Biodeterioration of varnished wood with and without biocide: implications for standard test methods

There is little information in the literature on the colonisation and biodeterioration of varnished wood and the effect of biocides on these processes. We investigated the effect of the incorporation of a pyridine biocide (2,3,5,6-tetrachloro-4{methylsulfonyl}-pyridine) on the deterioration of a superficial layer of alkyd varnish on pine panels after 30 days incubation in an environmental chamber. Three genera of filamentous fungi and bacteria and two genera of yeasts and algae were isolated from varnished wood panels exposed to the environment in southern Brazil and used in the inocula. The highest degree of surface changes occurred in the panels inoculated with the fungal mixture below the non-biocide-containing varnish. The method of inoculating the panels made a significant difference to the results, with higher deterioration occurring in panels inoculated beneath, as opposed to above, the varnish. The incorporation of 0.3% biocide into the varnish significantly reduced the degree of surface changes occurring after incubation with any of the inocula. The effect was particularly noticeable for the fungal inoculum (85% reduction). The greatest damage in biocide-containing panels was produced by the mixed inoculum. This suggests that, although fungi are the most aggressive organisms, biocide efficacy is best assessed with a mixed inoculum containing various microbial groups. The overall effect of the biocide was to reduce damage to the varnish layer by approximately 62%.