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
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
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 susceptible
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
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
already on board at ISU, whom you may call with
Greg Tylka, Iowa State plant pathologist, (515)
Tylka coordinated training sessions for over 400
crop professionals during the summer.
Palle Pedersen, Iowa State agronomist, (515)
Pedersen is Iowa State's extension expert on soybean
Alison Robertson, Iowa State plant pathologist,
(515) 294-1741, firstname.lastname@example.org.
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 assessment team.
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
Kathleen Delate is an associate professor of agronomy
and horticulture & ISU organic ag extension specialist