blue stain
A blue or grey discolouration in sapwood caused by mildew infection.
blue stain: fungal discoloration in freshly felled timber
Blue stain is a blue or grey discoloration that appears in the sapwood of freshly felled logs and freshly sawn timber, caused by growth of pigmented fungi in the wood cells. The stain penetrates only a few millimeters into the surface and does not substantially weaken the wood itself, but it makes the affected timber unsuitable for most end uses where appearance matters. The fungi responsible are not rot organisms; they do not consume the wood's structural components, though they do consume sugars and starches present in the sapwood.
Blue stain becomes visible within days to weeks of felling, depending on moisture content, temperature, and humidity. It spreads fastest in conditions above 15 degrees Celsius with relative humidity above 80 percent. The fungi thrive in the presence of free water on the wood surface and in the sapwood itself. Softwoods, particularly pine, spruce, and fir, are most vulnerable because their sapwood is wider and richer in nutrients than that of hardwoods. Logs stored in the open during wet weather, or timber in poorly ventilated stacks, are at highest risk.
Prevention and response
Control relies on reducing moisture and promoting air circulation. Green logs can be sprayed with fungicidal coatings containing registered timber preservatives, typically copper-based compounds or chlorinated phenols, applied immediately after felling. Freshly sawn boards must be kiln-dried or air-dried rapidly; slowing drying time increases the window for fungal colonization. Debarking logs promptly also reduces risk, since fungi often enter through bark wounds and the interface between bark and sapwood.
The stain is purely cosmetic in structural timber applications, and kiln-drying will halt further fungal growth. However, many buyers, particularly in export markets and in higher-grade products, reject stained lumber outright. The economic cost lies not in strength loss but in downgrading, rejection, or the expense of fungicidal treatment and rapid drying. For this reason, preventing blue stain remains a standard mill practice even though the defect is superficial.