Soybean rust is a destructive foliar disease of soybean caused by the fungus Phakopsora pachyrhizi. Asian soybean rust is an aggressive disease that destroys photosynthetic tissue, causing premature defoliation, early maturation, and lower yields. It was first found in the continental United States in November 2004.
The soybean rust pathogen can also infect some common leguminous crop plants and weeds found in Iowa, including yellow sweet lover, vetch, medic, lupine, green and kidney bean, and lima and butter bean. Corn and other grain crops are not hosts to Asian soybean rust.
The disease begins with small, water-soaked lesions, which gradually increase in size, turning from gray to tan or brown. As the plant matures and sets pods, the symptoms spread rapidly to the middle and upper parts of the plant. Lesions are found on petioles, pods, and stems but are most abundant on leaves.
Especially at the early stages, it is easy to confuse the symptoms of soybean rust with symptoms of three other soybean leaf diseases: brown spot caused by the fungus, Septoria gycines; bacterial pustule, caused by Xanthomonas axonopodis pv. glycines; and bacterial blight (also called angular leaf spot ), caused by Pseudomonas savastanoi pv. glycinea. Later in the season soybean rust may be confused with Cercospora leaf spot or sunburn, which also turns the leaves a bronze color.
Resistant varieties are not yet commercially available, but state and federal soybean pathologists and breeders are working to develop resistant soybean lines.
Partial resistance or tolerance, in which fewer lesions with a reduced number of spores develop, is likely to be the most effective resistance strategy. Planting cultivars with resistance to rust pathogens has been very successful in managing rust in wheat and corn, and is expected to be a successful approach with soybean rust.
Until resistant soybean varieties are in place, judicious use of fungicides will probably be a major tool to protect yield. Early detection is essential for effective management of soybean rust with fungicides. Monitoring soybean fields and adjacent areas will be important each growing season.
If you suspect rust is present in your field this season, contact a trained First Detector by going to or calling your local county Extension office. Check for current information for Iowa at http://www.ipm.iastate.edu/.
Fungicide information for Iowa is available at the Iowa soybean rust web site.
Rust surveillance and forecasting
The USDA Soybean Rust Mapping website compiles rust observation and forecasts based on a national soybean rust monitoring network and casting models, and input from state specialists. A second national soybean rust surveillance and forecasting system is the North American Plant Disease Forecast Center. Monitor these sites regularly during the growing season.
Printable fact sheets and diagnostic guides
Common soybean leaf diseases and soybean rust (pdf)
Four scenarios you may face this crop season (pdf)
Using Foliar Fungicides to Manage Soybean Rust (has links to printable pdf files for each chapter)
More rust topics
The February 27 issue of the Integrated Crop Management newsletter features Asian soybean rust. It has a review of 2005, the effects of rust fungicides on other soybean diseases, a review of the rust monitoring efforts, and factors that will affect the 2006 season...read the ICM newsletter.
The proceedings of the recent National Soybean Rust Symposium are available to read online at the Plant Management Network website...view proceedings.
The American Phytopathological Society published an online feature article about the development of Asian Soybean Rust in the southeast in 2005, and an outlook for the 2006 season...read APS feature article.