Soybean Rust: A Threat to Food Security in the United States
Soybeans are one of the most important agricultural products of the United States, and their nutritional value helps ensure a high-quality diet for households as well as their food security. Rupe and Sconyers (2008) mention that soybean is a major crop both in the US and in 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. Unfortunately, soybean rust is one of the most important soybean diseases worldwide, because of the lack of plant resistance, the explosive nature of the disease, and the high potential yield losses up to 30 to 80% (Rupe and Sconyers, 2008). Another author (Malvick, 2018) also is of this opinion, stating that under favorable conditions, the pathogen can cause yield losses greater than 50 percent.
Soybean rust is potentially one of the most significant diseases of soybean, and it can spread quickly, cause leaf spots and defoliation of soybean plants (Malvick, 2018). According to Rupe and Sconyers (2008), "soybean rust is a serious disease caused by two closely related fungi: Phakopsora pachyrhizi, sometimes referred to as the Asian or Australasian soybean rust pathogen, and Phakopsora meibomiae, the so-called New World soybean rust pathogen, which is found only in the western hemisphere. Except for a few minor characteristics, the two fungi appear morphologically identical, but P. pachyrhizi is much more aggressive on soybean than P. meibomiae. To date, P. meibomiae has not been documented to cause significant yield losses in Central and South America. The first symptoms of soybean rust caused by Phakopsora pachyrhizi begin as very small brown or brick-red spots on leaves (Figure 1, on left). Lesions remain small (2-5 mm in diameter) but increase in number as the disease progresses. Pustules (Figure 1, on right), called uredinia, form in these lesions, mostly on the lower leaf surface, and they can produce many urediniospores."³
Figure 1. Spots and pustules on leaves (Photos credit: Rupe and Sconyers, 2008)
As more and more lesions form on a leaflet, the affected area begins to yellow, and eventually the leaflet falls from the plant. Symptoms caused by P. meibomiae are similar to those of P. pachyrhizi. The two fungi species can only be distinguished by using DNA analysis protocols. Like other rusts, the soybean rust pathogens are obligate parasites that require a living host to grow and reproduce. They can survive away from the host as urediniospores for only a few days under natural conditions."³ Extended periods of wet weather, moderate temperatures (59 degrees F to 85 degrees F), and high humidity (over 75%) favor disease development (Malvick, 2018).
Rupe and Sconyers (2008) indicate that the soybean rust was first 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. For these authors, the first introduction of Asian soybean rust caused by P. pachyrhizi in the US occurred in Hawaii in 1994, luckily this did not impact the major soybean growing areas in the US. According to many authors including the Minnesota Department of Agriculture (MDA) and Rupe and Sconyers (2008), the serious soybean rust caused by P. pachyrhizi was reported in November 2004 in Louisiana and then, within a short time, in 8 other southern states (Figure 2, on left).
"This soybean rust probably entered the US from Columbia with hurricane Ivan, which made landfall in September 2004. Figure 2 (on right) 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, but some late finds were found on kudzu as far north as Kentucky and North Carolina."³ MDA also mentions that "in 2005 researchers were able to document the presence of soybean rust spores in rain samples collected in Minnesota but no disease was reported."²
Figure 2. Soybean rust distribution in US in 2004 (Map from Rupe and Sconyers, 2008)
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. Rust appeared for the first time as far north as Iowa in 2007, under the right conditions, soybean rust can spread quickly and is a threat to the major soybean growing areas of the US (Rupe and Sconyers, 2008).
Historically, 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 (in Africa). 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. It is probably from Columbia that the current soybean rust entered the US with hurricane Ivan, which made landfall in September 2004 (Rupe and Sconyers, 2008).
