Use of metham sodium to eliminate Phytophthora spp. from roading gravel

Lateritic gravel is used for road building in Western Australia, with about 8 million m3 required annually. The introduced, soil-borne pathogen Phytophthora cinnamomi, which is associated with mortality of native plants, has been widely spread by using gravel from infested pits. Uninfested gravel is in demand, and in short supply. The biocide metham sodium is registered by the Australian Pesticides and Veterinary Medicine Authority (APVMA) as a soil fumigant, but the registered application method is unsuitable for treating gravel. In moist soil metham sodium decomposes into the fumigant-like pesticide, methyl isothiocyanate (MITC). Field experiments in which metham sodium was applied during the construction of 4 m3 gravel stockpiles, showed Phytophthora spp. are killed within 2 weeks of treatment with 80 mL m3–1 metham sodium. MITC is sealed within the stockpile by surface crusting; it was only detected 5 m downwind within the first 24 h of treatment. It was not detected in gravel 28 days after treatment. This application method that can safely eliminate Phytophthora spp. from infested gravel has been registered by the APVMA, allowing scaling-up trials to be considered.