Understudied Fungicides Common in U.S. Streams Draining Agricultural Land

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U.S. Geological Survey (USGS) scientists have detected one or more fungicides in 20 of 29 streams across the United States that were sampled during 2005 and 2006 in areas where soybeans are grown. The number of fungicides used has increased significantly since 2002, and very little is known about their occurrence in the environment. For some of the fungicides this is the first data documenting their environmental occurrence. Nine of the twelve targeted fungicides were detected in at least one stream sample, and mixtures of fungicides were commonly detected. In addition to very little being known about their environmental occurrence, the fungicides in the study were selected due to their extensive use in orchards and increasing use in row-crop agriculture (corn and soybeans, for example). Streams in areas threatened by Asian soybean rust fungi outbreaks were among those selected for this study in order to provide background information on fungicide occurrence before major outbreaks and treatments. The study was made possible by the development of new analytical methods for the analysis of fungicides in water.

Study Results

Aerial spraying of fungicides on row crops in Iowa

Aerial spraying of fungicides on row crops in Iowa. A soybean field is in the foreground, and a cornfield is in the background. The soybean crop is the target of the aerial application. Fungicides are used to combat soybean rust, a fungal disease. Photo Credit: Dana W. Kolpin, USGS.

The fungicide azoxystrobin was detected most frequently (in 45 percent of 103 samples, with a maximum observed concentration of 1.13 micrograms per liter [µg/L]) followed by metalaxyl (27 percent, maximum concentration 0.067 µg/L), propiconazole (17 percent, maximum concentration of 1.15 µg/L), myclobutanil (9 percent, maximum concentration 0.032 µg/L), and tebuconazole (6 percent, maximum concentration 0.115 µg/L). The maximum observed concentrations of fungicides were typically less than concentration levels considered unsafe (toxicological benchmarks) for freshwater aquatic organisms and humans. The spatial and temporal occurrence patterns are related to the use of fungicides to prevent fungal diseases as well as treatments during the onset of disease. The results of this study provide baseline fungicide occurrence data for later comparisons should major outbreaks and treatments or other changes in use occur in the future.

Fungicide Applications are Increasing

Estimated 2002 annual use rate for the fungicide azoxystrobin

Estimated 2002 annual use rate for the fungicide azoxystrobin, and the maximum observed concentration of azoxystrobin in stream-water samples collected in 2005 and 2006.

Application of fungicides on the approximately 89 million hectares of corn, soybean, and wheat planted in the United States has increased from less than 2 percent of hectares in 2004 and 2005 (National Agricultural Statistics Service) to an estimated 25 to 30 percent in 2009 (Letter from Universities Regarding the Strobilurin, Pyraclostrobin, Supplemental Label, 2009). In addition, fungicides are commonly used on turf and a variety of fruit and vegetable crops, such as almonds, apples, grapes, lettuce, peanuts, potatoes, and tomatoes. Results of this study are being used to inform additional studies on the occurrence of fungicides in the environment, relations of occurrence to various fungicide-use settings such as potato production, and the potential effects of fungicides on amphibians and other aquatic organisms.