VIEW ARTICLE

Techniques

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 (1–4 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 86–91% 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 5–50 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 50–500 ng/ml range. The main advantage of this method is that many samples can be measured accurately in a short time.