There are two species of fungi that cause soybean rust, with Phakopsora pachyrhizi being the primary one of concern within the U.S. P. pachyrhizi is native to Australia and eastern Asia and since 1997 has spread to Africa, South America, and North America. It was first discovered in the continental U.S. in 2004 on soybean fields in Louisiana and it is suspected that spores of the fungus were carried within hurricane Ivan from that year.
|Spores of soybean rust (Phakopsora pachyrhizi) by Daren Mueller, Iowa State University, Bugwood.org|
The rust is able to survive winters in the southeast U.S. on kudzu and other alternative hosts and re-infect soybeans and other legumes when winter passes. It can also spread by wind each season and infect soybeans in areas where it is unable to overwinter, due to cold temperatures, and cause yield loss for soybean growers in the more northern states. When left untreated, soybean rust can cause 10-80% yield loss. This seems like a very wide range, but it is due to many environmental factors. The spores which travel by wind (urediniospores) must have at least six hours of leaf wetness to penetrate leaf surfaces and it should be 58-82F with the optimum conditions being 70-80F and either a heavy dew or light rainfall. As such, the infection and symptoms tend to occur in mid- to late summer.
|Advanced symptoms of soybean rust on unsprayed control plot within a fungicide trial. Soybean rust (Phakopsora pachyrhizi) by Edward Sikora, Auburn University, Bugwood.org|
Symptoms first appear on the top side of the leaves as tiny dark red or brown spots and then pustules will form on the undersides of leaves. The pustules will break open and the wind-blown spores will released. If the rust goes untreated it can cause leaves to fall off and induce premature maturity, leading to yield loss and reduced seed size.
|Symptoms of soybean rust (Phakopsora pachyrhizi) by Daren Mueller, Iowa State University, Bugwood.org|
For more information: BugwoodWiki on Phakopsora pachyrhizi
University of Arkansas publication: Asian Soybean Rust
Alabama A&M and Auburn Universities: Asian Soybean Rust in Alabama
More images from Bugwood: Soybean rust