Bioassay Appraisal of Vapam and Chloropicrin Fumigant-Treating for Controlling Internal Decay of Douglas-Fir Poles

Douglas-fir utility pole sections, 2.9 m (9.5 ft) long and 27-43 cm (10.5-17 in.) in diameter, from sound, untreated poles were used in trials at room temperature. The fumigants were introduced in holes at mid-height on the vertically-standing sections. Effectiveness was denoted by killing of a decay fungus, Poria monticola, growing in wood dowels systematically inserted at 1-foot intervals along the poles. Viability of the fungus was ascertained every second month by culturing on malt agar a segment of each dowel. Within 10 months the assay fungus was totally eradicated in the more effectively treated sections to the maximum distance assayed, 1.2 m (4 ft) above and below the treated zone. The indicated average rate of progress of the advancing front of lethally concentrated vapor was 0.3 to 0.6 m (1 to 2 ft) per month. Responding to gravity, movement of the fumigant vapors was somewhat greater downward than upward. Transverse diffusion was much slower than the vertical, evidenced by persistence of a pronounced radial gradient of chloropicrin vapor–with maximum concentration in the central heartwood–for as long as 20 months. The persistence of fumigant vapors in the pole sections was ascertained by two bioassays developed for the purpose. With both, P. monticola was grown on a malt-agar slant in test tubes with interiors exposed to any vapors given off by the funigated wood; continuing presence and relative amounts of toxic vapor were evidenced by reduced linear growth (or killing) of the fungus. By one method (“open tube”), the open end of the tubes was sealed over holes drilled in the sections. With the other method (“closed tube”), plug samples of uniform size were extracted from different depths in the sections and immediately placed in the dry end of the tubes, which were then capped. This second method was useful particularly for examining the radial distribution of the vapors. Lethal concentrations of vapor were still present in the chloropicrin-treated sections at the end of 20 months, but there was no evidence of residual vapor from the Vapam treatment. Fumigant treating not only can eradicate fungi from poles but it also may persist in poles for many months after treating–offering extended protection against reinvaslon by wood-destroying organisms.

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