Soft Rot Determines Service Life of L-joints with Low Borate Loading
P I Morris, J Wang, J K Ingram
As part of a research program on the potential use of borates by the Canadian wood products industry, Forintek set up an L-joint test of untreated and borate-diffusion treated hem-fir (western
hemlock and amabilis fir) in 1990. The treated material had a low initial retention of only 0.2% boric acid equivalent and analysis after 5 years revealed no detectable borate close to the tenon. Nevertheless the borate treated L-joints showed no signs of decay for 7 years. Only 12% showed signs of decay after 10 years, and there was very slow deterioration after that. In contrast 93% of the untreated L-joints showed signs of decay after 10 years and two thirds had failed. Microscopic evaluation after 17 years exposure showed soft rot in all of the 21 units remaining after removal of material for analysis. Soft rot was also present in all three remaining untreated units. The initial low loading of borate may have been sufficient to stop colonization by wood rotting basidiomycetes but insufficient to stop colonization by soft rot fungi. The soft-rot fungi were unable to rapidly decay the wood due to unsuitable conditions but they were able to exclude the basidiomycetes from the niche.