- Treating durable, second-growth or plantation-grown material with supplemental chemicals could improve durability while enhancing the uniformity of the products. To test this hypothesis, the effect of two candidate biocide treatments on the durability of teak, redwood, and western redcedar was assessed in a ground proximity field test established in Hawaii. The objective was to determine if 4,5-dichloro-2-n-octyl-4-isothiazolin-3-one (DCOI) or didecyldimethylammoniumchloride (DDAC) provided supplemental protection to either sapwood or heartwood of the three wood species. DDAC (4 kg/m3³) improved sapwood durability of teak, redwood, and western redcedar, while DCOI (0.6 kg/m³) improved sapwood durability of teak and western redcedar after 36 months of sample exposure. Western redcedar heartwood durability was improved with 4 kg/m³ of DDAC. There was a slight improvement in decay resistance for redwood heartwood samples treated with DDAC or DCOI; however, further exposure will be required to test the effects of biocide treatment on the heartwood of this species. Conversely, teak heartwood durability appeared to be reduced by biocide treatments.
Biocides retentions were assessed after 6, 12, 18, and 24 months of field exposure. All samples except teak heartwood had acceptable biocide retentions after 24 months of exposure, but there was evidence of biocide leaching. Teak heartwood samples had very low and poorly distributed biocide loadings. There was no evidence of natural extractives depletion due to biocide treatment.
Fungal diversity and community development were monitored at six month intervals over a 24 month period from wood samples in the same field test. Fungi were isolated and identified by coupling microscopic techniques with the comparisons of the internal transcribed spacer (ITS) regions of the fungal rDNA with sequences available on the National Center for Bioscience Informatics (NCBI) website. 2361 isolates were obtained, representing 241 unique ITS sequences that were grouped into 81 morphotaxa. The most represented groups were: 63 ascomycetes (15 molds, 26 soft rot fungi, 2 stain fungi, and 21 genera with unknown capabilities), 16 basidiomycetes (11 white rot fungi and 5 genera with unknown capabilities), and 3 zygomycetes. Nonmetric multidimensional scaling ordination revealed that the strongest differences in fungal community composition were related to time. Multi-response permutation procedures did not indicate a change in fungal communities as a result of biocide treatment, wood species, or wood type (sapwood/heartwood). Fungal communities in all wood types contained taxa with similar decay capabilities. This similarity in fungal capabilities suggested that the observed decay rates in samples of the same wood type were due to biocide treatments and not due to changes in fungal communities. Further studies are underway to better characterize the decay capability of the isolated fungi.