Graduate Thesis Or Dissertation
 

Interactions of the fumigant methylisothiocyanate with Douglas-fir wood and their influence on fumigant effectiveness

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/cz30pw36t

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  • Experiments investigated how the fumigant methylisothiocyanate (MIT), Douglas-fir wood at various moisture contents (MC), and the wood decay fungus Poria carbonica Overh., interacted to govern overall fumigant effectiveness. MIT decomposed in wood to form non-MIT residues at rates of about 0.16%, 0.9%, and 1.6% of the total bound MIT per week, in blocks fumigated at 0%, 12%, and 60% MC, respectively. Compounds formed during fumigation included N,N'-dimethylthiourea and 2,4- dimethy1-1,2,4-thiadiazolidine-3,5-dithione, which were toxic to P. carbonica, and elemental sulfur which showed minimal fungitoxicity. MIT not removed by extensive dry-aeration was rapidly volatilized at fungitoxic concentrations when wood was wetted. The susceptibility of P. carbonica in Douglas-fir heartwood blocks to MIT vapors and the amount of MIT sorbed by wood were dependent on wood moisture content. At constant, low MIT vapor concentrations (less than 1 ug/cc air), wood at 10% MC bound 5 times more MIT, but required 4 times the exposure period to control P. carbonica, than similarly treated wood above the fiber saturation point. Adsorption of MIT to wood was not substantially influenced by the amount of wood decay. Increasing wood moisture content from 10% to 30% during fumigation resulted in a rapid volatilization of previously bound MIT and an associated increase in fumigant fungitoxicity. In wood at 0% MC, the equilibrium MIT adsorption/desorption ratio was low (0.2), but increased to about 0.94 above 18% MC. Partition coefficients (bound/vapor) for MIT adsorption to wood increased as wood moisture increased from 0% to 12% MC, and then decreased with increasing moisture content up to about 30% MC. Steady-state diffusion coefficients for MIT in Douglas-fir heartwood were over 300 times higher for longitudinal than transverse movement. Diffusion coefficients increased with wood MC, although increasing wood moisture contents from 22% to 80% MC reduced longitudinal MIT diffusion about 3 fold. Radial movement of MIT was about 7 times faster in Douglas-fir sapwood than in heartwood. Treatment with waterborne chromated copper arsenate (CCA) did not influence MIT sorption or diffusion in sapwood at 15% MC, but impregnation with P-9 Type A oil restricted MIT movement and may provide a barrier to fumigant loss.
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