Soybean Rust—A Prime Example

Abstract

15 There are 65 records of invasive and exotic diseases recorded in the United States (http://www.invasive.org). Soybean rust (Phakopsora pachyrhizi) is one of those records. This paper provides a brief history of the geographic distribution of rust, overwintering establishment, surveillance and research activities, and potential hosts other than soybean and kudzu. Geographic Distribution. Soybean rust occurred throughout much of the Eastern Hemisphere for more than 100 years (Sinclair and Hartman 1995) before it was confirmed in Hawaii in 1994 (Kilgore and Heu 1994) (Fig. 1). The explosion of rust in the Western Hemisphere occurred soon after the report in Hawaii and included countries in Africa and South America (Miles et al. 2003). Soybean rust was reported in the continental United States in Louisiana in the fall of 2004 (Schneider et al. 2005). A team of first responders organized by APHIS searched for soybean rust in Louisiana and other locations (Fig. 2). Since that initial report, rust was reported in 15 other states (http://www. sbrusa.net/) including Illinois in 2006 (Hartman et al. 2007). The first report of the disease on another host in the United States was on kudzu in Quincy, FL, in 2004 (Harmon et al. 2005). The introduction of the rust fungus into the continental United States may have been facilitated by Hurricane Ivan, which hit the U.S. mainland in September 2004 (http://www.ceal.psu.edu/ivan04.htm). Overwintering. The primary overwintering site of the fungus in the continental United States is Florida and other southern states, where the fungus survives the winter on kudzu, which remains green below the frost line (http://www.sbrusa.net/). Rust has been reported on kudzu in Alabama, Florida, and Georgia, and on volunteer soybean in Texas preceding the 2007 planting of soybean. Although the scenario for upcoming growing season (2007) has yet to occur, there has been an increase in overwintering sites compared with previous years (Fig. 3). This will likely increase the fungal spore load and lead to increased spread of rust on soybean in the 2007 season. Surveillance and Research Activities. Since the introduction of soybean rust in the continental United States, there has been an increase of surveillance and research activities. Before soybean rust hit the continental United States, a review in 2003 outlined the importance of the disease and evaluated the risk of rust to the U.S. soybean crop (Miles et al. 2003). The main focus since 2004 has been sentinel and spore trapping networks, research programs (fungicides, resistance, detection technologies, forecasting and modeling, host range studies and pathogen biology), and information exchange including activities supported by the American Phytopathological Society, which organized two National Soybean Rust Symposia in 2005 and 2006. Hosts. A number of research areas could be summarized, but one interesting area that pertains to the “invasiveness” of P. pachyrhizi is the additional hosts that it may infect. There are many legumes that have not, at least to my knowledge, had any contact with P. pachyrhizi because the fungus only was found in the Eastern Hemisphere until the last decade. The fungus infects more than 95 species of plants from more than 42 genera including soybean and related Glycine species (Ono et al. 1992). Besides kudzu, there are a number of other legumes on which the pathogen may overwinter in the southern United States, as well as in the Caribbean Basin, Mexico, and Central America. A recent study in the United States reported the fungus on leaves of Phaseolus coccineus (scarlet runner bean), P. lunatus (lima bean), and P. vulgaris (kidney bean) had rust lesions when grown adjacent to a rust-infected soybean field in Quincy, FL) (Lynch et al. 2006). Further research indicates that another nine species may be hosts, with two of these species representing new genera, Glycyrrhiza and Teramnus (unpublished data). At this point in time it is not Soybean Rust—A Prime Example