Louisiana Soybean

Rust Detection

Q: Where has soybean rust been detected in the

continental United States?

A: Phakopsora pachyrhizi, or soybean rust, was 

confirmed on November 9 in two research plots 

associated with Louisiana State University’s research

farm located approximately three miles from the main

campus in Baton Rouge, Louisiana.

Q: What methodologies were used to positively

identify the pathogen affecting the suspect samples as soybean rust? And, what does that 

testing process involve?

A: USDA scientists with APHIS’ National Plant

Germplasm and Biotechnology Laboratory in

Beltsville, Maryland, confirmed the presence of 

soybean rust using polymerase chain reaction (PCR)

and morphological testing. PCR testing is simply a

DNA test. In using PCR techniques, DNA is 

extracted from spores or infected leaf samples, 

subjected to PCR, and then ground and purified

before being analyzed for the presence of key 

diagnostic sequences of DNA that distinguish soybean rust from related species. Morphological testing

is simply a microscopic examination of the spores or

lesions on infected leaf samples. The testing process

is generally completed within 24 hours.

Q: What is soybean rust?

A: 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. Affected plants are quickly defoliated,

resulting in reduced yields and seed quality.

Soybean rust has devastated soybean crops in many

parts of the world, with reported yield losses as high

as 80 percent in some afflicted areas of Africa and

South America. The Asian species of soybean rust is

the species detected in Louisiana.

Q: How is soybean rust spread?

A: Soybean rust is a fungus, spread primarily by

windborne spores that can be transported over long

distances. Seed-borne transmission has not been

documented. Clouds of spores are released if 

infected plants are disturbed by wind or by individuals

walking through rust-infested areas.

Q: What is believed to be the source of infection

for the recent detection?

A: APHIS has dispatched its Soybean Rust

Detection Assessment Team to work closely with

state officials to determine the nature and extent of

infection. An exact source of infection may never be

known, but a probable explanation is the spread of

the disease from South America to the United States

during the active hurricane season.

Q: What steps is APHIS taking to control or 

prevent the further spread of soybean rust in the

United States?

A: At present, APHIS has dispatched its Soybean

Rust Detection Assessment Team to determine the

nature and extent of infection. Because soybean rust

is spread via wind currents, quarantining the area will

not be effective to contain the disease. In preparation

for the possible introduction of the disease, APHIS

had been working with industry groups and its sister

agencies, Agricultural Research Service and

Cooperative State Research, Education and

Extension Service, to educate soybean producers

nationwide on how to identify the disease and report

suspected finds. With the soybean harvest in

Louisiana complete and the detection of the disease

in a research plot, this is the best possible time for

the introduction. Now that we have detected the disease, we can better prepare growers for the upcoming season.

Q: Is the detection of soybean rust expected to

impact the trade of U.S. exports?

A: No. USDA does not anticipate any effect on

exports. The disease occurs in most areas where

soybeans are grown: Asia, Australia, Africa, and

South America Hawaii. The New World Species is

also found in the Carribbean. Despite the presence

of soybean rust, many of these countries export 

soybeans.

Q: Can soybean rust be controlled to minimize

potential impacts?

A: Yes. Soybean rust can be managed with the 

judicious use of fungicides. However, early detection

is required for the most effective management of 

soybean rust. Fungicide applications can reduce

yield loss depending on the developmental stage of

affected plants, the time when soybean rust is 

detected, and the fungicide application method. For

efficacy information on fungicides labeled for use on

soybeans, consult university extension personnel in

your state.

Factsheet

Plant Protection and Quarantine November 2004

APHIS
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(202) 720–2600 (voice and TDD).

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Civil Rights, Room 326–W, Whitten Building, 14th and

Independence Avenue, SW, Washington, DC 20250–9410 or call

(202) 720–5964 (voice and TDD). USDA is an equal opportunity

provider and employer.

United States Department of Agriculture • Animal and Plant Health Inspection Service • Safeguarding American Agriculture