Evidence for the enhanced degradation of metalaxyl in UK carrot soils

摘要

Laboratory studies were carried out to determine the rates of degradation of metalaxyl in soils from nine fields in which metalaxyl had been used extensively as a soil treatment for the control of cavity spot disease in carrots. In all these fields, the fungicide had failed to control the disease in recent years. A further carrot field was sampled to which metalaxyl had never been applied. Soil samples were taken from 10 stations within each field and each sample was processed individually. Sub-samples of the 10 were also bulked to produce a composite sample for each field. The time taken for 50 % of the fungicide to be degraded (DT_(50)) was calculated using GENSTAT 5, fitting Gompertz or linear regressions to the data. Comparisons were made between the regressions fitted for the composite samples and the average of those fitted for the 10 stations. In fields where the fungicide failed, the DT_(50) values varied from 4 to 14 days. The composite samples produced DTso values that were comparable (4to 15 days). In the field where no metalaxyl had been applied the average DT_(50) for the 10 stations was 46 days compared with 43 days from the composite sample. A second study was carried out with soil samples from two fields adjacent to one another. One field had no previous history of metalaxyl application whilst the other had a history of application and failure in the control of cavity spot. The DT_(50) values for metalaxyl degradation in soils from these fields were 39.3 and 13.2 days, respectively.