Asian Soybean Rust -
Frequently Asked Questions I:
Background and General Information
Author: Elizabeth Bush, Special Research Associate, Department of Plant Pathology, Physiology, and Weed Science, Virginia Tech; Erik Stromberg, Extension Plant Pathologist, Department of Plant Pathology, Physiology, and Weed Science, Virginia Tech; Pat Phipps, Extension Plant Pathologist Tidewater Agricultural Research and Extension Center; and David Holshouser, Extension Soybean Specialist, Tidewater Agricultural Research and Extension Center
Publication Number: 450-301, posted March 2006
What is Asian soybean rust (ASR)?
ASR is a disease caused by the fungus Phakopsora
pachyrhizi. The disease causes numerous lesions to
appear on the plant that could lead to premature defoliation
and yield loss.
What will ASR mean for Virginia’s soybean
crop?
If ASR moves into Virginia, it is manageable. Correct
fungicide selection and application timing can minimize
yield loss. If conditions are favorable for the
disease and fungicides are not applied properly, yield
losses of 10 percent or greater can occur. The cost of
production will increase with fungicide application;
therefore, soybean profitability may decrease.
How much yield loss from ASR can farmers
expect?
Risk analyses indicate that ASR could cause yield
losses of greater than 10 percent in any United States
soybean-growing region. In the southeastern coastal
states, losses of up to 50 percent are possible because
of more favorable environmental conditions and earlier
infection due to the closer proximity to overwintering
sources of ASR. Actual losses will depend on
weather conditions during the growing season, where
the pathogen overwinters, and when the pathogen enters
Virginia.
What areas of the United States are most
at-risk for yield losses from ASR?
All soybean-growing areas of the United States are at
risk. In general, southern growing areas are at greater
risk than northern areas because of the longer growing
season, the presence of other ASR hosts such as
kudzu, and their proximity to areas where ASR can
overwinter. Therefore, the southeastern states closest
to an overwintering source are at greatest risk.
Additionally, a warm, humid climate favors ASR disease
development. Based on 30-year averages, very favorable
weather conditions for the disease occur in the
mid-Atlantic states, the Mississippi Delta region, and
the Ohio and Mississippi river basins in the Midwest.
Late-planted soybeans, such as those planted after
small grains, are at higher risk, since there is more time
for spore movement and inoculum buildup preceding
the crop’s entry into reproductive stages.
How did ASR reach the United States?
It is thought that ASR was carried to Louisiana and
other southeastern states by Hurricane Ivan, which hit
the Gulf Coast states in September 2004. Fortunately,
researchers at Louisiana State University discovered
ASR while monitoring soybean plots in November
2004. Further scouting efforts revealed that ASR was
widespread in the Gulf Coast states.
Will ASR overwinter in Virginia?
This is unlikely since ASR cannot survive without a
living annual or a perennial host (e.g., kudzu) with
green foliage. Virginia experiences low temperatures
that would kill most annual hosts and eliminate any
green foliage of perennial hosts. Areas that are likely
to support year-round occurrence of ASR are the
southernmost United States (e.g., Florida, south Texas)
and further south (e.g., Caribbean, Mexico, equitorial
South America) where freezing temperatures are rare.
The presence of green kudzu and several other hosts in
those southernmost areas creates a potential for ASR
to become established year round.
When will ASR reach Virginia?
No one can say for certain whether ASR will reach
Virginia in any given year. However, since ASR will
overwinter on kudzu in Florida, the likelihood that it
will reach Virginia is increased. Asian soybean rust requires
a living host to survive and produce spores. Its
host range includes many species of legumes, which
increases the potential of overwintering and moving to
new locations during the growing season. Additionally,
the spores of ASR are produced rapidly in large quantities
under favorable environmental conditions (i.e.
temperatures between 59 to 82°F and at least six hours
of free moisture on the leaves). These spores are readily
windborne and can travel long distances on wind
currents. This is why weather and wind currents are
being used to predict where and when ASR might appear.
