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