After 13 years, chioropicrin, methylisothiocyanate (MIT), Vapam, and
Vorlex continue to effectively control internal decay of pressure-treated
Douglas-fir transmission poles. The estimated retreating schedules for
application of these fumigants to treated wood may be as long as 10 years
for Vapam and 15 years for the others. The close-tube bioassay, developed
during this research, has proven to be an effective method for determining
the persistence of these fumigants in wood and should help in determining
when fumigant-treated poles should be retreated.
The use of gelatin to encapsulate MIT for wood treatment was ideal
because the capsules did not react with MIT, were MIT impermeable when dry,
permitting prolonged storage without significant fumigant loss, and
readily released MIT when moistened in the wood.
Although release of encapsulated MIT in the wood was enhanced by small
amounts of water, excess moisture appeared to hinder MIT diffusion into
wood. Assays of Vapam treated wood show that the amount of MIT released by
breakdown of Vapam is significantly lower than the expected theoretical
yield from this fumigant. Encapsulated MIT reduced the decay fungus population
in poles in service more effectively than applications of Vapam.
The effectiveness of a fumigant in controlling decay may be expressed
as the product of the fumigant concentration (C) and the time (T) the decay
fungus is exposed to the fumigant before it succumbs. A higher value
indicates a less effective treatment. In wood, the CT value obtained for
MIT was two times greater at 20% wood moisture content (MC) than at 40 and
75% MC. Although MIT was least effective at 20% MC, there was more MIT
bound to the wood than at the higher moisture levels. This suggests that
the MIT bound to the wood structure may be less effective against decay
fungi than the MIT in the air and water in the wood. Although the effectiveness
of MIT varied with the wood MC, it was nevertheless still very
fungitoxic over a broad range of moisture levels.
In chioropicrin treated wood, inhibition of invasion by decay fungi
was indicated by the lysis and vacuolation of the fungal hyphae in the
wood. Chloropicrin appears to hydrogen bond to wood and may form covalent
bonds with phenolic wood extractives and lignin, possibly increasing the
persistence of the treatment.
Controling decay of cedar sapwood
Three waterborne fungicides have been added to the previously selected
materials being tested on cedar pole sections at an O.S.U. test site as
potential substitutes for the pentachiorophenol in oil treatment currently
used. The effectiveness of all treatments will be evaluated later this
year using the Aspergillus bioassay and a modified soil block test. The
most effective treatments then will be tested on poles in service.
Later this year cores will be removed from the control bolt holes to
evaluate the extent of natural fungal colonization. If a sufficient level
of colonization has occurred, the effectiveness of the various chemical
treatments in preventing decay in field-drilled bolt holes in Douglas-fir
poles will be evaluated.
Detecting decay and estimating residual strength in poles
A serological technique for rapid detection of decay fungi was found
to cross react with non-decay fungi and thus lackedthe necessary specificity
for identifying decay fungi in wood. Additional work is needed to purify
the preparations to render them more specific to decay fungi.
Measurements of modulus of rupture (MOR), modulus of elasticity (MOE),
work to maximum load, specific gravity, radial compression strength (RCS),
and Pilodyn pin penetration of sound appearing wood containing decay fungi
were not significantly different from the corresponding values for wood
from which no decay fungi were isolated. However, tests of poles with more
advanced decay did show significant reductions in wood strength properties.
Specific gravity alone was not a good predictor of bending strength of wood
from decayed poles, but the use of both specific gravity and RCS tests
significantly improved the ability to predict the bending strength of these
Decay of Douglas-fir poles prior to pressure treatment
The continued study of the fungal infestation of poles during air
seasoning has demonstrated that there is a significant buildup of Poria
carbonica, the major pole decay fungus, with time. In general, as air
seasoning time increased, decay fungi infested more poles and occupied more
wood within each pole. Sampling of freshly cut poles in the forest this
past year showed that some contained potential decay fungi prior to
reaching the pole yard. Frequent isolation of decay fungus monokaryons
throughout the air seasoning period suggests that spores of these fungi
were infesting the poles at a relatively constant rate. The ability of the
basidiomycetes isolated from these poles to reduce wood strength will be
evaluated in rapid tests for toughness by impact breaking and changes in
the breaking radius of Douglas-fir test sticks.
The germination of basidiospores of P. carbonica was studied on
culture medium and laboratory techniques are being developed to follow
germination on a wood surface under varying environmental conditions to
further illucidate their role in the infestation of seasoning wood.
Exposure of sterilized pole sections at four Pacific Northwest air
seasoning sites for successive 3-month periods has been continued. The
dramatic increase in infection during Nov.-Jan. '81 at all locations
except Arlington WA, did not reoccur in that same time period during
1982. There was, however, a continuing low level of infection at all
sites during the year with a slight peak of infection in May-June '82 in
Arlington WA. The results of these tests are currently being computer
analyzed to more effectively study the patterns of fungal invasion of
wood as influenced by environmental factors.