Mammalian Cell Cytotoxicity Analysis of Soybean Rust Fungicides

Abstract

The identification of soybean rust, caused by Phakopsora pachyrhizi H. Sydow & Sydow, in the southern United States in November 2004 (Schneider et al. 2005), in the Midwest in 2006 (Hartman et al. 2007), and elsewhere has increased the concerns of the impact of the pathogen on US soybean production (Miles et al. 2007). The rapid spread of P. pachyrhizi and its potential to cause severe yield losses makes this among the most destructive foliar diseases of soybean. Yield losses of 20%–60% were reported in Asia, with losses of 80% reported from experimental plots in Taiwan (Hartman et al. 1991). Soybean rust could have a major impact on both total soybean production and production costs in the US. The primary control of soybean rust has been with fungicides (Miles et al. 2003). There are four fungicides commercially available in the US that are registered for use on soybean and labeled for soybean rust. These fungicides are Bravo , Echo , Headline , and Quadris . Headline and Quadris are strobilurin products; Bravo and Echo are chlorothalonil products. In addition, there have recently been Section 18 Emergency Exemption requests for additional fungicidal compounds (myclobutanil, tebuconazole, propiconazole, tetraconazole, and trifloxystrobin) and mixtures of compounds (trifloxystrobin and propiconazole; propiconazole and azoxystrobin; and pyraclostrobin and tebuconazole) submitted to the EPA by the Departments of Agriculture from many states (U.S. EPA 2006a). A number of commercial products (e.g., Bumper , UpperCut , Quilt , Laredo , and Stratego ) containing these requested fungicidal compounds have been approved and may now be registered (Section 3 registration status) for management of soybean rust on soybean (U.S. EPA 2006a). While these fungicides have been used on crops (e.g., fruits, vegetables, and cereals), many questions remain regarding the large-scale application of these compounds to soybeans (~74 million acres in the US) in the prevention and treatment of soybean rust. Indeed, there is growing concern about the use of these fungicides since little is known about the toxicity of many of these compounds and their impact on human health. Therefore, the goals of this project were to assess the toxicity of soybean rust fungicides (Table 1) using a mammalian cell cytotoxicity assay and to compare the cytotoxicity (%C1/2) of these fungicides to the published toxicity values of other agrichemicals (insecticides and herbicides).