6 Wood Discoloration

7.3 Soft Rot 145
Further infection symptoms are the blunt fracture and short-fibrous breaking
out of splinters when puncturing.
Within the cell wall, soft-rot fungi degrade cellulose and hemicelluloses.
Compared to the brown-rot fungi the cellulolytic agents diffuse, however, not
sodeepintothecellwall,butremainindirectproximityofthehyphae(Liese
1964). Lignin is not (or little) attacked at least in the initial stage, mainly
by demethylation, so that soft rot with regard to the decay type resembles
brownrot.IsolatedligninsandDHP’sarenotdemethylated.Inligninmodel
compounds, the β-O4 linkage and the aromatic ring were cleaved (Eriksson
et al. 1990; also Bauch et al. 1976).
The inhibiting effect of lignin was demonstrated by the result that a delignifying
pretreatment promoted the carbohydrate degradation (Zainal 1976).
Wood decay by soft-rot fungi is further affected by the quantity and type of the
lignin: Lignin-rich softwood with lignin predominantly made of coniferyl units
is more resistant than the lignin-poorer hardwood made of sinapyl-coniferyl
units (Nilsson et al. 1988; Eriksson et al. 1990). In conifers, wood decay occurs
preferentially in the late wood (Fig. 7.4b) with its relative low lignin and high
cellulose content.
Due to the intensive carbohydrate degradation, soft-rot fungi, just like
brown-rot fungi, already cause about 50% decrease of impact bending at only
5% mass loss, and cracks occur by the reduction of the dimensional stability.
Soft rot develops in trees, stored wood, and in outside used wood. Soft-rot
fungi can decay wood under extreme ecological conditions, which are unsuitable
for Basidiomycetes: constantly wet wood till almost water saturation, like
in harbor constructions and ships, but not permanently submerged, as well
as wood in soil contact, like poles, piles, sleepers (Liese 1959). Several soft-rot
fungi were found on rotting branches (Butin and Kowalski 1992). Soft-rot fungi
(and Basidiomycetes) under marine conditions were described by Kohlmeyer
(1977), Leightley and Eaton (1980) and Troya et al. (1991). The wood moisture
tolerance of the fungi reaches from dryness resistance to decay at almost water
saturation. For example, Chaetomium globosum and Paecilomyces spp. did not
show any inhibition of their decay ability in beech wood samples of 200%
wood moisture content (Liese and Ammer 1964). With altogether relatively
low oxygen demand, soft-rot fungi receive the necessary oxygen for the decay
of water-saturated wood in cooling towers by the sprinkling effect of the water,
which brings oxygen in solution. Thermophilic species and those with the ability
of heat resistance destroy wood in the inner of wood chip piles (Hajny 1966;
Smith 1975). Chaetomium globosum canstartgrowinginnutrientsolutions
with initial pH values from 3 to 11. Some soft-rot fungi decay woods with high
natural durability, like Bongossi or Teak. After 21 years of outdoor exposure
in soil, the heartwood of several hardwoods exhibited soft rot in about threequarters
of all the samples, about one-quarter white rot and only 3% brown
rot (Johnson and Thornton 1991). Soft-rot fungi are tolerant to chrome fluo-
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