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A Metalaxyl Bioassay for Large Numbers of Small Foliar Samples. M. G. Milgroom, Department of Plant Pathology, Cornell University, Ithaca, NY 14853; W. E. Fry, Department of Plant Pathology, Cornell University, Ithaca, NY 14853. Phytopathology 77:237-240. Accepted for publication 14 July 1986. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/Phyto-77-237.

A simple bioassay was developed for quantifying the fungicide metalaxyl in small samples of potato leaf tissue. Metalaxyl was extracted from leaf disks (14 cm2) by soaking in 100% methanol (2 ml) in 12 75 mm glass culture tubes. Disks were removed after 24 hr and methanol was evaporated by placing tubes in a water bath (75 C) for approximately 2 hr, which left crude leaf extracts in the tube. Extraction efficiency in this method was 8691% as efficient as maceration and centrifugation. One milliliter of molten cornmeal agar was pipetted into each tube, autoclaved, and poured into 35 10 mm plastic petri dishes. Radial growth of Phytophthora boehmeriae after 96 hr accurately quantified metalaxyl in the 550 ng/ml range. In samples determined to be > 50 ng/ml, the P. boehmeriae colony was removed and P. citrophthora was used in a similar way on the remaining medium to quantify metalaxyl in the 50500 ng/ml range. The main advantage of this method is that many samples can be measured accurately in a short time.

Additional keywords: fungicide bioassay.