White Rot

White-rot research has been reviewed by Ericksson et al. (1990) and Mess-ner et al. (2003). White rot means the degradation of cellulose, hemicellu-loses, and lignin usually by Basidiomycetes and rarely by Ascomycetes, e.g., Kretzschmaria deusta and Xylaria hypoxylon.

White rot has been classified by macroscopic characteristics into white-pocket, white-mottled, and white-stringy, the different types being affected by the fungal species, wood species, and ecological conditions. From microscopic and ultrastructural investiga-tions, two main types of white rot have been distinguished (Liese 1970).

In the simultaneous white rot ("corrosion rot"), carbohydrates and lignin are almost uniformly degraded at the same time and at a similar rate during all decay stages. Typical fungi with simultaneous white rot are Fomes fomentar-ws, Phellinus igniarius, Phellinus robustus, and Trametes versicolor in standing trees and stored hardwoods (Blanchette 1984a).

Wood decayed by F. fornentarius, T versicolor and some other fungi shows black demarcation lines (zone lines) (Fig. 7.2a), by which different species, or incompatible mycelia of the same species separate themselves from each other, or mycelia dissociate them-selves from not yet colonized wood ("marble rot", in German: "Marmorfaule"). The lines result from fungal phenol oxidases, whereby fungal compounds or also host-own substances are transformed to melanin (Li 1981; Butin 1995).

As a function of the moisture distribution in wood, or between different fun-gal species or incompatible genotypes, a compartmentalization of individual decay centers can result from black pseudosclerotic layers of firmly structured mycelium (Rayner and Boddy 1988; Eriksson et al. 1990).

Cell wall decay can start by microhyphae producing holes in the secondary wall (Schmid and Liese 1966), which flow together to larger wall openings with advancing decay. Usually, however, the hyphae grow inside the lumen with close contact to the tertiary wall. The hypha surrounded by a slime layer excretes the degrading agents, which are active only in direct proximity of the hvpha. Thus, a lysis zone develops under the hypha, and the hypha produces grooves in the wall which is gradually reduced in thickness, like a river erodes theg round (Schmid and Liese 1964; Liese 1970).

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