Sapstain / Blue stain fungi

Blueing, blue stain or sapstain are terms used to describe wood that shows blue to greyish-black stains on its surface, caused by wood-staining fungi. Sapstain is considered a fault in timber and will be taken into account when sorting. The fault is that the discolouration of the wood makes it unsuitable for some applications.
Sapstain fungi live on the content of wood cells. The wood itself is not destroyed (no rot formation).

Which fungi cause sapstain?

The discolouration is caused by fungi of the Acomycetes or the Fungi imperfecti (Deuteromycetes) groups. Today, there are between 100 and 250 different known species of blue-stain or sapstain fungi. Among the most significant species of sapstain fungi are Ceratocystis (from the Acomycetes) as well as Aureobasidium, Alternaria und Cladosporium (from the Deuteromycetes group). The blue staining is often the result of a mixed infestation. To determine the individual species, subcultures of the fungi and microscopic examination are required.

How is the wood attacked?

Sapstain damage occurs mainly in coniferous wood. Pine timbers are particularly susceptible, but also spruce, fir, or larch timbers, as well as certain deciduous wood types like beech or imported timbers, such as Limba, Ramin or Brazil pine can be attacked by sapstain fungi.
Infection of the wood can happen in different ways. Spores can spread through the air, or be carried by insects or rainwater. Infection can be different depending on the fungus species. Some fungi depend on insects, others depend exclusively on air. Sapstain fungi also have different requirements regarding habitat, for which important determining factors are temperature and wood moisture.

What different kind of sapstain fungi are there?

There are three main kinds: trunk wood sapstain, sawn timber sapstain and sapstain occurring in timber coatings.

The fungi causing trunk wood sapstain attach very damp wood and can infest logs lying in forests. Though freshly cut wood is not usually attacked, the slightest reduction in moisture can result in sapstain fungi spreading into the sapwood. Sapstain in trunk wood is mainly caused by the Ceratocystis and Ophiostoma species.
(Photo: protected wood - unprotected wood with sapstain)

Fungi that cause sapstain in sawn timbers tend to attack slightly drier wood, like freshly cut planks and boards. They mainly occur in storage places after the trunks have been cut, in planks and boards that are badly stacked and not sufficiently dried.

The final strain occurs in wood that has already been processed and used in some application, either coated or not. Moisture and fungal spores can penetrate the wood through cracks in the coating. The fungi grow underneath the coating and form fruiting bodies that can raise this coating layer and cause damage after the wood has been put to its final use.

Why does the wood turn blue?

The spores germinate and develop into hyphae. After dividing numerous times, the sapstain mycelia are formed. This is hyaline (glassy or transparent) to begin with and spreads through the wood interior, mainly in the rays. In the course of its development, dark brown pigments (melanins) are formed in the hypha. The blue colouring comes from the hyphae of sapstain fungi - which have been darkened by the melanins inside them - shimmering through to the wood surface. The blue appearance is therefore no more than an optical illusion, similar to that which occurs with cigarette smoke which also appears to be blue, though the ash particles are actually black. The colour of blue-stained wood depends on the concentration of the pigments and consequently from the number of hyphae in the wood. The more hyphae, the darker the colour.

How can sapstain be prevented?

The risk of infestation from trunk wood sapstain can be significantly reduced by the correct choice of felling time, optimisation of log storage and, above all, by processing the timber quickly.
During the drying phase of freshly felled logs in the saw mill, the risk of sawn timber sapstain is especially high. Temporarily effective, environmentally compatible anti-sapstain products protect the wood during the drying phase and prevent massive losses in its value.
Correct priming impregnation treatment, using the double vacuum process, for instance, will reliably prevent the occurrence of sapstain and consequently eliminate costly and time-consuming renovation of painted and coated timbers.

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Blue stain attack on sawn pine