Bioassay Estimation of Sapwood Protection in Spray-Treated Western Redcedar Poles

A bioassay was developed using samples of 160 butt-treated western redcedar poles in 8 lines in the Willamette Valley of western Oregon that had been sprayed (flooded) 5 to 12 years earlier with a pentachlorophenol petroleum solution. For the assay, Plugs 3/8 inch in diameter and 2 inches long were removed with a gasoline-powered plug cutter. The plugs were cut into halves lengthwise and then the sapwood portion of one of the halves was cut into 1/8- inch-thick wafers at progressive depths from the pole surface. Each wafer was lightly surface-sterilized by flaming and placed 2.5 cm (1 inch) from the margin of a culture of Poria monticola Murr., a decay fungus, growing on malt agar in petri dishes. Three wafers, uniformly spaced, were alloted to each dish. A template with concentric circles and three radial lines spaced 120 degrees apart standardized placement of the wafers and aided later in measuring the limitation on growth of the fungus imposed by the wafers: The dishes were incubated for 3 weeks at 25?C; any outgrowth of fungi was noted and the distances between the assay fungus and wafers (zone of inhibition) were measured. Larger zones of inhibition were considered indicative of higher levels of perservative protection. Outgrowth of fungi or growth of the test fungus over the wafer indicated virtual absence of preservative. The assays showed that sapwood near the base was better protected than sapwood at mid-height, which, in turn, was better protected than sapwood just below the crossarm. Sapwood on the south sides of the poles was better protected than sapwood on the north side. One line sprayed 12 years before with a 5 percent pentachlorophenol petroleum solution had very little residual sapwood protection and was in need of respraying. The other lines that had been sprayed from 5 to 10 years before with a 10 percent pentachlorophenol petroleum solution were judged to have comparatively high levels of sapwood protection. The residual protection of sapwood of western redcedar poles might appropriately be tested about 10 years after spraying and those lines not: in need of respraying should be bioassayed again approximately 5 years later. To make the bioassay additionally informative, research is planned to determine the correlation between preservative content of the sapwood and zone of inhibition.

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