Module 5 · Biological Agents
5.2 Wood-Decay Fungi
11 min read
Decay fungi need wood, oxygen, a suitable temperature and — above all — water. The moisture requirement is the practitioner's lever: below roughly 20% moisture content, decay does not establish and existing decay does not progress. Everything else about identification matters less than that single number, which is why the treatment for decay is almost always the removal of water rather than the application of a chemical.
Learning objectives
By the end of this lesson you will be able to:
- State the moisture threshold for decay and its practical consequences.
- Distinguish brown rot from white rot and explain what each degrades.
- Identify the main species by reliable features rather than folklore.
- Assess the extent of decay beyond what is visible.
Key terms
- Brown rot
- Fungi degrading cellulose and leaving lignin, so the residue is brown, shrinks and cracks into cuboidal blocks. Includes the common structural rots.
- White rot
- Fungi degrading lignin as well as cellulose, leaving timber lighter, fibrous and stringy rather than cracked into cubes.
- Mycelium
- The vegetative growth of the fungus — the sheets, strands and cottony masses. What you usually see before any fruiting body.
- Rhizomorph
- A thickened conducting strand able to transport water and nutrients, letting a fungus extend across and through inert materials.
- Fibre saturation point
- Around 28–30% moisture content, where cell walls are saturated and free water begins to appear. Decay becomes vigorous above it.
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