infects trees through wounded or damaged tissue initially, before attacking living tissue and, once established, can seriously weaken the affected tree’s structure by causing extensive internal damage often long before any external evidence appears.
Meripilus fungus infects and grows in the roots of mature broadleaf trees but rarely extends into the trunk. The first evidence of its presence may be crown dieback or defoliation.
It appears that there is no clear correlation between the appearance of the fungi and the severity of infection. In some cases, fungi can be present for a number of years with no apparent detriment to the tree but there is likely to be a greater risk of branch snapping and tree instability.
As the 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 fungi than Meripilus itself.
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.
How do I recognise it?
Meripilus produces multiple bracket or shelf like fungi, often in dense clusters of up to 1m across. These appear in late summer or autumn at the base of the trunk of an infected tree, or from the ground immediately above the infected roots. The fungi are yellowish brown with a creamy white underside. The flesh stains black if bruised or cut and disintegrates if exposed to frost.
Initial visible symptoms are likely to be foliage dieback and crown thinning associated with damage to the roots caused by the fungus. The fungus appears annually 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.
Some soil treatments, if used at the time of planting, may protect the roots and help resist infection. Maintaining optimum growing conditions by aeration, mulching and irrigation will also help resistance to the fungi.
Regular inspections/surveys of trees in your care should include assessment of crown growth. If crown thinning or dieback is identified the cause must be investigated immediately. Often these symptoms may be caused by drought or waterlogging but they could indicate more serious problems such as Meripilus.
Existing infections cannot be controlled and felling for safety reasons is usually the only option for infected trees, particularly in public access areas.
image courtesy of hortweek.com