Most studies suggest that enzymes produced by brown-rot fungi are too large to penetrate sound wood structures, even after decay begins. Thus, nonenzymatic agents have been proposed to initiate brown-rot decay. We have reported that the brown-rot fungus Postia placenta MAD-698 initiates a 2-fold decrease in wood pH within 7 days of colonization which is mediated by production of oxalic acid. Strain ME-20, essentially a non-decay isolate of Postia placenta, does not accumulate oxalic acid when colonizing wood. Our lab has been investigating potential inducers of oxalic acid production, and we have been comparing hemicellulose-like substrates with cellulose-like substrates in liquid culture. Potent inducers of oxalic acid include xylan, glucomannan, chitin, uronic acids, pectin and compounds containing acetyl sidechains. Although cellobiose also induces oxalic acid in MAD-698, utilization of cellulose is considered a relatively late event, therefore not influencing incipient decay. We conclude that oxalic acid induction is an early event in the decay and is initiated by hemicellulosic substrates and by-products.

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