the fungal decay of wood 1 1
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The Fungal decay of wood
By Noel Wallis, James Welburn, Charlene Wenn and Matthew Wheeler
Abstract
Fungi involved in wood decay
• Wood decay bacteria can be grouped in different ways:
• by their method of degrading wood
• by their general biology, pathogens, parasites or saprotrophs
• whether they are primary colonizers or secondary colonizers
• There are 3 different types of wood decaying bacteria
Soft-rot fungi• Degrade cellulose and hemicelluloses
• Occur in wood of high water content and high nitrogen content.
• Found in rotting window frames, wet floor boards and fence posts, etc
• Some are common decomposers of cellulose in soil • (e.g. Chaetomium species)
• They are the least specialised of the wood-rot fungi
Figure 3. Soft rot of woodUniversity of Minnesota (2006)
Brown-rot fungi• Degrade the cellulose and hemicellulose
• leave the lignin intact as a brown framework
• 6% of wood-decay fungi cause brown rots
• All these fungi are members of the basidiomycota. They include dry-rot fungus
• (Serpula lacrymans) and brown cubical rot of birch (Piptoporus betulinus)
Figure 2. Brown rot in woodAustralian Government (2006)
White-rot fungi• These fungi degrade all major wood components (cellulose,
hemicelluloses and lignin)
• Wood becomes more fragile but remains white as it decays.
• Caused by the two major root-rot pathogens of trees, the honey fungus
• (Armillaria mellea) and butt rot (Heterobasidion annosum), and also by many saprotrophic fungi, including turkey tail mushroom (Coriolus versicolor).
Figure 1. White rot in wood.Doe Joint genome insitute (2006)
Type of decay •Wood rotting fungi are defined as;
•“those which can bring about significant weight lass and structural change in woody tissues”.
•There are 3 types of wood decay each taking its name from the general appearance of the decayed wood. They are known as;
•Brown rot
•White rot
•Soft rot
Type of decay
Host fungi Especially in coniferous trees
Fungi Basidiomycetes -especially from the family of the Polyporaceae
Degradation
Cellulose and hemicellulose degradation
Consistency
Fragile, powdery, brown cracks and clefts
Strength Drastic reduction of bending and impact strength
Brown Rot
only cellulose and other wood carbohydrates are utilized so the wood remains predominantly brown
in colour.
Type of decay
Simultaneous rot Selective delignification
Host Broad-leaved trees Broad-leaved trees and conifers but seldom in
conifers
Fungi Basidiomycetes and Ascomycetes
Degradation Cellulose, lignin and hemicellulose
First lignin and hemicellulose, later cellulose
also
Consistency Brittle Fibrous (stringy)
Strength Brittle fracture. At the initial stage: great
reduction of impact bending strength
Ductile fracture. At the initial stage: slight increase in impact bending strength
White Rot
Oxidation of lignin causes the wood to take on a white
or bleached appearance.
Type of decay
Conventional picture New information
Host Broad-leaved trees and conifers. Especially on
wooden structures.
Extensive decay in living broad-leaved trees.
Fungi Deuteromycetes & Ascomycetes
Basidiomycetes
Degradation Cellulose and hemicellulose. Lignin
slightly.
Cellulose and hemicellulose. Lignin
strongly.
Consistency Brittle
Strength Between brown and white rot
High stiffness
Brittle fracture
Soft RotWood loses mechanical strength and
becomes wet and spongy.
Fungal succession in decay of wood
Fungal succession in decay of wood
Fungal succession in decay of wood
Method by which soft-rot fungi breakdown wood
• Pectolytic enzymes produced, breaking down pectic acid, leading to partial degradation of cell wall
• Degradation of wall leads to cell secretions of nutrients, explaining appearance
• Hyphae then penetrate in the cell, growing broader, releasing cellulase enzymes
• This causes generalized decay of cellulose in a rhomboidal shape
Method by which brown-rot fungi breakdown wood
• Cellulase enzymes created by fungi, have little impact on the cellulose
• Breakdown relies on weaker hemicelluloses. This breakdown is actually an oxidation process in which hydrogen peroxide is formed.
• Hydrogen peroxide is strong oxidiser and readily causes generalized decay, of cellulose
• Leaves the lignin intact, maintaining the general cell shape
Method by which white-rot fungi breakdown wood
• White rot capable of breaking down all major components of the wood.
• The fungi use conventional cellulase enzymes to breakdown hemicelluloses and cellulose, exposing lignin.
• The lignin is complex polymer but the fungi only require a few enzymes to break it down.
• Again an example of an oxidation process with strong oxidisers such as hydrogen peroxide, lignin peroxidase and laccase.
• The resultant molecules can become fungi toxic to the fungi so it detoxifies it by polymerisation.
Conclusion
References• Australian Government, Department of the Environment and Heritage.
(2006). Brown rot in wood. [Online] URL: http://www.anbg.gov.au/fungi/images-captions/brown-rot-wood-0121.html
• Site accessed 19/01/2007• Blanket, R. 2006. Soft Rot• http://forestpathology.coafes.umn.edu/archaeologicalwood.htm• Site accessed 19/01/2007• Brown rot jpg. 2007.• http://webs.wichita.edu/mschneegurt/biol103/lecture21/Brown_rot.gif• Site accessed 19/01/2007• Deacon. J. (1997). The Microbial World: Armillaria mellea and other wood-
decay fungi. [Online] URL: http://helios.bto.ed.ac.uk/bto/microbes/armill.htm• Deacon, J. 2006. Fungal biology. (Fourth edition). Blackwell Publishing.
pp 217-235• Dix, N. J. & Webster, J. 1995. Fungal Ecology. Chapman & Hall, London. P.
161• Doe Joint Genome Institute. (2006). Why sequence white rot fungus?.
[Online] URL: http://www.jgi.doe.gov/sequencing/why/whiterot.html• Ingold, C., & Hudson, H. 1993. The biology of fungi. (Sixth Edition).
Chapman and Hall publishers. Pp146-160
References• Schwarze, F. W. M. R., Engels, J. & Mattheck, C. 2000. Fungal Strategies
of Wood Decay In Trees. Springer, New York. pp. 185• University of Minnesota. (2006). Microbes in trees and wood. [Online] URL:
http://forestpathology.coafes.umn.edu/microbes.htm• World of fungi. 2007• http://www.world-of-fungi.org/Mostly_Mycology/Lucy_Goodeve-
Docker_bioremediation_website/whiterotfungi.htm• Site accessed 17/01/01