Monitoring the northward movement of the pathogen
is critical. Therefore, an extensive sentinel plot network
was implemented in 2005. This network proved
effective in monitoring ASR movement in the United
States. Movement into Virginia will likely differ each
year since the pathogen will not overwinter in Virginia.
Furthermore, dry weather will greatly inhibit rust development.
Can migrating birds carry rust spores to
the United States?
No research has examined this question; however, the
viability of spores that may travel in this way is unlikely.
Temperature, high moisture, ultraviolet light conditions,
oil on the birds’ feathers, and duration of the trip
would negatively impact spore survival.
How far and how fast can the ASR pathogen
spread?
ASR dispersal is highly dependent on environmental
conditions and weather patterns. Enviromental conditions
affect ASR spore production. Long-distance
dispersal is dependent on wind patterns and weather
conditions. Most spores from other rusts travel 20 to 30
miles a day from south to north in the United States. It
is expected that ASR could move similarly.
How long will it take for ASR to be transported
by wind currents from southern to
northern production areas?
Opinion varies on the time required for ASR to reach
northern production areas. This could take one to several
years. The movement of ASR from south to north
in the United States will be different each year and will
depend on where ASR is able to survive the winter,
the size of the ASR population that overwinters, and
weather patterns and conditions. Research shows that
wind patterns are more favorable for disease movement
from south to north in El Niño years.
Can the spread of ASR be prevented?
Since ASR is carried by wind currents and has numerous
hosts there is no way to quarantine and halt the
spread of this disease.
Are models being developed to predict
the spread of ASR through the U.S.?
Yes. Three models for predicting ASR movement
based on weather patterns, environmental conditions,
and/or other parameters were developed and tested in
2005. Each have strengths and weaknesses, but performed
reasonably well in their first year. Using data
collected from sentinel plots, these models continue to
be refined. The goal is to link these models together
and thereby utilize the strengths of each. At this stage,
prediction models are not substitutes for a coordinated
monitoring and tracking system.
Are imports of soybean meal, planting
seed, or whole soybean likely to bring
rust into the U.S.?
No, ASR is not seed-borne, although there is a very
slight chance that spores could be associated with debris
in seed. For soybean meal there is virtually no
chance of contamination since meal is heat-treated,
which kills any ASR inoculum. The American Soybean
Association has worked closely with the United States
Department of Agriculture, Animal and Plant Health
Inspection Service, Plant Protection and Quarantine
(USDA-APHIS-PPQ) to reduce the risk of accidental
introduction of ASR.
Will crop insurance cover losses due to
ASR?
Unavoidable loss of production due to plant disease (e.g.
ASR) is a covered peril under the Coarse Grains Crop
Provisions, provided the loss is due to natural causes
and not agroterrorism. Therefore, soybean losses due to
ASR are insurable. However, the producer must follow
good farming practices and rust management recommendations
of agricultural experts, such as Extension
agents and certified crop consultants. Farmers should
document advice received and actions taken. If there
are no effective control measures available or insufficient
amounts of chemicals are available for effective
control, the resulting yield loss would be covered.
However, damage due to insufficient or improper application
of available disease control measures will not
be covered. Failure to purchase and apply recommended
control measures will result in uninsurable causes
of loss being assessed. Producers must be knowledgeable
of any pending outbreaks and the control methods
recommended by local agricultural experts. For more
information and updates on crop insurance, visit the
USDA Risk Management Agency’s website at http://
www.rma.usda.gov/news/soybeanrust/.
If ASR is found on my farm, will I be restricted
from moving my crops?
U.S. producers will face no restrictions on cropping activities
(planting, cultivating, harvesting, baling, pesticide
applications, etc.) due to the presence of ASR in
their fields.
Concern about restrictions most likely originated from
the fact that ASR was on the USDA’s select agent list.
