Copper tolerance of brown-rot fungi: time course of oxalic acid production

The increase in the use of non-arsenical copper-based wood preservatives in response to environmental concerns has been accompanied by interest in copper-tolerant decay fungi. Oxalic acid production by brown-rot fungi has been proposed as one mechanism of copper tolerance. Fifteen brown-rot fungi representing the genera Postia, Wolfiporia, Meruliporia, Gloeophyllum, Laetiporus, Coniophora, Antrodia, Serpula, and Tyromyces were evaluated for oxalic acid production bi-weekly in southern yellow pine (SYP) blocks treated with 1.2% ammoniacal copper citrate (CC). Eleven fungi were designated copper-tolerant based upon weight loss in CC-treated blocks. After 2 weeks, these fungi produced 2–17 times more oxalic acid in CC-treated blocks than in untreated blocks. After 10 weeks, weight loss ranged from 32% to 57% in CC-treated SYP. Four fungi were copper sensitive, producing low levels of oxalic acid and minimal weight loss in CC-treated blocks. Rapid induction of oxalic acid appeared to correlate closely with copper tolerance. We conclude that the brown-rot fungi tested that were able to exceed and maintain an oxalic acid concentration of 600 μmol/g effectively decayed SYP treated with CC.