- 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
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