Chlorothalonil (tetrachloroisophthalonitrile) is a highly effective fungicide and wood preservative. Chlorpyrifos (O,O-diethyl O-(3,5,6-trichloro-2-pyridinyl)phosphorothioate) is an insecticide and is not generally considered to possess significant antifungal activity. However, a review of the literature revealed that chlorpyrifos effects in various ways the activity of microorganisms and is even registered for control of a fungal plant disease. Laboratory and field research trials were designed to study the wood preservative effectiveness of combinations of chlorothalonil plus chlorpyrifos. In some tests the antifungal activity of the insecticide chlorpyrifos was confirmed; however in a fungus cellar test, chlorpyrifos by itself provided little protection. In laboratory termite testing, chlorothalonil was sufficiently effective that in most experiments the addition of chlorpyrifos had little effect, while in field tests the combination consistently outperforms chlorothalonil alone. In tests against various marine borer species, the combination of chlorothalonil plus chlorpyrifos proved superior to chlorothalonil alone. The synergy of chlorothalonil plus chlorpyrifos is demonstrated in field stake tests, where after eleven years in Florida the combination has given superior performance over stakes treated with pentachlorophenol or chlorothalonil alone. Chlorpyrifos alone provided little protection against decay in these field stake tests. Chlorpyrifos is a useful additive to wood preservative fungicides, conferring additional protection against both decay fungi and wood destroying insects.

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