Estimating Benzimidazole Residues in Thatch and Turfgrass by Bioassay

Bioassays using pellets of agar, thatch-agar and turfgrass-agar were developed using benzimidazole-sensitive Penicillium expansum, to detect the fungicide methyl 2-benzimidazole carbamate (MBC) which is the major fungitoxic degradation product of benomyl [methyl 1-(butylcarbamoyl)-2-benzimidazole carbamate] in thatch and turfgrass clippings. These bioassays were used to estimate the amount of fungicide that was biologically available and hence by subtraction from that applied, the amount that remained bound and biologically unavailable. The limit of quantitation was 0.5 mg kg. From 19.9% to 93.2% of the applied fungicide was bound by thatch and 46.2% to 56.9% was bound to turfgrass clippings depending on the concentrations used. In vitro degradation studies showed that MBC had a half life of approximately 2.5 weeks at 23C in non-sterilized thatch.