Light microscopy of radiata pine (Pinus radiata D. Don) field test stakes (20x20x500mm3) exposed in wet acidic (pH 3-4) soil for 12 - 24 months showed predominance of an unusual type of decay characte-rised by tunnelling attack of wood cell walls. After two years decay was moderate to severe in wood treated to ground contact CCA specifications and also equivalent retentions of creosote, and a number of new generation preservatives. Relative to other New Zealand temperate test sites and also an Australian tropical site, the New Zealand acidic soil test site was very aggressive. Correlative scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM) were used to elucidate the micromorphology of this attack. Tunnels of diameter 0.2-5 µm were present throughout all layers of the cell wall, and their orientation was not related to cellulose microfibril orientation. They also showed no preference for particular cell wall layers, indicating a lignin degrading capability. CLSM images showed that living, connecting fungal hyphae were present in the cell lumina and tunnels. This type of attack was predominant in wood that was highly saturated with water whereas wood that was less moist was predominantly attacked by classical white rot. Ongoing isolation and incubation studies in conjunction with further microscopy should enable identification of the fungal species involved.

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