Variation in concentrations of the Fungicides tebuconazole and dichlofluanid following successive applications to green house-grown lettuces

Residual levels and degradation rates of tebuconazole and dichlofluanid in lettuce plants grown in a greenhouse under agricultural conditions typical of northwestern Spain were studied. Lettuce plants were sprayed four times with a homogeneous 0.2% aqueous solution of Folicur Combi (wettable powder containing 40% dichlofluanid and 10% tebuconazole) at a rate of 2500 g/ha. Samples were collected 1, 5, and 9 days after the first two applications and at times ranging from 1 to 27 days after the last two applications. All samples were stored in a refrigerator at 4 degrees C. Fungicide levels were determined by solid-liquid extraction (SLE) followed by gas chromatography and mass spectrometry detection (GC-MSD). Recovery was good for tebuconazole (98%) but low for dichlofluanid (29%); precision was good (< 10% for both analytes), and quantification limits were low (< 1.5 mg/kg). Seven days after the last application, dichlofluanid levels were below the maximum allowed limit established in Spain (10 mg/kg), but tebuconazole levels were above the corresponding limit (5 mg/kg). Tebuconazole concentration dynamics was accurately fitted by zeroth- or combined first- and zeroth-order models (depending on variety), but modeling of the behavior of dichlofluanid was less satisfactory, probably due to its instability.