Significance

Soybean rust is one of the most important soybean diseases worldwide. Soybean, a major crop both in the US and the world, is high in vegetable oil and protein (approximately 20 and 40%, respectively) and provides 57% of the vegetable oil consumed worldwide and 68% of the vegetable protein. According to the American Soybean Association, the US produced 38% of the world soybean crop in 2006, worth over $19 billion, with Brazil and Argentina producing 24 and 19% of the world crop, respectively. Because of the lack of plant resistance, the explosive nature of the disease, and the high potential yield losses (30 to 80%), soybean rust has long been viewed as a serious threat to soybean production in both North and South America. The threat of soybean rust was so serious that Phakopsora pachyrhizi was included on a list of ‘select agents’ in the 2002 USA Bioterrorism Act, along with other biological agents such as those that cause anthrax and hemorrhagic fever. Select agents are pathogens of humans, animals, or plants that have the potential to be used as weapons of terrorism.

Soybean rust caused by P. pachyrhizi was first reported in Japan in 1902 and was limited to Asia and Australia until 1997 when it was found in Uganda. From Uganda it spread to Zimbabwe (1998) and then to South Africa (2001). In 2001, soybean rust was found in Paraguay. Since most of the world’s soybean production occurs in North and South America, the introduction of rust into Paraguay posed a significant threat. Soybean rust was reported in Brazil and northern Argentina in 2002. By 2003, soybean rust was occurring in most soybean producing areas in Brazil as well as Bolivia. In the summer of 2004, rust was reported in Columbia, and in November of that year it was found for the first time in the continental US in Louisiana and then, within a short time, in 8 other southern states (Figure 21). Soybean rust probably entered the continental US from Columbia with hurricane Ivan, which made landfall in September 2004. Figure 22 illustrates the estimated spore load carried by hurricane Ivan and the geographic distribution of spore deposits. Since then, P. pachyrhizi appears to have established itself permanently on kudzu in Florida, and the area where it is found during the growing season has increased. In 2005, soybean rust was active primarily in the southeastern US (Figure 23), but some late finds were found on kudzu as far north as Kentucky and North Carolina. Soybean rust was found in Mexico in the early spring of 2006 and in an isolated field near Brownsville, TX.. Throughout the first half of the 2006 growing season, soybean rust was confined to the southeastern US, but it was not very active even there because of the unusually hot, dry weather. However, as rainfall increased, so did soybean rust, especially in Louisiana and along the southeast coast of the US. By the end of the season, soybean rust was found along the Mississippi River into southern Illinois and Indiana and as far north as West Lafayette, IN (Figure 24). In 2007, drought in the southeastern US limited soybean rust development throughout most of the season, but high rainfall in Louisiana, Texas, Oklahoma, and Kansas favored rust in these areas (Figure 25). Rust appeared for the first time as far north as Iowa in 2007. Although these late-season infections did not cause yield loss, it did show that under the right conditions, soybean rust can spread quickly and is a threat to the major soybean growing areas of the US.

Fig. 21	Fig. 22	Fig. 23
Fig. 24	Fig. 25

Note: The first introduction of Asian soybean rust caused by P. pachyrhizi in the US occurred in Hawaii in 1994, but this did not impact the major soybean growing areas in the continental US. Also, soybean rust was found in Puerto Rico in the mid-1970’s, but later analysis showed that this rust was caused by P. meibomiae and not P. pachyrhizi.

The introduction of Asian soybean rust into the continental US sparked a nationwide effort by research and extension scientists to prepare for rust epidemics. These efforts include a network of sentinel plots that track the occurrence of soybean rust nationally on a weekly basis, a coordinated effort to label fungicides for soybean rust, training of “first detectors” on how to recognize soybean rust, and other extension and research activities. As a result, this disease has sparked immense media interest. Some news headlines follow.

“Soybean Checkoff Builds Defense Against Rust”

“RUST BELT CINCHES UP”

“Sen. Feingold urges action against soybean rust”

The Asian soybean rust work that has been conducted in the United States to date would not have been possible without a concerted and cooperative effort from Land Grant Universities, the U.S. Department of Agriculture, State Soybean Check-Off Boards, and Industry.

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by The American Phytopathological Society