Further observations on the enhanced degradation of iprodione and vinclozolin in soil

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Abstract
The effects of soil pH on rates of degradation of iprodione and vinclozolin were measured in a silty clay loam soil. Little degradation of either fungicide occurred at pH 4.3 or 5.0, and degradation at pH 5.7 was slower than at pH 6.5. In both of the higher‐pH soils, the rate of loss of a second application of either fungicide was faster than that of the first, and a third application degraded even more quickly. In soil with pH 6.5, for example, the times for 50% degradation of iprodione following the first, second and third applications were about 30, 12 and 4 days, and for vinclozolin were 30, 22 and 7 days respectively. Iprodione degraded very rapidly in a sandy loam that had been treated three times previously with this fungicide and also degraded rapidly in the same soil pretreated three times with vinclozolin. Vinclozolin degraded rapidly in the vinclozolin pre‐treated soil, but its rate of loss in the iprodione pre‐treated soil was only slightly faster than in the previously untreated control. Studies of iprodione degradation in 33 soils from commercial fields demonstrated a clear trend towards faster rates of loss in soils with an extensive history of iprodione use. The time for 90% loss from previously untreated soils varied from 22 to 93 days. It varied from 16 to 28 days in soils treated once previously and from 5.2 to 23 days in soils treated twice previously. In soils that had received three or more previous doses, the time to 90% degradation varied from 3.8 to 15 days.