Wood blue staining is most commonly caused by fungi belonging to the Ophiostoma genus (class Ascomycetes), as well as representatives of other fungal groups such as Aureobasidium pullulans (de Bary) G. Arnaud, Phialophora asteris (Dowson) Burge & I. Isaac, Cladosporium herbarum (Pers.) Link, Discula pinicola (Naumov) Petr., Alternaria humicola Oudem., and others. Blue staining can occur in both softwood and hardwood species. The effect of these fungi changes the natural color of the wood (sapwood). Such discoloration often appears on cross-sections of freshly cut, bark-on round timber and on slowly drying sawn wood. Due to the nature of these color changes, the physical and mechanical properties of the wood are usually not affected. Discoloration can be superficial, up to 2 mm deep, or deeper, spreading into the inner layers of the wood. Studies have shown that the strength of healthy wood and wood affected by blue-staining fungi does not differ. Affected wood can be used for the same purposes as healthy wood; the difference is only in its visual appearance. The altered color is caused by pigments (melanins) present in the mycelium of the fungi living in the wood.
From Ophiostoma fungal genera Ophiostoma piceae (Münch) Syd. & P. Syd. is one of the main fungi responsible for blue staining in softwood. This fungus spreads via spores carried by insects, wind, and water. In bark-on logs, initial blue-stain spots often appear along beetle galleries. The optimal temperature for fungal growth is +20–+25 °C. At lower temperatures (+5–+7 °C), mycelial growth slows. Below -10 °C or above +48 °C, fungal growth stops, though the fungus remains viable and can resume activity when favorable conditions return. However, in wood with moisture content below 18–20%, the fungus cannot grow or develop. To create unfavorable conditions for the fungus, sawn timber should be dried to 18–20% moisture or less. The fungus cannot spread in very dry or excessively wet wood; optimal growth occurs when wood moisture is between 33 % and 82 %.
Sometimes blue-staining fungi are confused with mould fungi—but they are not the same. Mould fungi develop only on the wood surface and can be removed, whereas blue-staining fungi spread inside the wood. Blue-staining fungi are also not wood-decay fungi.
Latin name
Currently, up to 250 species of blue-staining fungi are known. Accurate identification of the species requires laboratory testing. One of the main species is named: Ophiostoma piceae (Münch) Syd. & P. Syd. Another species frequently mentioned in the literature is Aureobasidium pullulans (de Bary) G. Arnaud.
Exact Lithuanian name
There is no specific Lithuanian name—these fungi are generally referred to as blue-staining fungi. For precision, the Latin names are used.
When and how it appears
It develops under favorable conditions—moisture, temperature, oxygen availability, and the presence of necessary nutrients—once the wood becomes infected. Wood can be infected in several ways: spores can enter through the air, insects, or water, depending on the species of blue-staining fungus. Once inside, the spores germinate and form hyphae, which determine the color of the stained wood.
Can it be avoided by cutting the tree at a specific time?
Yes—cutting the tree during a cold period can prevent blue staining. As mentioned, at low temperatures (+5–+7 °C) mycelial growth slows, and below -10 °C, fungal growth and development stop completely. Additionally, the cut wood should be delivered for processing as quickly as possible (in summer, no longer than 15 days).
Do wood storage conditions have an effect?
Yes—storage conditions significantly affect blue staining in wood. Key factors for fungal development include temperature, moisture, and oxygen availability. Fungi do not thrive when wood moisture is below 18% or the temperature is below -10 °C. Excessive moisture is also unfavorable, as it can create oxygen deficiency within the wood. During storage, treating the log ends with chemical agents can help prevent fungal growth. Fungicides should be applied no later than 24 hours after cutting, as delays reduce their effectiveness. It is also advisable not to store infected and healthy wood together.
Does it appear only in wood with moisture content above 18%?
Yes—blue-staining fungi generally develop when wood moisture exceeds 18%.
Can it develop only under specific conditions?
Yes—if conditions are unfavorable, the fungus will not develop. Essential factors for its growth include moisture, temperature, oxygen, and nutrients.
Does it appear at other moisture levels in wood?
Optimal conditions for fungal growth occur when wood moisture is between 33% and 82%. Moisture below 18% inhibits fungal development. Excessive moisture also limits fungal activity due to oxygen deficiency within the wood.
Is it dangerous?
Yes—it is dangerous to a growing tree, as the fungus consumes water, which can cause the wood to dry out. It is not harmful to human health. Mould fungi, however, can pose health risks.
Does it damage the wood?
Blue-staining fungi do not affect the mechanical properties of wood. There is no significant difference in strength between healthy and stained wood. This fungus does not cause wood decay; it grows within the wood cells without damaging them.
Only the visual appearance of the wood changes.
Does it affect only the wood’s appearance?
Yes—these fungi change the color of the wood. This is a purely visual defect.
