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Soybean Rust

What is Soybean Rust?  

Soybean rust is a serious disease causing crop losses in other parts of the world. Two fungal species, Phakopsora pachyrhizi (also known as the Asian species) and P. meibomiae, cause soybean rust and are spread primarily by windborne spores that can be transported over long distances. Asian soybean rust, P. pachyrhizi, the more aggressive of the two species, was first reported in Japan in 1903 and was confined to the Eastern Hemisphere until its presence was documented in Hawaii in the mid-nineties.

Soybean rust symptoms are similar for P. pachyrhizi and P. meibomiae species. Symptoms begin on the lower leaves of the plant as small lesions that increase in size and change from gray to tan or reddish brown on the undersides of the leaves. Tan lesions, when mature, consist of small pustules surrounded by a slightly discolored deadened area with masses of tan spores on the lower leaf surface. Reddish brown lesions have a larger reddish brown deadened area, with a limited number of pustules and few visible spores on the lower leaf surface. Once pod set begins on soybean, infection can spread rapidly to the middle and upper leaves of the plant.

Host Range: P. pachyrhizi is capable of infecting more than 90 species of legumes (plants having pods that split open when dry); however, the number of legumes infected in nature is unknown. Kudzu, an invasive species that is a fast-growing vine, is widespread in the United States and could serve as a reservoir for the soybean rust pathogen. Some other common hosts are yellow sweet clover (Melilotus officinalis), vetch (Vicia dasycarpa), medic (Medicago arborea), lupine (Lupinus hirsutus), green and kidney bean (Phaseolus vulgaris), lima and butter bean (Phaseolus lunatus), and cowpea or backeyed pea (Vigna unguiculata). See the Soybean Rust Data Sheet for a more complete list of hosts.

Distribution of Asian Soybean Rust

Economic Impications  

In recent years, soybean rust has reduced yields and raised production costs for soybeans in every major production region of the world except the United States. The recent and rapid spread of the pathogen in South America prompted the Economic Research Service (ERS), in April 2004, to publish a study of the economic and policy impacts of its windborne entry into the United States.

This analysis demonstrates that, while soybean producers and consumers do realize some new costs as a result of soybean rust, the U.S. agricultural sector as a whole is minimally affected after adjusting to the presence of this new pest. Such resilience, seen in response to past shocks to the agricultural system, is explained by the availability of substitute crops (in production) and commodities (in consumption), as well as the technological savvy to mitigate pest losses.

In the first year of SBR infestation, assuming that U.S. producers are able to treat with fungicides upon SBR detection, the expected value of losses (given that a rust outbreak occurs) across all U.S. agricultural producers and consumers would range from $640 million to $1,341 million, depending on the severity of infestation. These losses, which represent less than 1 percent of net economic benefits derived from agricultural activity, demonstrate the flexibility and resilience of the U.S. agricultural system as a whole.

In the medium term, for all the yield loss scenarios:

  • Soybean acreage declines in the most susceptible, higher cost soybean production regions, and is supplanted by alternative crops (largely cotton in Southern regions).
  • Soybean acreage increases in regions less susceptible to SBR (Southern Plains, Northern Plains and Lake States).
  • Acreage increases do not offset acreage declines; U.S. soybean acreage declines and U.S. soybean prices increase.

This information has been extracted from the "Soybean rust economic assessment" (USDA-ERS). To read the entire assessment, click here.


Soybean Rust Control Methods  

If soybean rust is present in your area, a preventative soybean fungicide application should be considered. Consult your local Extension agent or crop advisor before applying a fungicide.  The contact information for your local extension office can be obtained by consulting your telephone directory or the following Web site:

For soybean rust fungicide best practices, refer to the IPM Centers Web site that will list a number of points to follow when considering a fungicide application. 


Last Updated:10/2/2009 1:43 AM

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