6 Wood Discoloration

182 8 Habitat of Wood Fungi
The type and intensity of a biological attack can be recognized by different
macromorphologic changes of the wood tissue. Typical discolorations occur
on and inside wood that is colonized by molds, blue stain, and red-streaking
fungi. Brown- and soft-rotten woods differ in color and shape of the brown
and soft-rotten cubes, and white-rotten wood between simultaneous and white
pocket rot.
Various mechanical and physical wood changes occur when wood-inhabiting
microorganisms colonize wood. Wood mass (weight) loss is a commonly
used measure of decay capability. The basic calculation is: [(weight before −
weight after): weight before] ×100%. The extent of decay in a specimen that
was sampled from attacked wood can be determined the same way, if its dry
weight is compared to that of a comparable healthy control: mass loss ML
(%) = [(DW1 −DW2) :DW1] × 100 (DW1 = dry weight of control, DW2 =dry
weight of decayed sample).
Mass loss of wood samples exposed to fungi is likewise used to determine the
efficacy of wood preservatives and to examine the natural durability of wood
species. There is a permanent discussion if fungal pure cultures or artificial
mixed cultures should be used in laboratory tests (Kolle flask method, soilblock
test, vermiculite method) or if soil contact decay tests are preferable.
Laboratory tests are reproducible as they are based on defined test fungi.
Field stake tests result in a severe exposure condition as the natural microbial
composition may contain microorganisms that degrade wood, biodegrade
organic wood preservatives or modify inorganic preservatives making them
more susceptible to leaching (Nicholas and Crawford 2003). In Europe, the
Kolle flasks method with malt extract agar and defined wood blocks of 5 ×
2.5 × 1.5 cm in size from Scots pine sapwood and European beech is used for
Basidiomycetes according to the standard EN 113 (Fig. 7.5; Table 7.6). In this
method, specified isolates of certain fungal species, e.g., Coniophora puteana
Ebw. 15, have to be used. The wood decay capacity of the test organisms is,
however, erroneously named “virulence”, although it concerns fungi and not
viruses.Soft-rotfungi tests are performedinplasticcontainers with vermiculite
(grainy substance of aluminum iron magnesium silicate) as moisture and
nutrient depot. The standard soil block test AWPA E10 uses either 14-mm or
19-mm wood cubes that are exposed to white- and brown-rot fungi that were
previously inoculated onto wood wafers on top of a sterile moist soil bed in
a bottle. Soil bed testing based on the methodology described in the European
Pre-standard ENV 807 uses 100 mmlong ×10mmrad ×5mmtang specimens that
are exposed to the naturally soil-inhabiting microorganisms (v. Acker et al.
2003). Field stake tests use stakes or posts of the selected wood species that
are half buried vertically in soil and inspected for decay at intervals. Wood
assembly above-ground tests (post-rail, L-joint, lap-joint), all including some
type of joint that effectively traps rainwater, simulate decking, door frames or
joinery (Zabel and Morrell 1992; Nicholas and Crawford 2003).
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