In November I created a post about a decay pathogen I had spotted on a mature beech tree. Following on from that post I would like to discuss the different types of parasitic rot that effect trees, starting with brown rots.
There are two cellular components in wood that are significant in allowing trees to achieve and maintain their size and stature. These are cellulose and lignin. Cellulose provides flexibility. It is the cell wall component that allows trees to withstand loading forces such as heavy snowfall and strong winds, and allows them to adapt to their environmental stimulus. Lignin provides rigidity and is the cell wall component that allows them to support themselves.
If a tree were to loose its cellulose it would become inflexible and the wood texture would be brittle. Loose the lignin and the tree can flex but looses its rigidity, its strengthening material. In arboriculture we say that cellulose provides tensile strength and lignin provides compressive strength.
Parasitic fungal decay pathogens (biological agents causing disease, and feeding on living wood) live off components of the cell wall and are selective about what cellular component they degrade, dependant on the rot type that they cause. Fungal pathogens are also selective about what part of the tree they degrade for example root rot, stem rot or branch rot.
First we will consider brown rots; brown rots degrade the cellulose in wood. The result as discussed above is a loss of tensile strength. The tree looses its ability to flex and the woody degradation caused by brown rots may result in brittle fractures.
If the fungal pathogen that causes a brown rot is associated with degradation of the roots of a tree, or the base of the stem, then the significance can be severe.
Arboriculturalists study decay pathogens in great detail. Fungal pathogens can be species specific, or effect a variety of tree species, sometimes being of a varying significance across the species. Aditionally trees have their own coping mechanisms and defences against pests and diseases which can seal off areas of decay and develop reactionary growth to compensate for the increased stress on their structure.
This is one of my favourite decay fungi, and brown rot fungus, Phaeolus schweinitzii. (Sorry, no common name!)
The fungus is associated with degradation of the root systems and the decay can further extend up the main stem. Brackets are usually found on the ground around the root plate, growing directly off a root, or on the stem base or main stem.
It should be noted that the bracket, or fruiting body, of a fungus is just that, the fruit. The fungus itself lives within the tree.
As mentioned it is vital that arborists have a good understanding of the significance of pests and diseases, and good identification skills, to allow appropriate measures to be prescribed however from what we have discussed in this post it is clear that this fungus creates a serious risk of tree failure given its rot type and the part of the tree it degrades. This is what it can do to trees.
In the next post we will consider white and soft rots. For further reading I recommend the following book which is my favourite and most thumbed tree book: Principles of Tree Hazard Assessment and Management - David Lonsdale 1999 (DTLR ISBN 0 11 7533556)
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