Copper is widely used as the primary component in wood protectants because it demonstrates a broad range of biocidal properties. However, a key concern with using copper in wood preservative formulations is the possibility for brown-rot basidiomycetes to resist the toxic effect. Many brown-rot basidiomycetes have evolved mechanisms, like the production and accumulation of oxalate, which helps these fungi to tolerate copper-treated wood by detoxifying copper. The purpose of this study was to determine if untreated wood and copper-treated wood soaked in sodium oxalate influenced the rate of decay by brown-rot basidiomycete fungi. Both untreated and 1.2 % ammoniacal copper citrate-treated test blocks were subjected to an additional sodium oxalate treatment via two soaking methods (ten minute and two hour). Test blocks were exposed to two Fibroporia radiculosa isolates (FP-90848-T and L-9414-SP) and one isolate of Gloeophyllum trabeum isolate (MAD 617) and evaluated for weight loss at four and eight weeks. Decay was between 40-43% weight loss at week eight for F. radiculosa L-9414-SP when untreated blocks were soaked with sodium oxalate. F. radiculosa L-9414-SP demonstrated decay of 38% at week eight when copper citrate-treated blocks were soaked with sodium oxalate. F. radiculosa FP-90848-T decay was much lower for untreated blocks soaked with sodium oxalate (12-13%) and only slightly higher on copper-treated blocks soaked with sodium oxalate (19%) by week eight. G. trabeum MAD 617 decay was between 40-46% when untreated blocks were soaked with sodium oxalate. G. trabeum was unable to successfully decay the copper citrate-treated blocks soaked with sodium oxalate (0.5%) by week eight. The copper-tolerant and copper-intolerant test fungi used in this study demonstrated no major increase in decay when untreated and copper-treated wood was amended with oxalate.

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