Although found to be very effective in laboratory tests, alkylammonium compounds (AAC's) have failed to perform as well in field stake tests. Examination of leachability showed that this was not the cause. The present study investigated the possibility that staining fungi, (which have been observed to rapidly infect the field stakes), degrade the AAC wood preservative. Soil-blocks were treated with alkyldimethylbenzylammonium chloride, and sterilized, using gamma radiation. Half of the blocks were exposed to a mixed suspension of staining fungi, which had previously been isolated and identified from failed dialkyldimethylammonium chloride-treated stakes. After incubation for ten weeks they were conditioned, and one half leached in a static seven day leaching cycle. Half of the blocks not exposed to the staining fungi were also leached in a similar manner. All the blocks were then sterilized prior to exposing to one of three decay fungi, Lentinus lepideus, Gloeophyllum trabeum and Poria placenta. Following the test, Lentinus lepideus was found to be least tolerant to the AAC wood preservative, while Poria placenta was the most tolerant fungus. The Poria pre-exposure of the soil blocks to staining fungi greatly increased the toxic threshold and toxic limits of AAC to both Gloeophyllum trabeum and Poria placenta. The increases were almost independent of leaching, suggesting that degradation of the AAC is taking place rather than rupture of the AAC-wood substrate bonding. The increase in the toxic threshold to ca 9 kg/m³ would clearly have caused failure of the stake material in test at Westham Island, since the maximum concentration in that test was only ca 10 kg/m³.

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