6 Wood Discoloration

6.2 Blue Stain 125
There are various textbooks and keys to identify molds (e.g., Wang 1990;
Kiffer and Morelet 2000; Samson and Hoekstra 2004; Samson et al. 2004).
The attachment of a species to the molds is not always strict. There are overlappings
with blue-stain and soft-rot fungi since fungi traditionally implicated
in wood discoloration can cause soft rot if the conditions are suitable (e.g., Alternaria
alternata, Cladosporium herbarum, Aspergillus fumigatus) and many
soft-rot fungi are highly melanized (e.g., Phialophora spp.). That is, a fungus
all may show the typical superficial mold growth and is treated in textbooks
on molds, but also effected blue stain, or produced weight loss in soft-rot tests
(Seehann et al. 1975; Daniel 2003).
6.2
Blue Stain
Blue stain (synonymous sap stain) is a blue, grey or black, radially striped
wood discoloration of sapwood, which can be caused by about 100 to 250
(Käärik 1980) fungi belonging to the Ascomycetes and Deuteromycetes. Seifert
(1999) and others differentiated three groups of blue-stain fungi: – Ceratocystis,
Ophiostoma and Ceratocystiopsis species (Upadhyay 1981; Perry 1991;
Gibbs 1999), – black yeasts such as Hormonema dematioides, Aureobasidium
pullulans, Rhinocladiella atrovirens, andPhialophora species, – dark molds
such as Alternaria alternata, Cladosporium sphaerospermum,andC. cladosporioides.
Yang (1999) differentiated dark staining fungi, such as Ophiostoma
piliferum on jack pine, Ceratocystis minor on white pine, and C. coerulescens
on white spruce, and light staining fungi, such as O. piceae, C. adiposa and
Leptographium sp. Frequently, like in the Ophiostoma species, the teleomorph
is a perithecium (Figs. 2.14, 6.3E). Blue stain occurs in conifers, particularly
in pine, but also in spruce, fir, and larch, in hardwoods, like beech and birch,
and in tropical woods. The stain may be superficial or penetrate deeply into
the wood. In heartwood species, only the sapwood discolors, since blue-stain
fungi live mainly on the content of the parenchyma cells. Figure 6.3 shows
some details of blue stain.
The hyphae are brown colored due to melanin (Zink and Fengel 1989) and
relatively thick (Fig. 6.3C). Some species like A. pullulans develop dark-brown,
thick-walled chlamydospores (Fig. 6.3D). The blue-black color of the wood
develops as optical effect due to refraction of light. Hyphae penetrate into stem
wood from cross sections or radially through bark fissures and move via the
medullary rays. Easily accessible nutrients (sugars, carbohydrates, starch, proteins,
fats, extractives) are taken up from the ray parenchyma cells. Xylanase,
mannanase, pectinase and amylase have been detected in several blue-stain
fungi (Schirp et al. 2003a). From the rays, the hyphae penetrate into the longitudinal
tracheids with mechanical pressure through the torus of the bordered
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