Decay micromorphology was studied systematically for diversely preservative treated Pinus radiata and Fagus sylvatica 20 x 20 x 500 mm stakes across 13 in-ground field test sites, during a 6.5 year exposure. Sites were selected to maximise occurrence of a diverse range of decay types. Presence of clamp connections in close associated with tunnelling coupled with macroscopic features similar to white rot was a strong indication that white rot basidiomycetes were the causative agents of hyphal tunnelling. Hyphal tunnelling is a fungal decay type that does not fit neatly within the decay types brown rot, soft rot (Types 1, 2 and diffuse) and white rot (simultaneous and preferential). Overly rigid decay classification boundaries that place emphasis on taxonomic affinity and biochemical reactions under laboratory conditions, fail to accommodate the diversity of decay types that occur under field conditions. Classical simultaneous white rot and type 2 erosion soft rot were almost completely absent from preservative treated pine and beech across 13 highly diverse sites. It is possible that fungal erosion decay was of minor significance in preservative treated wood because the decay fungi that cause it in other situations (e.g. in untreated wood) adopt a tunnelling or cavitation mode in preservative treated woods i.e. they adopt a stress tolerant strategy in response to the high lignin content and/or preservative content of the lumen wall. It is known that some basidiomycetes produce cavitation with features very similar to soft rot cavitation and this study and the work of others indicates that basidiomycetes also produce a mixture of features typically associated with soft rot and those fungi which produced hyphal tunnelling. It is possible that some of these basidiomycetes possess multiple decay capabilities.

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