The U.S. Department of Agriculture’s Animal and Plant Health Inspection Service has confirmed the presence of soybean rust on soybean leaf samples taken from two plots associated with a Louisiana State University research farm Nov. 6.
While this is the first instance of soybean rust to be found in the United States, the detection comes at a time when most soybeans have been harvested across the country. As a result of the harvest, the impact of the fungus should be minimal this year.
Soybean rust is caused by either of two fungal species, Phakopsora pachyrhizi, also known as the Asian species, and Phakopsora meibomiae, the New World species. The Asian species, the one found in Louisiana, is the more aggressive of the two species, causing more damage to soybean plants.
USDA will dispatch its soybean rust detection assessment team, composed of scientific experts and regulatory officials, to the site within 24 hours. The assessment team will work closely with Louisiana State Department of Agriculture representatives to assess the situation and conduct surveillance around the detection site to determine the extent of the disease spread.
Soybean rust is spread primarily by wind-borne spores capable of being transported over long distances. At this point in time, based on predictive models, APHIS believes that the detection in the U.S. is related to this year’s very active hurricane season. While the harvest for this year is complete, during next year’s planting season, producers will need to watch for symptoms of the fungus such as small lesions on the lower leaves of the infected plant that increase in size and change from gray to tan or reddish brown on the undersides of the leaves.
USDA and the soybean industry have been cooperating on awareness efforts and will amplify those efforts now that the disease has been found in this country. Lesions are most common on leaves but may occur on petioles, stems, and pods.
Soybean rust produces two types of lesions, tan and reddish brown. Tan lesions, when mature, consist of small pustules surrounded by slightly discolored necrotic area with masses of tan spores on the lower leaf surface. Reddish brown lesions have a larger reddish brown necrotic area, with a limited number of pustules and few visible spores on the lower leaf surface.
Once pod set begins on soybean, infection can spread rapidly to the middle and upper leaves of the plant.
Soybean rust can be managed with the judicious use of fungicides. However, early detection is required for the most effective management of soybean rust. Monitoring soybean fields and adjacent areas is recommended throughout the growing season.
Fungicide applications can reduce yield loss, depending on the plant developmental stage, time when soybean rust is detected, and fungicide application method. Efficacy information for producers on fungicides is available through state university Extension services.
For more information, visit APHIS’ soybean rust “hot issues” Web site at