Asian Soybean Rust

Asian soybean rust is a foliar disease caused by the fungus Phakopsora pachyrhizi.  It is an aggressive disease capable of causing defoliation and significant yield loss. Asian soybean rust is found in most soybean growing areas of the world, and was found for the first time in North America in November 2004.

2007 Season
According to the USDA Pest Information for Extension and Education (PIPE) website, soybean rust was found in 19 states and 301 counties (thought to be a conservative estimate) in 2007. Although disease pressure was high in some southern areas, notably Alabama, Louisiana, and Georgia, rust epidemics were generally held in check by widespread freezing temperatures in the south in April that reduced the amount of spores, and also by the 2007 drought. The northernmost find in the U.S. was in Hancock County, Iowa in October, and by November it was also detected for the first time in Ontario, Canada.

Key points to know about Asian soybean rust

• Soybean rust will not survive over the winter in the North Central region because it can't live and reproduce without green living tissue.
• To cause outbreaks in the north, the fungus has to spread successively north via southern winds during the spring and summer each year.

• Check the USDA Soybean Rust Maps frequently during the growing season

  Overwintering sites of soybean rust will be restricted to areas with very mild winters, such as the gulf coasts of Florida, the southern areas of Texas, or in Mexico.
• Weather conditions in the North Central region will not always be moist enough to favor widespread or severe rust development, even when spores are present.
• The most susceptible crop stages are between early flower stage (R1) and mid-seed development stages (R5).
• Resistant soybean varieties are not yet available. However, host resistance is expected to be an effective, long-term solution for soybean rust.
• Until resistant commercial varieties are in place, the management of rust will be dependent in the next few years on judicious use of fungicides.
• A national system of real-time soybean rust monitoring and forecasting is in place to help growers with spray decisions. This information is posted at the USDA Pest Information for Extension and Education (PIPE) website.

Urediniospores of the soybean rust fungus.

Spread of rust and the disease cycle

Spores of the soybean rust pathogen are transported readily by air currents and can be disseminated rapidly hundreds of miles in a few days. Weather conditions will determine when and where the spores travel from south to north.

Rust spores, called urediniospores, are able to penetrate the plant cells directly, rather than through natural openings or through wounds in the leaf tissue. Thus infection is relatively quick: about 9 to 10 days from initial infection to the next cycle of spore production.

Rust is a multi-cyclic disease. After the initial infection is established, the infection site can produce spores for 10 to 14 days. Abundant spore production occurs during wet leaf periods (in the form of rain or dew) of at least 8 hours and moderate temperatures of 60 to 80° F.

Soybean rust has a wide host range

Phakopsora pachyrhizi has a wide host range, which is unusual for a rust pathogen. There are more than 20 species of alternative hosts for soybean rust in southern U.S., including the common weed kudzu. Some other common hosts are yellow sweet clover, vetch, lupine, green and kidney bean, and lima and butter bean.

The name Asian soybean rust is used to distinguish the soybean rust disease caused by Phakopsora pachyrhizi from a mild form of soybean rust, Phakopsora meibomiae , which is endemic to the Caribbean region.

Information on these pages is from the cooperative effort of researchers throughout the North Central states. Information from this site can be copied and distributed for educational use. Please credit the source with our name and URL: NCSRP Plant Health Initiative at www.planthealth.info. Please do not use copyrighted photos without permission.