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Sunday, 22 January 2017

Brown Rot



Brown Rot is caused by Basidiomvcetes, which metabolize the carbohydrates cellulose and hemicelluloses of the woody cell wall by non-enzymatic and enzymatic action and leave the lignin almost unaltered , whereby the brown color develops. 




Brown-rot fungi do not produce lignin-degrading enzymes. There are how-ever reports of lignin peroxidase and manganese peroxidase in some brown-rot fungi, and lignin loss or metabolization by brown-rot fungi have been reported. Particularly in later stages of decay, the highly lignified middle lamella/primary walls were observed to undergo attack. Also, the penetration of the wood cell wall by bore holes removes lignin in the process, all suggesting that low molec-ular weight lignin degrading agents and potentially even lignin degrading enzymes max occur in some brown-rot fungi, at least with localized activity (Goodell 2003).

 Laccase activity was also found in Coniophora puteana (Lee et. al. 2004), and in Glocophyllum trabeum and Oligoporus placenta (Goodell 2003). Non-enzymatic, low molecular agents produced by the brown-rot fungi are responsible for initial stages of cell wall attack (Goodell 2003; Chap. 4). Of about 1,700 wood-degrading Basidiomycetes in North America, only 120 species (7%) caused brown rot, and of these 79 (65%) were polypores (Eriksson et al. 1990; Ryvarden and Gilbertson 1993). White-rot fungi distribute broader over the different basidiomycetous groups and some belong to the Ascomycetes (Rayner and Boddy 1988). Most brown-rot fungi affect conifers (Ryvarden and Gilbertson 1993), while white-rot fungi occur more frequently on hardwoods. 

Brown rot occurs in standing trees, felled and processed wood as well as in sapwood and heartwood. In the northern hemisphere, the majority of timber used in construction is from conifers. Thus, a large part of wood in outdoor and indoor service is destructed due to the action of brown-rot fungi. Brown rot is usually uniformly distributed over the substrate. A brown cubical pocket rot is caused by Laurelia taxodii in cypress and by Oligoporus amarus  in incense cedar. Decay pockets are localized and surrounded by firm wood (Zabel and Morrell 1992). 

A woody substrate both may show brown rot and white rot; a standing tree of Picea engelmannii exhibited "white pocket rot" by Phellinus pini in the heartwood , and after wind throw the healthy areas became brown-rotten (Blanchette 1983). Brown-rot wood debris is extremely stable due to its content of slightly modified lignin and has remained unaltered in the soil for centuries. In conifers forests, this humic material may comprise up to 30 vol % in the upper layers (Swift 1982; Ryvarden and Gilbertson 1993). 

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