Fungal deterioration of sawn softwood lumber.
The colonisation of freshly sawn Corsican pine lumber by sapstain and mould fungi was
investigated at a sawmill in Hampshire, UK. Three repeat trials encompassing the different
seasons of the year were carried out over two years. Results show that fungal colonisation of
sawn lumber is dependent on the effect of time of year. Sawlogs were stored for different
intervals up to 16 weeks before conversion to boards. Boards were then stored for up to 12
weeks after milling and sampled every 4 weeks to determine the effect of timber ageing on
fungal colonisation up to 28 weeks after felling. The metabolic activity of wood cells over the
period after felling of the original tree was also measured. It was evident that the defacement of
boards reached maximum levels after 12 weeks exposure irrespective of seasonal influences.
Initial levels of fungal growth on lumber were reduced if the boards were milled from logs
stored for a period prior to conversion. Investigations into the metabolic activity of the wood
cells revealed significant levels of respiration taking place up to 28 weeks after felling of the
original tree including 12 weeks post-conversion into boards. Boards were used to make a
nested stack arrangement allowing plastic tanks top be placed in the centre. The tanks contained
a sub-sample of the full-size boards in order to investigate insect activity and effects of gammairradiation.
A total of 115 insect species representing 16 of the 34 British orders were collected
during the trials. Seventy-two percent of these insects were collected from within the stacks of
lumber and investigations using sealed tanks containing boards showed that the insects could
influence the fungal colonisation of sawn lumber. Despite the relatively short length of the
trials, a succession of insect colonisation from fungivores through to predators and detritivores
was recorded. Boards, which were sterilised by gamma-irradiation, were preferentially
colonised by mould fungi and subsequent internal staining was confined to the outer surface.
Trials with short-length billets allowed the wood-colonising ability of selected sapstain fungi to
be investigated under controlled conditions following sterilisation by gamma-irradiation or
autoclaving, and storage at 30°C and 20°C. Lesion formation in gamma-irradiated tissue was
solely due to the fungus potentially conditioning the wood for colonisation. Colonisation
studies also revealed that different fungi exhibit different strategies enabling them to infect
timber. Pathogenic species demonstrated a relatively fast initial growth rate to establish
themselves before triggering any host anti-fungal responses in the wood. The characteristic
lesions created in the billets were investigated using light and electron microscopy to reveal
hyphal invasion and or/ wood cell modifications. Respiratory activity of the lesions was
elucidated using radioactively labelled glucose allowing the metabolic pathways to be
ascertained and demonstrated that wood tissue in the apparently healthy regions adjacent to the
lesions reacted as if infected. Future work considers the possibility of biocontrol, using insects
in combination with gamma-irradiation of sawn lumber and also further investigations into the
reaction zones produced by the fungus growing in the wood.