Since ASR was determined to have been naturally introduced
into the United States, it has been removed
from the select agent list (as of March 10, 2005).
Those involved in ASR research are required to
have permits for interstate movement of cultures of
ASR, but there are no restrictions on crop movement.
Would I be permitted to harvest the soybean
vegetation for silage or hay and use
it for feed on my farm?
If there are no restrictions related to forage listed on the
fungicide labels that have been applied to that crop, the
soybean crop may be used for forage. If a fungicide has
been applied, refer to the specific product label regarding
the required preharvest interval between fungicide
application and harvest. In addition, be sure to check
the fungicide label for restrictions for the use of the hay
for livestock feed or residue as for bedding.
Is there anything I can do now to prepare
for soybean rust?
Continue to stay informed. If you suspect ASR is
present in your fields this season, contact your local
Extension office, a certified crop advisor, or one of
Virginia’s Primary Diagnostic Centers for identification
of ASR or other soybean foliar diseases. If ASR
is confirmed and has not previously been found in
Virginia, the sample will undergo a second level of testing
by APHIS Plant Protection and Quarantine (PPQ)
and Center for Plant Health Science and Technology
(CPHST).
Check the Virginia Asian Soybean Rust home page,
http://www.ppws.vt.edu/ipm/soybeanrust/index.htm,
and hotline, (757) 657-6450 x130, frequently for updates
on soybean rust detections, movement, and upto-
date fungicide recommendations.
How are my soybean checkoff dollars
being used to help find solutions to this
problem?
The Virginia Soybean Board committed funds to provide
for a soybean rust monitoring program during the
2004 growing season. Over 75 fields were monitored
through this program throughout most of Virginia’s
soybean production regions on a bi-weekly basis between
June and September. In 2005, the Virginia Soybean Board funded fungicide efficacy and cultural
control research. In addition, the Virginia General
Assembly allocated $50,000 in 2005 for ASR monitoring
and research during the 2005 and 2006 growing
seasons. These funds were matched by Virginia Tech’s
College of Agriculture and Life Sciences. ASR was not
found in Virginia in 2004 or 2005.
The United Soybean Board has funded research on
ASR movement, occurrence, soybean resistance to
ASR, and screening of commercial soybean cultivars.
Soybean checkoff dollars have funded much of the information
on ASR available to soybean growers. State
and national associations have been at the forefront
of preventing accidental introduction of ASR into the
United States, promoting federal funding for research,
and encouraging the EPA and chemical companies to
ensure the availability of fungicides.
In addition, USDA has dedicated over $1 million to establish
sentinel plots throughout the United States.
Other ASR Resources:
Asian Soybean Rust website at Virginia Tech -
http://www.ppws.vt.edu/ipm/soybeanrust/index.htm
USDA Soybean Rust Tracking site -
http://www.sbrusa.net/
North American Plant Disease Forecast Center -
http://www.ces.ncsu.edu/depts/pp/soybeanrust/
The Southern Plant Diagnostic Network -
http://spdn.ifas.ufl.edu/soybean_rust.htm
Related Publications
Asian Soybean Rust - Frequently Asked Questions
II: Identification, Biology, and Ecology, Virginia
Cooperative Extension publication 450-302
Asian Soybean Rust - Frequently Asked Questions
III: Control with Fungicides, Virginia Cooperative
Extension publication 450-303
Asian Soybean Rust - Frequently Asked Questions
IV: Cropping Systems and Cultural Practices,
Virginia Cooperative Extension publication 450-304
Asian Soybean Rust - Frequently Asked Questions
V: Monitoring, Tracking, and Scouting, Virginia
Cooperative Extension publication 450-305
Asian Soybean Rust - Frequently Asked Questions
VI: Sprayer and Nozzle Technology, Virginia
Cooperative Extension publication 450-306
Acknowledgments
The authors would like to express their appreciation for
the review and comments made by Scott Hagood, Sam
Johnson, Brian Jones, and Steve Rideout.
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