Factors affecting degradation rates of five triazole fungicides in two soil types: 2. Field studies

The criteria for registering pesticides persistent in soil are still a matt er of debate. Amongst modern pesticides, several triazole fungicides are ve ry persistent, though no deleterious effects on soil microbial processes ha ve been reported. The behaviour of five such compounds (flutriafol, epoxico nazole, propiconazole, triadimefon and triadimenol) has been examined in tw o field trials utilising different agronomic treatments. These fungicides w ere applied in June 1996 at rates of 0.5 kg ha(-1), and soil cores were tak en to 20 cm depth at intervals over 2.5 years and analysed by extraction an d high-pressure liquid chromatography. Triadimefon was quite rapidly reduce d to triadimenol. Triadimenol, flutriafol and epoxiconazole were all very p ersistent with DT50 > 400 days, whilst propiconazole had DT50 c 200 days; b ehaviour was similar in the Rothamsted clay loam and Woburn sandy loam. Onl y flutriafol, the most polar and hence weakly sorbed of these fungicides, w as appreciably leached, with traces reaching the 15-20cm deep soil layer. S prays applied to plots of fallow soil suffered loss of up to 50% of applied compound in the first four weeks, a loss eliminated by shallow incorporati on, indicating an early role for surface loss processes such as photolysis and/or volatilisation. A young barley crop intercepted about one-third of t he spray, though subsequent rain caused some wash-off. After one to two yea rs, amounts of each compound remaining in the plots were similar for the th ree agronomic treatments, especially for flutriafol, though with a tendency for the incorporated plots to have the most chemical and the barley plots the least. Computer simulation of behaviour in the field using the model CA LF, utilising sorption and degradation measurements made in laboratory incu bations with these same soils together with daily climate measurements, ove restimated persistence especially for flutriafol, epoxiconazole and triadim enol. This was due both to lack of inclusion of surface loss processes in t he model, which caused initial deviations in the plots not receiving cultiv ation after spraying, and to a longer-term underestimation of breakdown in the field. This latter was especially noticeable for triadimenol, which was not detected after 2.5 years despite predictions of c 50% remaining. Thus field measurements of behaviour are desirable, as simulations based on labo ratory measurements can overestimate persistence. (C) 1999 Society of Chemi cal Industry.