Development of threshold values for boron compounds in above ground exposures: Preliminary trials
J J Morrell, C M Freitag, S Unger
Boron is increasingly used both as an initial and remedial treatment for protecting wood against fungal and insect attack. While establishing lethal dosages for boron against insects through feeding tests is relatively simple, establishing thresholds for fungal attack poses more of a challenge. Tests using boron dispersed in agar artificially surround the fungus with both boron and excess nutrients and poorly replicate the wood matrix. Soil or agar block exposures more closely replicate exposure, but these tests also expose the wood to leaching and employ large masses of actively growing fungal mycelium. In practice, however, fungal attack may be initiated by either spores or hyphal fragments that may be far more sensitive to boron. In an effort to develop more accurate thresholds for boron in wood exposed in non-soil contact, we evaluated the ability of hyphal fragments of two fungi, Gloeophyllum trabeum and Trametes versicolor to colonize and decay sapwood and heartwood blocks of Douglas-fir (Pseudotsuga menziesii) treated to retentions between 0.03 and 0.46% boron oxide (wt/wt) in modified petri dish tests. As expected, weight losses were lower in untreated heartwood samples, reflecting the moderate durability of Douglas-fir heartwood. Thresholds for sapwood and heartwood for both fungi were between 0.08 and 1.0% BAE and there appeared to be little effect of the heartwood on the resulting toxic threshold values for boron. These values illustrate the sensitivity of hyphae to boron and imply that higher concentrations may not be necessary for protecting wood from fungal attack in true above ground exposures protected from rainfall.