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A Sensitive Bioassay for Quantification of Metalaxyl in Soils. Ana Maria Bailey, Graduate research assistant, Department of Plant Pathology, University of California, Riverside 92521; M. D. Coffey, associate professor, Department of Plant Pathology, University of California, Riverside 92521. Phytopathology 74:667-669. Accepted for publication 3 February 1984. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/Phyto-74-667.

A sensitive bioassay was developed to detect low concentrations of metalaxyl in soils. Aqueous extracts of metalaxyl-treated soils were incorporated into cornmeal agar prior to autoclaving. The quantitative estimation of the amount of metalaxyl in soils was based on a highly significant positive relationship between the radial growth of an isolate of Phytophthora boehmeriae and the log concentration of the fungicide in the agar. The isolate of P. boehmeriae was chosen for its high sensitivity to metalaxyl as manifested in a linear growth response on cornmeal agar over the range of 2 to 30 ng/ml. The sensitivity and quantitative nature of the bioassay were confirmed by comparison with data obtained for scintillation counting using a common source of ¹⁴C-metalaxyl.