Figure 3. Late stage of infection by soybean rust (Photo: MDA)
"Soybean rust has long been viewed as a serious threat to soybean production in both North and South America. 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; because it has the potential to be used as weapons of terrorism."³
For its management, the early detection of this disease is the greatest tool currently available. At present, Rupe and Sconyers (2008) state that "the most reliable early detection method is the use of "sentinel plots." These are small plots (primarily soybean, but kudzu or another susceptible host also may be used) planted several weeks before the commercial crop, and often use early maturing cultivars. Both the early planting and the early maturity of the cultivars results in the sentinel plots flowering 1 to 3 weeks before the commercial crop. Since soybean rust usually develops after flowering (Malvick, 2018 and Rupe and Sconyers, 2008), the disease can be observed in these sentinel plots a week or two before being found in adjacent commercial fields. This early warning gives growers in the area time to apply a protective fungicide treatment. Sentinel plots have been established throughout the soybean and dry bean production areas of the US. Information from these plots is uploaded weekly into a dedicated USDA website where maps are generated showing rust activity in the country."³
Another method of early detection of soybean rust described by Rupe and Sconyers (2008) is spore trapping, in which two strategies are being assessed. One traps windblown spores on glass slides coated with petroleum jelly. The spores are examined microscopically, and the presence and number of soybean rust-like spores noted. At this time, microscopic examination can only identify spores that resemble the soybean rust pathogen because it is not currently possible to identify P. pachyrhizi with certainty by simply examining the urediniospores. More conclusive identification of urediniospores of P. pachyrhizi is being developed by using labeled antibodies and polymerase chain reaction (PCR) protocols. The other spore trapping approach involves collecting and filtering rainwater and then uses PCR to determine the presence of P. pachyrhizi on the filters. It is thought that long-distance spread of urediniospores occurs when storms pick up the spores and then deposit them in rainwater at distant locations. Because this technique uses species-specific molecular markers, positive findings are thought to be more reliable. In 2005 and 2006, both air and rain sampling found P. pachyrhizi or P. pachyrhizi-like spores over a wide area, far from where soybean rust was active. While neither approach can determine if the spores arrived alive, they do indicate that this pathogen has the potential to spread widely and quickly.
There are three basic management tactics that Rupe and Sconyers (2008) considered can play a role in reducing soybean rust epidemics: fungicides, genetic resistance, and cultural practices. At present, fungicides are the only highly effective tactic, and to be effective, selecting the right fungicide and applying it at the right time are crucial. Several fungicides are registered in the US for soybean rust control, and most can be classified into three groups: chloronitriles, strobilurins, and triazoles.
Regarding genetic resistance, Rupe and Sconyers (2008) explain that Soybean plants respond to infection by P. pachyrhizi by producing either tan, red-brown, or no lesions at all. It is thought that these responses represent susceptible, moderate, or highly resistant reactions, respectively. There are four known dominant genes for resistance to soybean rust, Rpp1 through Rpp4. While these dominant genes confer high levels of resistance and are relatively easy to incorporate into new soybean cultivars, they are not effective against all races of P. pachyrhizi. The long-term management will probably depend more on resistance, in combination with fungicides and changes in cultural practices. For these authors, in most areas of the US where rust must be introduced each year for an epidemic to occur, some cultural practices may help manage soybean rust; thus, changing planting and harvest dates may avoid disease: planting early with an early maturing cultivar may avoid the rust until the crop has either been harvested or is so far along that the disease will have little impact on yield.
References
Malvick, Dean. 2018. Soybean rust. The University of Minnesota. Available at: https://extension.umn.edu/pest-management/soybeanrust
Minnesota Department of Agriculture (MDA). N.D. Soybean Rust. Available at: https://www.mda.state.mn.us/plants/plantdiseases/soybeanrust
Rupe, John and Sconyers, Layla. January 1, 2008. Soybean Rust. The Plant Health Instructor. Volume: 08. The American Phytopathological Society (APS). Available at: https://www.apsnet.org/edcenter/pdlessons/Pages/SoybeanRust.aspx and DOI: 10.1094/PHI-I-2008-0401-01
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