Application of the Logistic Function to Toxicity Testing of Wood Preservatives

A series of red pine sapwood samples treated with various concentrations of pentachlorophenol deposited from toluene was subjected to soil-block test using T. trabea as wood destroying fungus. Data was treated in two ways: l) Percent decay was plotted with respect to concentration of pentachlorophenol in lbs./cu.ft. and the threshold value was obtained. 2) The logistic function ln(q/l-q), where q is per cent weight loss of specimen, was plotted with respect to the logarithm of concentration and was found to be linear, having a regression coefficient of close to 1. Although the great advantage of the threshold value is that it is the most convenient single point on which to compare preservatives, it appears that the logistic function can be used to describe the wood-preserving value of fungicides of both strong and weak toxicity. This method, then, is of wider applicability than is the threshold value, which can only be found for strong fungicides. It also appears that operational losses, which can be easily detected on the logit-log dose plot, leave the logistic constants unchanged although they can alter the apparent position of the threshold value. Another practical advantage of the logistic method of expressing the results of decay tests is that it gives information concerning the rate of change in effectiveness of a fungicide before the threshold value is reached; one fungicide might permit very slow decay but another might permit rapid decay in the region below the threshold.

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