November 23, 2004: The Iowa State University (ISU)
Organic Ag Program has been asked to respond to the discovery of soybean
rust in the United States (as of November 19, 2004, the disease has
been reported in 5 states in the South) and how it relates to organic
growers. Iowa has approximately 60,000 acres of organic soybeans and
all growers are equally concerned about the prospect of rust appearing
in Iowa in 2005.
Soybean rust (Phakopsora pachyrhizi) is a very important
exotic disease threatening commercial soybean production in this
country. Environmental conditions in the US are expected to be very
favorable for rust development. Experts have said that should the
disease be introduced here, it would likely spread throughout the
main US soybean production area in one season.
The rust pathogen survives and reproduces only on live hosts. Soybean
rust is not seed-borne, although spores could travel as contaminants
in seed lots from infested areas. Urediniospores, the spore type
associated with soybean rust, are very easily wind disseminated.
Although soybean is susceptible to rust at any growth stage, symptoms
usually first appear on the lower leaves, at or after flowering.
The pathogen prefers prolonged wet and cool conditions. Two types
of foliar symptoms are manifested: a) large tan lesions with abundant
spore production and b) smaller red-brown lesions, which yield fewer
spores (see photos).
So far, all commercial varieties appear to be
Varietal screening done in quarantine by the USDA reveals that
virtually all the existing commercially grown soybean cultivars
are susceptible. Sources of resistance to the prevalent natural
populations of soybean rust have been identified. A number of different
synthetic fungicides are known to be effective in rust management
but organic management strategies have not yet been sufficiently
studied due to the absence of the disease in the US.
Starting in 2005, all available organically-approved materials
(copper, sulfur, hydrogen peroxide, and other naturally based materials)
will be tested for efficacy against soybean rust. Tests will be
conducted in cooperation with states where the disease has already
been detected. The chances of finding a material as effective as
the already identified synthetic fungicides is not good, however.
Organic farmers, like conventional soybean farmers, will need to
do a risk/benefit assessment and determine if economics favor spraying
any materials if the disease is found in Iowa. ISU will determine
costs of materials for organic producers and help in developing
best methods for dealing with this disease if it is found here.
Longer crop rotations and compost applications can assist with general
disease management, but the long-term effect of these strategies
for soybean rust is still unknown.
The early arrival in the US from points south (South America and
Africa) caught many by surprise. "The number of hurricanes
and precipitation events this year increased the possibility of
spore dispersal," X.B. Yang, plant pathologist at ISU, said.
"Rain is a factor in the survival of spores. This hurricane
season was a once-in-40-year event, which may help explain how soybean
rust was blown into the United States this year."
Soybean rust may or may not find its way to Iowa organic soybean
fields next year. "Soybean rust will not overwinter in Iowa,"
Yang said. "We will have to wait for the spores to travel from
the south every season. It's too early to make predictions, but
we need to make predictions based on spring rust occurrence in the
south and early summer weather systems, such as tropical storms,
that may influence its travel."
Stay vigilant, learn the symptoms and report
any suspicious leaves immediately
on board at ISU, whom you may call with questions:
Greg Tylka, Iowa State plant pathologist, (515) 294-1741,
Tylka coordinated training sessions for over 400 crop
professionals during the summer.
Palle Pedersen, Iowa State agronomist, (515) 290-3212,
Pedersen is Iowa State's extension expert on soybean
Alison Robertson, Iowa State plant pathologist, (515)
Robertson is an Iowa State extension plant pathologist.
X.B. Yang, Iowa State plant pathologist, is one of
the leading experts in the world and is on his way to
Louisiana as member of the USDA soybean rust detection
The Organic Ag program will continue to provide updates
as information is gained through USDA, ISU and other
University sources. A soybean rust website has been
established at http://www.soybeanrust.info. Other web-based
information may be accessed at:
ISU has developed a rapid analysis system to speed up reporting
of soybean rust. The system is simple: if a farmer finds a suspicious
leaf, he or she can submit a sample to designated first detectors
at no cost. First detectors send suspect samples to triage personnel,
who are Iowa State Extension field staff, for further diagnosis.
The triage person then forwards suspect samples to the Iowa State
University Plant Disease Clinic. Those living outside of Iowa can
contact their local extenction agent for the best method of reporting
rust in their area.
ISU is reacting to the threat of soybean rust with great speed.
As part of the new effort, the university will create two new faculty
positions working on methods to combat soybean rust in the colleges
of agriculture and liberal arts and sciences, as well as a postdoctoral
scientist position. The Iowa Soybean Promotion Board will provide
$500,000 over three years to help fund the salaries of the new faculty.
Extensive surveys of soybean and various legume hosts for Asian
soybean rust will be carried out in Iowa beginning in 2005.
Kathleen Delate is an associate professor of agronomy and horticulture
& ISU organic ag extension specialist