The appearance of fungal fruiting bodies on the trunk or branches of a tree often causes alarm,
frequented by the public. The fear of litigation due to injury or death caused by falling trees means that it is all too easy to reach for the chainsaw, but
particularly if the tree in question is in an area
necessary? As we will see, many factors need to be taken into account before an informed decision can be
JOHN SCRACE,
Plant Pathologist, explains more
Tree decay fungi
Firstly, remember that many of the fungi producing fruiting bodies on or around trees are of no concern, and a great number are actually beneficial. Many species (known as mycorrhizal fungi) form a close association with the roots of trees, enabling them to extract more water and nutrients from the soil than would be the case if the fungus were not present. Others are part of the natural recycling process of dead wood, freeing up nutrients for re-use by trees and other plants. Without these saprophytic fungi we would quickly disappear under a mountain of dead wood and leaves!
how often is this actually
The fungi that can cause problems are those that colonise the roots, stem base (butt) or branches of living trees to cause decay, although the majority of these fungi can also act as harmless saprophytes on dead wood. The fruiting bodies produced by these fungi range from typical mushrooms or toadstools (technically the same thing) with a stalk and cap, to brackets projecting from the trunk and even thin, flattened encrustations on the wood or bark.
A distinction is usually made between
root / butt rots, and ‘top rots’ that affect the upper trunk and branches. Some species of fungi, however, can cause both types of symptom. A further distinction is made between the types of decay caused by different fungi. Some cause a ‘white rot’ by decaying the lignin, leaving affected wood white and stringy or spongy. Others cause a ‘brown rot’ by selectively degrading cellulose - this results in a
reached as to the best course of action.
firmer decay with characteristic cracking across the grain. In either case, if the structural integrity of the tree is compromised, there is the potential for branches to be shed, the trunk to snap, or the whole tree to fall.
The life-cycles of many of the decay fungi are still not fully understood. In most cases it is thought that spores from the fruiting bodies infect roots, the stem base or branches through wounds. The majority of the decay fungi are incapable of infecting, through intact, healthy bark (honey fungus, described below, is an exception).
Common Species
It is beyond the scope of this article to give a comprehensive description of the many species of fungi capable of causing decay in trees. Here are brief descriptions of a few of the more commonly encountered or easily recognisable species:
Giant Polypore (Meripilus giganteus)
A common problem on beech; occasionally affects other trees. It attacks the roots, and can often cause extensive decay before any reduction in vigour (i.e. crown thinning, branch dieback) is noticed. By the time fruiting bodies are produced there is often a real risk of the tree falling. The fruiting bodies are large, overlapping brown fronds, produced at the
Talking
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