Soft Rot

The term "soft rot" was originally used by Findlay and Savory (1954) to describe a specific type of wood decay caused by Ascomycetes and Deuteromycetes which typically produce chains of cavities within the S2 layer of soft- and hardwoods in terrestrial and aquatic environments (Liese 1955), for example when the wood-fill in cooling towers became destroyed despite water saturation, and when poles broke, although they were protected against Basidiomvcetes.

About 300 species (Seehann et al. 1975) to some 1,600 examples of ascomvcete and deuteromvcete fungi (Eaton and Hale 1993) cause soft rot, e.g., Chaeromium globosurn (Takahashi 1978), Hurnicola spp., Lecythophora hoffrnannii, Monodictys putredinis, Paecilornyces spp., and Thielavia terrestris. Soft-rot fungi differ from brown-rot and white-rot Basidiomycetes by grow-ing mainly inside the woody cell wall trate, starting from the tracheidal lumina., by means of thin perforation hyphae of less than 0.5 pm thickness into the tertiary wall and re-orientate then as thin hyphae after L-bending in one direction or after T-branching in both directions along the microfibrils in the secondary wall (soft rot type 1, Nilsson 1976). In longitudinal wood sections, hyphal activity is recognizable in the polar-ized light by rhombus-shaped cavities in the secondary wall of different size and arrangement (Levy 1966; Butcher 1975), which may be lined up like a string of pearls : The thin hypha stops its growth and the eca\ritevnidsotghieun_ developed around the hypha by the release of enzymes rana:es) along what is described as the proboscis hypha.

Within the cavity, hyphal thickness increases to about 3 pm. From the tip of the cavity, the next fine hypha starts its growth, which results in the next cavity, and continuous enlargement of existing cavities and the formation of new cavities lead to total destruction of the S2 layer (Eriksson et al. 1990; Daniel 2003).

SEM and TEM showed that the hyphae are normally associated with a variety of granular and fibrillar materials including extracellular slime, melanin and lignin breakdown products. In Lecythophora mutabilis, CCA was concentrated in the granular material (Daniel 2003). Several causes are discussed for oscillating hyphal growth and cavity formation . In cross sections, the cavities appear hole-shaped ("initial stage") and in-crease with advancing decay to larger wall openings. Finally, it comes to circular detaching of the tertiary wall ("advanced stage"). Because of their high lignin content, the tertiary and primary walls are attacked in the end ("late stage").

It remains an incomplete skeleton of middle lamella/primary walls ("destruction stage"). In the soft rot type 2, which particularly occurs in hardwood (Zabel et al. 1991), the hyphae erode particularly from the lumen the tertiary wall and penetrate till the middle lamella/primary wall. As rare variant, diffuse and irregular cavities in the secondary wall were described (Anagnost et al. 1994).

Soft rot develops also in monocotyledons (bamboos: Liese 1959; Sulaiman and Murphy 1992). In a broader definition for soft rot, each significant fungal decay of the woody cell wall by non-basidiomycete fungi was suggested, which however contrasts to the white-rot causing Ascomycetes.

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