Brown Rot And Soft Rot Fungi

Only a few organisms are capable of degrading the aromatic polymer lignin, the most efficient of which are fungi. Three groups of fungi are capable of lignin degradation (Eriksson et al. 1990): White rot, brown rot, and soft rot fungi (Table 1).

Brown rot fungi belonging to the basidiomycetes extensively degrade cell wall carbohydrates and only modify the lignin (Eriksson et al. 1990). Demethylation is the most obvious consequence of attack on lignin by these fungi. The brown rot fungi grows mainly in the cell lumen next to the secondary wall and cause a generalized, diffuse rot (Blanchette 2000). The residual wood is brown and often cracks into cubical pieces when dry. Brown rot fungi have an obvious preference for coniferous substrates (gymnosperms), which are softwoods. A survey of substrate relationships reported that 19% of North American basiodiomycetes are brown rot fungi,

Table 1 Lignin degrading fungi, their actions, and distribution

Organism

Subdivision

Examples

Actionsa

Distribution

White rot

Basidiomycetes

Phanerochaete sp.,

Mineralize lignin to CO2 and H2O;

Predominantly degrade wood

fungi

Pleurotus sp.,

some species preferentially remove

from deciduous trees

Bjerkandera sp., Trametes

lignin (selective delignification)

(angiosperms), containing

sp., and Phlebia sp.

whereas others degrade lignin and cellulose simultaneously

hardwood

Brown

Basidiomycetes

S. lacrymans P. betulinus,

Modify lignin by demethylation,

Preference for coniferous

rot fungi

G. trabeum, and

limited aromatic hydroxylation, and

substrates (gymnosperms),

P. placenta

ring cleavage

which are softwoods

Soft rot

Ascomycetes,

Chaetomium sp.,

Some lignin modification

Active generally in wet

fungi

Deuteromycetes

Ceratocystis sp., and Phialophora sp.

environments as well as in plant litter; attack both hardwood and softwood

a All groups degrade cellulose and hemicellulose that serve as actual carbon and energy source.

a All groups degrade cellulose and hemicellulose that serve as actual carbon and energy source.

among which 60 out of 71 (85%) occur primarily on conifers, and that they are mainly softwood degraders (Gilbertson 1980). They commonly cause decay of timber in buildings (Blanchette 2000). One of the most destructive brown rot fungi is Serpula lacrymans, which is well adapted to attacking timber in service and can spread rapidly on wood and traverse non-nutritional surfaces. Commonly, this type of decay has been referred to as dry rot. This term, apparently first used to describe any deterioration of dead wood or wood in service is misleading because moisture must be present for the decay to occur.

Soft rot fungi are taxonomically classified in the subdivisions, Ascomycota and Deuteromycota. They are active in environments that are too severe for white- or brown-rot fungi, generally in wet environments, but they also decompose plant litter in soils (Blanchette 1995). Soft rots are relatively unspecialized cellulolytic fungi in the genera Chaetomium, Ceratocystis, Phialophora, etc., that readily degrade cellulose and hemicellulose, but only modify lignin. They penetrate the secondary wall of the wood cell, forming cylindrical cavities in which the hyphae propagate. The rot is of limited extent, being closely associated with the fungal hyphae, because the cellulase enzymes do not diffuse freely through the wood.

Two distinct types of soft rot are currently recognized (Blanchette 2000). Type I is characterized by longitudinal cavities formed within the secondary wall of wood cells and Type 2 results in erosion of the entire secondary wall. The middle lamella is not degraded in contrast to cell wall erosion by white rot fungi, but may be modified in advanced stages of decay. As decay progresses, extensive carbohydrate loss occurs and lignin concentrations increase in the residual wood.

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