Treating wood with the water-borne sodium salt of N'-N-naphthaloylhydroxylamine (Na-NHA) protects wood against decay and termite damage. Initial testing indicated little or no inhibition of sapstain fungi, molds, or soft-rot fungi by Na-NHA, suggesting that the mechanism by which this compound protected wood was complex and not that of a broad-spectrum biocide. Previously, we (Green et al, 1997) suggested that the protective mechanism was due to Na-NHA complexing with calcium ions to disrupt fungal metabolism, and/or binding of Na-NHA to the calcium in pit membranes, which acts as a physical barrier to fungal colonization. More recent experiments suggest an alternate mechanism. Specifically, pKa measurements of Na-NHA show that the sodium salt will be essentially fully protonated when impregnated into SYP which is naturally acidic (pH at or below 5.5). Furthermore, the protonated form (H-NHA) was more fungicidal than Na-NHA against two white- and two brown-rot fungi, with the bioactivity approaching that of several commercial organic fungicides. The moderate fungicidal activity of H-NHA is not surprising; the compound is a polyaromatic hydrocarbon much like many of the compounds in creosote. By precipitating mainly at the pit membranes, H-NHA or Ca-NHA serves as a fungicidal barrier to inhibit fungal hyphae from spreading to adjacent cells through the pits. Studies of H-NHA combined with three commercial organic biocides showed that only one combination was synergistic, which further suggests that NHA protects wood by a biocidal mechanism rather than being simply a compound is extremely water insoluble and will thus not leach from wood in outdoor exposure. NHA may also have metal chelating and/or antioxidant properties which further assists in protecting wood. non-biocidal additive.

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