Soybean Rust

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,

reducing pod set and pod fill, which results in

reduced yields and seed quality.

Q. How does the fungus damage soybeans?

A. Scientists believe the soybean rust pathogen can

have as many as five stages in its life cycle, though

only three have been observed. Spore–bearing bodies of those three stages—uredinia, telia, and

basidia—grow on the underside of infected leaves,

below the epidermal layer. Infected leaves take on a

brown, stippled look before losing chlorophyll and

eventually dying. Under a powerful lens, fungal 

bodies look like little brown volcanoes.

Q. What does soybean rust look like on the

plant?

A. Soybean rust is difficult to identify in the early

stages of infection, as symptoms are very small,

poorly defined, and occur in the lower–middle canopy

of the plant, where it is humid. Early symptoms

appear as a yellow mosaic discoloration on the upper

surfaces of older leaves. As the disease progresses,

leaves turn yellow, and brown or reddish pustules

appear, generally on the bottom surface of the leaf.

Soybean rust infection may be mistaken for spider mite damage or foliar diseases such as Septoria

brown spot, bacterial blight, or bacterial pustule. It is

important to note that soybean rust pustules frequently lack the yellow halo associated with bacterial

pustule. Unlike lesions from spot diseases, soybean

rust pustules are raised, mainly on the underside of

the leaf.

Q. How is soybean rust transmitted?

A. The fungus is 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–infected areas. Though tourists, live plant material, or mechanical transport on planes or ships could

deliver soybean rust spores to the continental United

States, spores blown in on wind currents are the

most likely source of introduction for soybean rust to

the U.S. mainland.

Q. How mobile is soybean rust?

A. Soybean rust spores can be carried long distances by wind currents. In 1998, spores were blown

1,350 miles down Africa from Uganda to Zimbabwe.

Between 2001 and 2003, the disease spread more

than 1,500 miles, from Paraguay to near the equator,

infecting as much as 90 percent of Brazil’s soybean

acres on the way. Researchers believe soybean rust

is as mobile as the sugarcane rust that blew into the

Dominican Republic in 1978. In fact, many believe

that soybean rust spread via wind currents from Asia

to Africa, then to South America. Plant pathologists

believe soybean rust is likely to spread much like

wheat stem rust or Southern corn rust when P.

pachyrhizi reaches North America.

Q. Where is soybean rust found?

A. As of April 2004, soybean rust had not been

found in the continental United States. Asian soybean rust, P. pachyrhizi, the more aggressive of the

two species, was first reported in Japan in 1903 and

was confined in the Eastern Hemisphere until its

presence was documented in Hawaii in 1994. Today,

the disease occurs on some islands and most continents where soybeans are grown:

• Asia—China, Japan, the Philippines, Taiwan, 

India, and Eastern Russia

• Australia

• Africa—Uganda, Zimbabwe, Zambia, Rwanda, 

and other African nations

• South America—Paraguay, Brazil, Argentina, and

Bolivia

• Hawaii

• The Caribbean (P. meibomiae only) 

Q. How destructive is soybean rust?

A. 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. In Australian test plots where no

fungicides were applied, yield losses reached 60 to

70 percent. In 2003, Brazilian producers lost $1.3 billion to soybean rust, a figure representing lost yield

and the cost of fungicides applied to combat further

losses.

Factsheet

Plant Protection and Quarantine May 2004

APHIS
Q. What fungicide application schedule will work

best for controlling soybean rust?

A. Researchers are designing a model to predict

areas in the United States most susceptible to soybean rust. They believe this model will help to identify

where soybean rust will most likely appear in the

United States. The disease is difficult to detect in the

early stages of an introduction. However, experience

in South America, Africa, and Asia indicates that early

detection and treatment are essential.

Q. What is the U.S. Department of Agriculture’s

(USDA) Animal and Plant Health Inspections

Service (APHIS) doing to prevent the introduction

of soybean rust here in the United States?

A. National and international scientists and

researchers, including USDA experts, agree this disease will most likely be introduced into the United

States by natural means through airborne spore dispersal; however, APHIS is working to delay the

human–assisted introduction of the disease through

its safeguarding program.

Q. What are the elements of APHIS’ soybean rust

safeguarding program?

A. APHIS has recently updated its strategic plan to

minimize the impact of soybean rust introduction. The

plan focuses our efforts on protection, detection,

response, and recovery. To view the APHIS Soybean

Rust Strategic Plan, visit our Web site at

www.aphis.usda.gov/ppq/ep/soybean_rust/sbrplan12-

03.pdf.

Q. Because the focus of the strategic plan is to

minimize the impact of soybean rust, what actions

has APHIS taken?

A. APHIS is assessing the risks associated with the

introduction of imported soybean seed, meal, and

grain.

Because of our excellent working relationship with

the American Soybean Association (ASA), APHIS

co–sponsored a soybean rust conference in January

2004 to disseminate information and to train more

than 300 soybean producers, handlers, and consultants in soybean rust detection. Representatives from

five major pesticide companies were also on hand to

discuss fungicide mitigation.

Our academic partners at the University of Illinois

and Iowa State University are developing models to

predict the regions of the United States most susceptible to soybean rust. They believe this model will help

to identify where soybean rust will most likely appear

in the United States.

APHIS’ pest detection staff is working with

USDA’s Cooperative State Research Extension and

Education Service (CSREES) and its National Plant

Diagnostic Network (NPDN), providing disease recognition and pathogen diagnostic tools.

• Several States are establishing soybean rust 

monitoring sites along the Eastern seaboard and 

within Southern States.

• We’ve also assembled a soybean rust detection 

assessment team composed of scientific experts 

and State and Federal regulatory officials who 

would come together to assess the nature and 

extent of the first outbreak of soybean rust in the 

continental United States.

Q. What’s APHIS’ current policy on imported soybean commodities?

A.

• While soybean meal, grain, and propagative 

seed are allowed into the United States, 

soybean plants are prohibited. Because the 

soybean rust pathogen is not a seed–borne 

disease, and it primarily affects the plant’s 

stem, pods, and leaves, we’ve restricted the 

importation of all soybean plants from countries 

with the disease to further protect U.S soybean 

crops.

• Imported soybean seed intended for planting must

be clean.

• Grade soybean meal is a processed product.

During processing, the meal is heat–treated, 

therefore eliminating spore viability. If foreign 

matter is removed prior to processing, and added 

back into the meal after processing, the matter 

must first be heat–treated.

• Soybean grain is allowed into the United States 

with up to 2 percent foreign matter.

• To date, there is no evidence supporting an 

assertion that soybean grain shipments, meeting 

our current import requirements, present a risk to 

U.S. agriculture.

Q. Given what we know about the devastation

caused by soybean rust in Brazil and Argentina,

why isn’t APHIS protecting U.S. soybean farmers

by imposing a ban on imported soybean meal,

grain, and seed from those countries?

A. It’s APHIS’ mission to safeguard U.S. agriculture

and natural resources. It wouldn’t be prudent to

impose a ban on imported soybean meal, grain, or

seed without scientific data to support such a ban.

Q. Are soybean shipments inspected at ports of

entry?

A. Yes. Department of Homeland Security Customs

and Border Patrol officers inspect soybean seed and

grain shipments to ensure compliance with import

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

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