Relative Fungal Efficacy Results From The Soil Block Test With A Long Incubation Period Of Three Commercial Copper Wood Preservatives

Development of new wood preservative systems is a lengthy and expensive endeavor. This study examined soil block experiments with a longer incubation time than typical to make the test harsher, using commercial particulate copper azole (PCA), soluble copper azole (CA-C), and soluble alkaline copper quaternary (ACQ-D) systems. Both the CA-C and PCA systems performed poorly compared to ACQ-D with two copper tolerant fungi, even at retentions well above the specified ground-contact residential level, i.e., Use Category 4A (UC4A). With the one copper sensitive fungus examined, PCA and ACQ-D prevented decay at about half the UC4A retention. To compare these laboratory efficacy results, data from an outdoor accelerated post test with a copper tolerant fungus gave efficacies of ACQ > CA-C >> PCA, with PCA having significantly more decay than the two soluble copper systems. Further confirmation was obtained from recently published E7 stake data conducted at a site with extensive copper tolerant fungi, which suggested that ACQ performed better than PCA and CA-B after about 3 years of exposure in agreement with the harsh soil block results with the two copper-tolerant fungi. Conversely, results from four other sites without a history of extensive copper tolerant fungi are interpreted that all three systems generally performed adequately, which is compatible with the soil block test with the one copper sensitive fungus examined.