Bioassays to Detect Dissipation and Efficacy of Benomyl on Turf
Paper disc, soil-agar pellet, turfgrass-agar pellet, thatch-agar pellet, and sample agar mixture bioassays were developed and evaluated for detection of the fungicide benomyl and its major fungitoxic product methyl 2-benzimidazole carbamate (MBC) in leachate, soil, turfgrass clippings, and thatch. The bioassays could detect benomyl and MBC residues with a limit of detection of 0.2 μg/g and a limit of quantitation of 0.5 μg/g. In vitro soil and thatch degradation studies showed a half-life for MBC of approximately 18 days in thatch and 4 to 5 weeks in soil, depending on soil type. An adsorption equilibrium with benomyl in solution was reached within 1 h for thatch and 2 to 4 h for soils. MBC adsorbed by thatch was as much as twice that adsorbed by a Fox sandy loam which may represent a typical soil found on golf courses in Southern Ontario. When MBC was applied at 10 μg/g, up to 90% of the chemical in thatch and 68% in soil were not extractable with water. Methanol could extract up to three times the fungicide from soil or thatch as could water. Laboratory and field experiments showed that the wetting agent Aqua-Gro (AG) (polyoxyethylene ester and ether of cyclic acid and alkylated phenols) significantly reduced adsorption of benomyl by creeping bentgrass thatch. With AG, significantly less fungicide was initially adsorbed and significantly more fungicide was later desorbed from the thatch layer by 20 mm of water irrigation. Aqua-Gro increased movement, uptake, and biological availability of the fungicide and resulted in a higher residue level of fungicide in the grass clippings. Tersan 1991 (50% benomyl) applied at 2 kg/ha with AG gave control of dollar spot disease as good as the full rate (3 kg/ha) without AG. Field studies also showed that core cultivation 1 or 7 days before fungicide application gave better and longer-lasting uptake of the fungicide by turfgrass and resulted in better control of dollar spot disease. Core cultivation one day before fungicide treatment gave the best results both in long-lasting uptake of benomyl and control of dollar spot disease.