Adapted from ‘Trees & Fungi: Their Complex Relationship’ by Professor Lynne Boddy – Part 4
Trees contain resources that some fungi seek to exploit. Most trees are resistant to most pathogens most of the time. Even if a disease attacks, usually the damage is localised. Trees can respond to such an attack, usually with a whole plant response, e.g. releasing a chemical such as nitrous oxide to counter.
Some fungi, however, have their own weapons for attack. Some produce enzymes to kill the individual cells of the host, causing the cell wall to break down and leak. This causes damping-off in seedlings.
Some fungi attack a plant via the vascular system. This is a network of cells under the bark which sustain a plant. It includes the xylem vessels, which are long, thin cells, like nerve cells, which transport water from the roots to the shoots. Some fungi enter the xylem vessels and feed on the nutrients within to proliferate.
The host often responds by blocking the individual vessel. This stops the fungus from spreading within the host, but also limits the flow of water to the shoots.
Biotrophic pathogens do not seek to kill the host, but to remove nutrients from cells. They can delay leaf cell death in autumn, so the foliage photosynthesises for longer. The host remains alive but with reduced vigour.
Whilst trees in good vigour are generally able to resist fungal attack, when they are under stress, such as suffering from lack of water during droughts, they are vulnerable to attack. During droughts, water update can be reduced, and individual xylem vessels may become dry, as there is insufficient water to maintain flow.
Once dry, the vessels can provide a network by which fungi and other pathogens access the host.
Some species of fungi are pathogenic, e.g. some species of Armillaria (Honey Fungus). They are often a problem. However, most wood decay fungi are colonisers of dead wood.
They may still prove problematic because as they decay dead wood, they can reduce the strength of the remaining fibres, leading to trunk or root plate failure.