Investigating Potential Water Quality Impacts of Fungicides Used to Combat Soybean Rust in Indiana

Abstract

Asian soybean rust (ASR) is a foliar plant disease caused by the fungus Phakopsora pachyrhizi that is potentially devastating for US soybean production. It was first detected in soybean fields in the Midwestern US in October 2006 but did not cause any damage to soybean production then because most of that year’s crop had been harvested by the time it appeared. In coming years, it is possible that ASR might enter soybean fields in the Midwest during the growing season and cause significant damage. The only current option for managing soybean rust is to use fungicides, many of which have been approved for use on soybeans by the US Environmental Protection Agency under emergency conditions. Since soybean fields traditionally have not received widespread applications of fungicides, it is important to understand the potential environmental impacts of using large quantities of fungicides to combat a potential ASR outbreak. Currently, the impacts of the fungicides used to combat soybean rust on surface and groundwater resources and on “off target” species are not fully known. In this study the National Agricultural Pesticide Risk Analysis hydrologic/water quality model was used to predict fungicide concentrations at edge of field and soil water concentrations at bottom of the root zone as a result of fungicide applications to control soybean rust in Indiana. It was also used to evaluate the likelihood of exceeding threshold chronic exposure concentrations of concern for human and aquatic organism health and identify areas of Indiana that are most vulnerable to contamination by fungicides. The model outputs for the different fungicides show spatial variations of fungicide losses in edge of field runoff and to bottom of root zone soil water or shallow groundwater at 5%, 10%, 25%, and 50% probability of exceedence, indicating that some fungicides may be present in concentrations above threshold ‎values of concern for fish and humans. This provides a basis for developing approaches to minimize potential environmental impacts of fungicides, such as prioritizing implementation of best management practices in the most vulnerable areas.