6 Wood Discoloration

6.2 Blue Stain 127
40 ◦ C. The moisture span reaches from fiber saturation close to umax.Inmany
species, the optimum is between 30 and 120% (Käärik 1980; Schumacher and
Schulz 1992). For log colonization, moisture loss in the felled tree of 10–15%
is sufficient. Blue stain occurs during seasoning or transportation of green
lumber before the wood is dried and is enhanced at relative humidities above
90% (Seifert 1999).
Blue-stain fungi were arranged into different ecological groups (Butin 1995):
In blue stain of stems (primary blue stain), spores of Ophiostoma species (moisture
optimum 50–130%), particularly Ophiostoma piceae (Harrington et al.
2001) and also Discula pinicola are transferred by wind in bark wounds (forest
work or wood transport) as well as by bark beetles particularly in un-debarked
pine stems which are allowed to dry out slowly over weeks or months while lying
in the forest (Neumüller and Brandstätter 1995). Hormonema dematioides,
A. pullulans, and a Leptographium species were the most frequently isolated
stain-fungi from bark and sapwood of living Pinus banksiana trees. There were
indications that none of the well-known log-staining fungi was associated with
healthy living jack pine trees, and it was deduced that prompt transportation
of logs from forests to sawmills and sanitary treatment of log storage yards
helps to reduce the severity of log staining before sawing (Yang 2004). The
most aggressive sapstain species on fresh Pinus contorta logs was Ceratocystis
coerulescens, followed consecutively by Leptographium spp., C. minor, O. piliferum,
O. piceae, O. setosum, C. pluriannulata, andA. pullulans (Fleet et al.
2001). Discula pinicola is the main cause of the so-called internal blue stain,
which is characterized by a central wood discoloration without any external
staining. A comparison of the growth of several blue-stain fungi in freshly cut
pine billets has been performed by Uzunović and Webber (1998). The bluestain
fungal composition on Pinus radiata logs harvested in New Zealand and
shipped to Japan showed differences between summer and winter transport
(Thwaites et al. 2004).
Blue stain of sawn timber (secondary blue stain) is caused e.g., by Cladosporium
species (moisture optimum 50–100%) and Strasseria geniculata (Butin
1995) in sawn timber that is not completely dry or badly stacked in timber
yards (Schumacher et al. 2003).
The classical distinction in primary and secondary blue-stain fungi was not
confirmed however by the frequent occurrence of D. pinicola both in stored
pine stems and in boards (Schumacher and Schulz 1992). Battens of Sitka
spruce were stained by O. piceae when the surface moisture content in a stack
was 22% or more (Payne et al. 1999).
Tertiary blue stain (moisture optimum 30–80%) results frequently from A.
pullulans and Sclerophoma pithyophila on timber that has been converted into
products, was painted and re-imbibe moisture while in service, like wooden
façades, window frames, garage doors and garden furniture. Through damages
of the coating in window wood e.g., by nails or due to inappropriate
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