8. Type of decay Brown Rot only cellulose and other wood carbohydrates are utilized so the wood remains predominantly brown in colour . Fragile, powdery, brown cracks and clefts Consistency Drastic reduction of bending and impact strength Strength Cellulose and hemicellulose degradation Degradation Basidiomycetes -especially from the family of the Polyporaceae Fungi Especially in coniferous trees Host fungi
9. Type of decay White Rot Oxidation of lignin causes the wood to take on a white or bleached appearance. Ductile fracture. At the initial stage: slight increase in impact bending strength Brittle fracture. At the initial stage: great reduction of impact bending strength Strength Fibrous (stringy) Brittle Consistency First lignin and hemicellulose, later cellulose also Cellulose, lignin and hemicellulose Degradation Basidiomycetes and Ascomycetes Fungi Broad-leaved trees and conifers but seldom in conifers Broad-leaved trees Host Selective delignification Simultaneous rot
10. Type of decay Soft Rot Wood loses mechanical strength and becomes wet and spongy. Between brown and white rot High stiffness Brittle fracture Strength Brittle Consistency Cellulose and hemicellulose. Lignin strongly. Cellulose and hemicellulose. Lignin slightly. Degradation Basidiomycetes Deuteromycetes & Ascomycetes Fungi Extensive decay in living broad-leaved trees. Broad-leaved trees and conifers. Especially on wooden structures. Host New information Conventional picture
The first mode of attack brings pectolytic enzymes which are produced from the tips of soft rot hyphae. This breaks down the pectic acid, holding the cells together, leading to cell separations and a partial degradation of the hosts cell wall. This degradation of the cell wall causes secretions of watery nutrient sources out of the cell giving rise to its soft watery appearance. Once a weak point in the cell wall is in place, the hyphae can penetrate through and then produce broader hyphae once inside. Then they produce the cellulase enzymes which cause generalized decay of cellulose in a rhomboidal shape
Brown rot fungi can operate in dryer conditions as they obtain their water from the breakdown of cellulose. Cellulase enzymes created by fungi, have little or no impact on the cellulose that it is attacking therefore the breakdown relies on the breakdown of the weaker hemicelluloses. This breakdown is actually an oxidation process in which hydrogen peroxide is formed. Hydrogen peroxide is a strong oxidiser and readily causes generalized decay, leaving the lignin intact, maintaining the general cell shape
White rot is capable of breaking down all major components of the wood. The fungi appear to use conventional cellulase enzymes to breakdown the hemicelluloses and cellulose, exposing the lignin. The lignin is very difficult to breakdown and white rot fungi is the only known living organism to do so. The lignin is a complex polymer but the fungi seem to only require a few enzymes to break its different chains down. Again this is an example of an oxidation process with strong oxidisers such as hydrogen peroxide, lignin peroxidase and laccase. The reactions themselves are very complicated but result is a ‘combustion’ of lignin framework. The resultant molecules can become fungi toxic to the fungi so it detoxifies it by polymerisation.
