the surface the wood is invariably sound. Soft-rotted wood is characteristically
darkened in colour and has a spongy soft texture when wet. A further charac-
teristic is the extensive longitudinal and cross-cracking that occurs at the sur-
face when the wood dries out. Wood exposed in seawater suffers from attack
by soft rot fungi. The speed of infestation and decay of the wood depends on
its natural durability, whether the material is sapwood or heartwood, and on
the environmental conditions at the site. Following submergence, the condi-
tions within the wood, i.e.water saturation and reduced oxygen levels, promote
the growth of lower ascomycete and mitosporic fungi. Members of these two
groups of fungi are traditionally recognised as agents of soft rot decay, but the
development of microscopic features characteristic of soft rot have also been
reported in wood infected by basidiomycete fungi.
It is now estimated that there are 444 formally described higher marine fungi
and of these, 360 species from 177 genera belong to the Ascomycota, 74 species
from 51 genera belong to the mitosporic fungi and 10 species from seven gen-
era belong to the Basidiomycota. A great majority of these fungi appear to have
adapted to an aquatic habitat by virtue of the elaborate appendages on their
spores. The spore appendages are not only considered important in aiding buoy-
ancy and therefore dispersal in the sea, but they also have an important role in
aiding attachment of spores to suitable substrata, notably wood surfaces. Follow-
ing attachment and spore germination, colonisation and decay by these fungi
will take place. Studies to assess the decay capabilities of marine fungi suggest
that around three-quarters of these could cause mass loss in wood.
The soft rot decay process begins with hyphal colonisation of wood via the
rays and in hardwoods also via the vessels. Soft rot can take two forms: cav-
ity attack (type 1) and erosion of the cell wall (type 2). Cavity attack is the
classical form of soft rot and when viewed in longitudinal sections of decayed
wood, appears as chains of biconical or diamond-shaped cavities, and more
or less cylindrical cavities with conical ends (Figure 9). Entry into the wood
cell wall by hyphae growing in the lumen of the wood cell occurs through
transverse penetration of a fine hypha into the S 3 layer. On reaching the S 2
layer, the fine penetration of hypha is realigned parallel to the direction of the
cellulose microfibrils. The fine hypha grows in an axial direction, but eventu-
ally apical growth ceases. Enzymes released from the hyphal surface start to
degrade the wood cell wall creating a cavity, and gradually the width of the
hypha increases. After a short period of time, a second phase of apical growth
begins with the emergence of a fine hypha from the tip of the cavity. This
hypha then grows in the cell wall for a period of time, but again growth ceases.
Cavities develop in the cellulose-rich S 2 layer of the secondary wall and fol-
low the helical orientation of the cellulose microfibrils; in badly attacked
wood, a series of cavity chains aligned in parallel can be observed before the
cell wall begins to collapse. Chains of cavities are therefore formed as a result
278 Chapter 11