Soybean Rust

Soybean rust : A threat to profitable soybean production

See more: photos of Soybean Rust

Soybean rust continues to be a serious disease of soybeans in the US. Before the November 10, 2004 announcement of a positive identification of the Asian soybean rust spores from two fields in Louisiana, North America was the only soybean producing continent unaffected by the disease.

The fungus that causes Asian soybean rust, Phakospora pachyrhizi, originated in tropical and subtropical regions of Asia and Australia and most likely spread to several African countries via wind currents. In 2001, it was discovered in South America and moved north above the equator in 2004. Once it moved north of the equator it moved to North America on wind currents as it had before.

Phakopsora pachyrhizi is an obligate parasite, meaning that it must have live, green tissue to survive. The host range of the soybean rust fungus is quite broad experimentally but so far soybean and kudzu are the most important. Kudzu is the most important overwintering reservoir for the fungus and allow for the increase of spores, in areas free from freezing temperatures. The pathogen is well adapted for long-distance dispersal, because spores can be readily carried long distances by the wind to new, rust-free regions.

For vegetable growers in the region, soybean rust does not appear to be a problem on snapbeans or llima beans. Out breaks of soybean rust have been limited to soybean, kudzu, and jicama bean in Mexico.

Early symptoms of soybean rust appear as chlorosis and brown flecking on the lower leaves in the canopy. Developing lesions can be confused with symptoms caused by other foliar diseases, such as bacterial blight, downy mildew and especially Septoria brown spot. The key diagnostic features of soybean rust are the cone-shaped pustules that form mostly on the undersides of the leaves and the dusty, light-tannish colored spores that erupt from the pustules. When untreated, soybean rust, causes yield losses due to premature defoliation, fewer seeds per pod and decreased number of filled pods per plant.

You need three things to have a disease the susceptible host (which we have), the fungus (do not know where or when it will arrive), and weather for infection to take place(the biggest question for us as well as for the South).

Important soybean rust facts:

  • Since no hosts of soybean rust survive the winter in DE, soybean rust spores will not over-winter in Delaware. Since they will survive in the frost-free areas of Florida and the Gulf coast, this is where they have survived since 2005.
  • The spores of soybean rust will be dependant on the northern movement of winds from the South during the spring and summer to spread the disease. Once here the spores will need proper temperatures and moisture to infect soybeans.
  • Levels of rust infection will vary from year to year because of these facts.

Soybean Rust – Quick Facts

  • Early detection is a must along with an aggressive fungicide control program. The sentinel plot system that has been established can provide an early warningof infection in nearby states.
  • When soybean rust has been confirmed to be present in the region and conditions are favorable for infection a grower needs to implement a control program.
  • Be knowledgeable about fungicides for control some are preventative some curative.
  • With eight registered fungicides and 9 section 18 registrations in place it is unlikely that there will be any fungicide shortages.
  • Research suggests that row width does not affect soybean rust incidence or severity.
  • Soybean rust resistant varieties of soybean are not commercially available.
  • Plant infection is quick, resulting in new crops of spores in 9-10 days under ideal conditions. Infection periods could last for up to 15 weeks, but not likely to occur in DE
  • Spore germination requires 6 hours of moisture within a temperature range of 59-82° F.
  • Soybean rust spores can infect the leaf cells directly, not needing any help from leaf feeding insects or mechanical damage.
  • Soybean rust spores pose no health threat to either humans or animals. The rust spores infect the leaves of soybeans and not the seeds that are processed for animal or human consumption.
  • Kudzu did not turn out to be universally susceptible to soybean rust as was feared. Infection levels vary among populations of kudzu as does the amount of sporulation.

Grower Education:

  • Be informed on issues that will affect your management decisions for the coming season.
  • Review and understand the fungicides that will be used for control until resistant varieties are developed, which may take 8-10 years. See the list of fungicides for use in DE for soybean rust.
  • Know in advance who to contact in case you have suspect samples of soybean rust.

Authored by Bob Mulrooney , Extension Plant Pathologist, University of Delaware.

University of Delaware
Cooperative Extension System