Submitted to: Journal of Environmental Quality
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/28/2005
Publication Date: 7/1/2005
Citation: Potter, T.L., Strickland, T.C., Joo, H., Culbreath, A.C. 2005. Accelerated soil dissipation of tebuconazole following multiple applications to peanut. Journal of Environmental Quality. 34(4):1205-1213. Interpretive Summary: About 75% of the nation's peanut crop is produced within the Atlantic Coastal Plain extending from Virginia to Alabama. Peanuts are well adapted to the region's soil and climatic conditions but are subject to high disease pressure. Fungicides are used widely and account for a substantial portion of grower variable costs. Soil-borne pathogen control in particular requires careful active ingredient selection. Fungicide persistence in soil between applications must be long enough to provide an effective dose for pathogen control; however, persistence may have negative consequences if a product remains available for runoff or leaching for extended periods. In this study, we examined persistence of a favored active ingredient, tebuconazole, during its normal agronomic use on peanuts. In combined field-laboratory studies, the time for half of the compound to dissipate decreased from 43 to 5 days after 3 applications. This type of behavior, often termed accelerated degradation, is due to adaptation of soil microbiological communities that degrade the pesticide. Although reduced pest control efficacy may result, the environmental risk is also reduced. While the potential for accelerated tebuconazole degradation was indicated in laboratory studies, it was not observed to a significant extent in the field due to dry soil conditions that apparently reduced microbial activity. Thus when conditions are dry growers may not needed to apply tebuconazole as frequently. The study provides a model for assessments needed to comprehensively evaluate the environmental behavior of fungicides that are applied repeatedly during the growing season. Benefits to growers include data which can be used to optimize fungicide application timing and frequency and reduce input costs. Environmental benefits are anticipated due to reduced fungicide use.
Technical Abstract: Peanut growers in the Southeastern USA typically make multiple fungicide applications during the growing season. Effective disease management and assessment of runoff and leaching risks requires data documenting active ingredient (AI) dissipation rates. This was accomplished for a favored AI, tebuconazole, by laboratory incubation (FC, 30oC) of soil collected from peanut plots after 4 applications at 2-week intervals. Untreated plot samples spiked in the laboratory served as controls. Results indicated microbial community adaptation and accelerated degradation in field treated samples. DT50 decreased from 43 to 5 days after two sprays while net degradates accumulation equivalent to 16 to 66% of the parent was observed. Using best-fit equations (r2=0.84 to 0.98) for prior sample incubations, tebuconazole and degradates soil concentrations were predicted for each successive samples. After the 2nd application measured tebuconazole was 1.6 fold less the sum of predicted and applied. More than 50% of the difference was explained by tebuconazole leaching below the 2-cm sampling depth. Close agreement in this sum and the measured value after the 3rd application suggested that foliar washoff replenished any tebuconazole that leached since the prior application. Predicted degradates concentration was 2-fold higher and tebuconazole 2-fold lower than measured in the sample collected after the 4th and final application. Dry soil conditions which inhibited microbial activity provided the most probable reason. The study showed that although accelerated degradation was indicated in laboratory studies, impacts may be offset by field conditions. Further assessment of the stability of dissipation rates between seasons and the fate and metabolism of selected degradates appear necessary fully assess tebuconazole's behavior during use in peanut production.