The importance of arresting sapstain pre-infection in radiata pine in New Zealand, to prevent downstream sapstain in both logs and sawn timbers has been clearly demonstrated. Efforts are being made to improve the efficacy of diffusible antisapstain agents through improved understanding wood/fungicide/fungus interactions. The mechanism of action of methylene bisthiocyanate (MBT) was investigated using Sphaeropsis sapinea ((Fr.:Fr.) Dyko and Sutton) and radiata pine (Pinus radiata D. Don). A laboratory method using an assembly of wood wafers was designed to allow determination of the importance of MBT diffusion through the liquid and vapour phases on fungitoxicity. For comparison, fungitoxicity of the essentially non-diffusible fungicide oxine copper was also determined. Scanning electron microscopy was used to examine the morphology of fungal mycelium and propagules after exposure to MBT. Results indicated that established hyphae of S. sapinea were more tolerant of MBT than propagules (hyphal fragments). For example, hyphal fragments colonised wood treated with 0.04% weight/volume (w/v) of MBT whereas established hyphae of S. sapinea on a wood wafer colonised an adjacent wafer treated with 0.16% w/v MBT. Diffusion through both the liquid and vapour phases contributed to the fungitoxicity of MBT. The vapour action was shown to act in the absence of liquid diffusible MBT (no direct contact with MBT treated wafer) but the combined effect of liquid and vapour phases was greater. Microscopy identified changes in the morphology of S. sapinea treated with MBT. At a concentration of 0.02% w/v of MBT, many hyphae were collapsed and plasmolysed, although some healthy hyphae were present.

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