Fungitoxicities of NaMDC decomposition products to decay fungi colonizing Doulgas-fir and ponderosa pine wood Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/2r36v163h

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  • Metham sodium or NaMDC is one of the four major soil fumigants that are currently registered for controlling internal decay of wood products in use. Fumigants are used primarily in utility poles but are also applied to piling, timbers and other large wood members. NaMDC must decompose to fungitoxic compounds to become effective. A variety of volatile and non-volatile decomposition products are produced during NaMDC breakdown in wood but the role these components play in the overall fungitoxicity of metham sodium is poorly understood. Volatile methylisothiocyanate (MITC) is believed to be the primary fungitoxic NaMDC breakdown product but it has been suggested in previous studies that some synergistic activity between MITC and other volatile NaMDC decomposition products in wood might occur, resulting in an enhanced MITC fungitoxicity. A fumigation apparatus was designed to examine the toxicity of two NAMDC volatile decomposition products, carbon disulfide and methylisothiocyanate, against six decay fungi (basidiomycetes) and one mold (ascomycete) established in Douglas-fir and Ponderosa pine. Fumigations were performed with the individual compounds at different concentrations for a period of ten days to determine sublethal dosages of each chemical. Fumigant effects were assessed by grinding blocks and plating a portion of the ground material in malt extract agar. The resulting number of colony forming units (CFU's) provided a measure fungal survival. A stimulus in the number of CFU's was observed for most species treated with lower fumigant concentrations suggesting that some fungi had the ability to metabolize sulfur-containing fumigants when growing in a low sulfurcontaining environment. Large variations in number of CFU's were exhibited among the fungi but CT₉₀ values showed that the CS₂/MITC mixture was more fungitoxic to most fungi than fumigations with the individual fumigants. The results indicate that NaMDC decomposition products can interact synergistically to enhance fungal control in wood. The effect may help to explain the effectiveness of NaMDC as a wood fumigant despite its relatively low MITC yield. Further studies using other decomposition products would help to better define other interactions in this process.
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