Copper tolerance of brown-rot fungi: Oxalic acid production in southern pine treated with arsenic-free preservatives

The voluntary withdrawal of chromated copper arsenate (CCA)-treated wood from most residential applications has increased the use of non-arsenical copper-based organic wood preservatives. Because the arsenic component of CCA controlled copper-tolerant fungi, scientists have renewed interest in and concern about the decay capacity in the important copper-tolerant group of brown-rot fungi. We have demonstrated that the primary means of inactivating copper in preservatives is by excess production of oxalic acid (OA). Oxalic acid production is a key metabolic indicator of brown-rot decay, and our objective was to estimate the production of OA in five commercial or experimental arsenic-free preservatives. Ten aggressive brown-rot fungi, chosen from previous studies and representing the genera Antrodia, Coniophora, Gloeophyllum, Postia, Serpula, Tyromyces, and Wolfiporia, were tested against southern yellow pine (SYP) blocks that were vacuum-treated with ground contact retentions of copper naphthenate, amine copper azole, alkaline copper quat type D (ACQ-D), N,N-naphthaloylhydroxylamine (NHA), and copper borate in a 12-week soil-block test. After determination of block weight loss, blocks were also tested for the presence of OA. Weight loss ranged from 0.3% to 8.3% for treated blocks and from 16.4% to 59.6% for untreated controls. We conclude that SYP treated with these five preservatives limited OA production and prevented decay, and thus confirmed the efficacy of the co-biocides against copper-tolerant fungi.