A Novel Type of Multiple Cavity Attack in Wood Cell Walls of Heat-treated Timber Exposed in Seawater – Preliminary Observations
R A Eaton, C Björdal, T Nilsson
Samples of untreated and heat-treated Norway spruce and Douglas fir were submerged in Langstone Harbour, Portsmouth for 4 years at a depth of ca. 0.3 metres. The heat-treated samples had been prepared using the Plato process. Samples were initially assessed for the severity of marine borer damage and were then examined microscopically for evidence of microbial decay. Longitudinal and transverse wood sections removed from the surface of undamaged regions of heat-treated samples revealed the presence of an unusual and novel form of decay in the cell walls of tracheids. The decay pattern produced was reminiscent of Type 1. fungal soft rot attack. However the characteristic chains of cavities in the S2 cell wall layer were absent. Instead, invasion of the cell wall by fine penetration hyphae emerging from wider lumen hyphae resulted in the fine hyphae growing into the S2 region of tracheid walls horizontal to the vertical axis. Thus the fine penetration hyphae grew around the cell wall rather than adopting the helical orientation of cellulose microfibrils in the S2 layer, typical of soft rot cavity formation. Horizontal penetration hyphae appeared to traverse a quarter to one half of the cell wall circumference, producing several lateral branches at right angles. These were orientated more or less parallel to the vertical tracheid axis. Up to 15 fine lateral hyphal branches were observed both above and below the horizontal penetration hypha giving the overall appearance of a double-sided ‘comb’. The fine tunnels in the wood cell wall formed by the lateral hyphae enlarged and became wider and eventually coalesced leading to cell wall collapse. There is no evidence so far that tunnels elongate further once they have formed, or that fine proboscis hyphae emerge at the tips. Marine fungi have been isolated from the heat-treated samples, but so far re-infection of sterile, unexposed, heat-treated wood with pure fungal cultures has not reproduced the decay pattern.
The micromorphology of this novel form of decay is discussed in the context of wood cell wall architecture following heat-treatment of timber.