Wood used in outside applications is susceptible to weathering and photo degradation, which often leads to surface discoloration, loss of brightness and surface deterioration. Research has shown that acetylated wood is more resistant against brown rot, white rot and soft rot, and more dimensionally stable than untreated wood. However, acetylated wood seems still to be disfigured by surface moulds and staining fungi. Samples of acetylated Southern Yellow pine at three different treatment levels; low, intermediate and high acetyl content were exposed outdoors at Ås (Norway) for a period of 9 months. Corresponding samples from the same material where tested according to a modified version of EN 15457 (agar plate test) using A.pullulans as test fungi. In the outdoor test unmodified wood had higher mould ratings than acetylated wood, and there was a tendency that samples with low acetyl content had lower mould ratings than samples with higher acetyl content. In the laboratory test, non-leached samples with high acetyl content had significantly higher mould rating than samples with lower acetyl content and unmodified samples. In this study a concentration of free acetic acid of 0.77% seemed to restrict growth of A.pullulans on samples of a high treatment level.

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