Soybean rust is a serious disease that has caused significant crop losses in many soybean growing regions of the world. Until its discovery in Louisiana in November 2004, soybean rust had never been detected in the continental United States. The fact that it is spread by wind-blown spores places major soybean regions in this country at risk of introduction in the coming years. The purpose of this publication is to alert Minnesota’s soybean growers to this risk, provide disease characteristics for soybean rust identification and contact information to report suspected introductions.
Soybean rust has a wide host range of over 30 legume species. There are two rust causing species in soybeans. The more virulent species, P. pachyrhizi has long been distributed throughout much of Asia & Australia, and more recently Africa and South America. This species was found near Baton Rouge, Louisiana, in November 2004. Yield loss in infested regions typically runs between 10-90 percent.
Early detection of soybean rust is difficult as symptoms may be confused with other soybean foliar diseases. The earliest symptom of infection is a yellow mosaic discoloration on the lower leaves of maturing plants at or near flowering. Advancing infection spreads to middle and upper portions of the plant, causing leaves to turn yellow and lesions to appear as small brown pustules. Advanced symptoms include numerous tan or reddish brown surface lesions, necrosis and eventual defoliation.
When the pathogen is present, disease development is favored by extended periods of leaf wetness and temperatures between 50ºF and 80ºF. Because present soybean cultivars are all rust susceptible, development of rust resistant varieties is a national priority. At present, early detection and fungicide applications are the main management tools in reducing losses. Several safe and effective fungicides are approved for use against soybean rust.
Soybean rust has the potential to seriously impact Minnesota agriculture. Soybeans are Minnesota's most valuable crop, generating $1.32 billion in 2000.
When soybean rust does arrive in Minnesota, fungicide options for control will be available. Chlorothalonil (Bravo, Echo), azoxystrobin (Quadris), and pyraclostrobin (Headline) are currently labeled for use in soybeans. Other fungicides have been approved for the control of soybean rust on soybeans as FIFRA section 18 emergency exemptions. Labels of fungicides that have been approved for control of soybean rust are available.
Photo credits: Early symptoms photo courtesy Clive Levy, Commercial Farmers Union of Zimbabwe; Advanced symptoms photo and close-up of pustule photo courtesy Arnold Tschanz, USDA, APHIS, PPQ, Riverdale, MD; Pustules photo courtesy Reid Frederick, USDA, ARS, Ft. Detrick, MD.
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