Copper tolerance of brown-rot fungi: Oxalic acid production in southern pine treated with arsenic-free preservatives☆
Introduction
Copper-based biocides have provided protection against fungal decay and termite damage to wood since the 1930s. For most of the latter half of the twentieth century, the wood preservation industry was dominated by chromated copper arsenate (CCA). The copper provided broad-based protection against fungi and termites, and the chromium helped to fix the copper and arsenic into the wood. The arsenic provided supplemental protection against copper-tolerant fungi (Lebow et al., 2000), which have been shown to represent most of brown-rot genera tested so far (Green and Clausen, 2003).
Increasing environmental concerns led to voluntary withdrawal of arsenic-containing wood preservatives in the residential markets, lumberyards, and home supply stores, thus reducing the use of CCA-treated wood by 70% after 2003. CCA has been replaced in the homeowner maintenance market by a new generation of copper-organic preservatives often supplemented with water repellants. This new generation of preservative systems is arsenic- and chromium-free (Schultz and Nicholas, 2003).
Removal of arsenic from residential wood preservatives raises concern about copper-tolerant wood decay fungi. Studies have shown that many copper-tolerant fungi are cupriphilic in a choice test; thus, inhibition depends upon the co-biocidal components (Illman et al., 2000). Nearly all these copper-tolerant fungi are likely to be brown-rot fungi, and their distribution and prevalence are unknown. We have been evaluating the relationship of oxalic acid (OA) production to the time course and severity of decay by this group of fungi and have shown that copper induces rapid OA production by copper-tolerant brown-rot fungi (Clausen et al., 2000; Clausen and Green, 2003; Green and Clausen, 2003). In laboratory studies, Pohleven and others (Pohleven et al., 2002) evaluated Norway spruce with commercial copper-based preservatives common to the European market. When wood treated with copper-based biocides was challenged with several copper-tolerant Antrodia spp., individual strains exhibited varying degrees of copper tolerance. Of the preservatives tested, copper amine and chromated copper boron inhibited decay by these fungi. The objective of our study was to estimate the potential of known copper-tolerant fungi to produce OA in southern yellow pine (SYP) treated with arsenic-free preservative formulations in the ASTM soil-block test.
Section snippets
Fungal cultures
The strains of brown-rot fungi evaluated in this study (Table 1) were maintained on 2% malt extract agar (Difco Laboratories, Detroit, Michigan).
Preservative treatments and decay test
SYP sapwood blocks (1 cm3) were conditioned to 70% relative humidity (RH) at 27 °C and weighed. The blocks were then vacuum treated according to modification of AWPA standard E10-01 (AWPA, 2003a) with one of five preservatives: water-borne copper naphthenate (0.15% elemental copper concentration prepared from a stock solution); amine copper azole (1%
Decay test
The decay capacity of the 10 brown-rot fungi on untreated SYP was compared to that on preservative-treated SYP (Fig. 1). Overall, mean weight losses of treated wood blocks fell below 10%. Thus, no obvious failures of these preservatives were observed within the limits of this laboratory testing when compared to the mean weight loss of the control SYP at over 40%. In a previous paper, weight losses were 32%–57% in copper citrate (CC)-treated SYP after 10 weeks (Green and Clausen, 2003).
Oxalic acid analysis
In a
Conclusions
Copper organic-containing preservatives (especially ACQ-D and Cu-azole) are currently the primary replacements for CCA in residential lumber markets. This immediately raises concern about the effect of copper-tolerant brown-rot fungi in the service life of treated wood. The results of this paper indicate that arsenic-free preservatives adequately inhibit our most aggressive isolates of copper-tolerant fungi under conditions favouring the fungi in laboratory tests.
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