Brown rot frequently occurs in buildings in which wood products
are in contact with a source of moisture. One of the most destructive
fungi causing timber decay is Serpula lacrymans. This brown-rot fungus
hasthe capacity to spread rapidly through wood and across nonnutritional
surfaces ( Jennings and Bravery 1991). Fungi that cause brown rot are a
significant threat to the conservation of ancient and historic buildings.
Brown-rot fungi are also responsible for the decay of wooden objects,
such as those from ancient Egyptian tombs along the Nile Valley that
were apparently affected by intermittent flooding or from other sources of
moisture that migrated into the tombs (Fig. 2c, d). The severe compromise
of wood integrity after an attack of brown rot presents difficult conserva-
tion problems (Blanchette et al. 1991; Blanchette et al. 1994). The extensive
degradation of cellulose caused by these fungi leaves such an extremely
weak framework of residual wall material that fragmentation occurs with
only slight pressure or agitation (Fig. 2a–d). Dry rotis a common but
inappropriate term that has been used instead ofbrown rot. Although the
wood is often dry when found, moisture was needed for the decay to be
initiated. The surfaces of older decayed wood usually crack and check
when brown rot has been the degradative agent; the result is dried, cubical
zones of brown wood.Soft rot
Soft rot in wood often resembles brown rot macroscopically but differs
remarkably in its microscopic characteristics. Soft rot may be localized to a
shallow zone on wood surfaces or be more diffuse, depending on environ-
mental conditions and the length of time over which decay has occurred.
It may be associated with water-saturated environments or with relatively
dry environments where lack ofmoisture or interacting alkaline conditions
appear to inhibit other, more aggressive brown- and white-rot fungi
(Blanchette et al. 1990; Blanchette and Simpson 1992). Microscopic obser-
vations of soft rot in many wood species reveal cavities within the sec-
ondary wall (Fig. 3a–d). Fungal hyphae colonize cell lumina and produce
fine hyphae that penetrate into the cell wall. Once inside the wall, the
hypha aligns its growth along the same axis as the microfibrils and initiates
a localized degradation of the cell wall. In transverse sections, holes are
observed within the S 2 region of the secondary wall (Fig. 3a, b). These
degraded zones are actually chains of cavities with conical ends formed by
oscillatory growth patterns from the soft-rot fungus (Fig. 3c, d). Cellulose
and hemicellulose are extensively degraded, and some lignin is lost, but
substantial amounts of modified lignin remain in the degraded wood. In
some woods, particularly low-density hardwoods, another form of soft-rot
attack may occur. The fungus enters cell lumina and progressively erodes
all secondary wall layers from the lumen toward the middle lamella region
(Blanchette et al. 1990; Nilsson et al. 1989). The middle lamella is not
degraded, leaving a highly lignified framework of lamellae between cells.
Significant strength losses are associated with advanced stages of soft rot,
but reductions in strength during incipient to intermediate stages of decay
are not well documented (Kirk and Cowling 1984; Zabel and Morrell 1992).White rot
White-rot fungi have the capacity to degrade all cell-wall components
(Fig. 4a–d). Preferential degradation of phenolic extractives, as well as of
lignin, often results in a mottled or overall bleached-white appearance.A G W D C M I 59