Correlation between oxalic acid production and copper tolerance in Wolfiporia cocos

The increased interest in copper-based wood preservatives has hastened the need for understanding why some fungi are able to attack copper-treated wood. Due in part to accumulation of oxalic acid by brown-rot fungi and visualization of copper oxalate crystals in wood decayed by known copper-tolerant decay fungi, oxalic acid has been implicated in copper tolerance by the formation of copper oxalate crystals. Nineteen isolates of the brown-rot fungus Wolfiporia cocos were evaluated for oxalic acid production and weight loss on wood treated with 1.2% copper citrate. Twelve of 19 isolates that caused moderate to high weight losses in copper citrate-treated wood produced low oxalic acid in liquid culture, whereas isolates with high oxalic acid production had low weight losses in treated wood. Seven W. cocos isolates demonstrated enhanced weight loss in Cu-treated wood. Wood weight loss was unaffected by the presence of copper citrate for two W. cocos isolates and weight loss was lower for 10 isolates compared to weight losses in untreated wood. Citrate did not significantly influence oxalic acid production in liquid culture. Previous hypotheses linking oxalic acid and copper tolerance were based upon observations of single isolates of Postia and Tyromyces. Although most isolates produced more oxalic acid in copper citrate-treated wood than in untreated wood, we found no statistical relationship between the amount of oxalic acid production in liquid culture or wood and copper tolerance in W. cocos. Production of oxalic acid does not seem to be the factor controlling copper tolerance in W. cocos. The diversity seen within W. cocos demonstrates that caution should be used when reporting results, so that generalizations are not based on the behavior of a single isolate.