Farm News, IA
10-05-07
Soybean rust confirmed in Iowa
By KELLI BLOOMQUIST, Farm News news editor
Officials confirmed the presence of soybean rust infection from plant samples
taken last week from a field in Dallas County.
This is the first confirmed case of the disease found during the growing season in
Iowa. But Iowa State University plant pathologists stress that the timing of the
discovery is fortunate for soybean growers as it is so late in the season.
“We knew this discovery was a real possibility because of the spore delivery from
the south,” said Bill Northey, Iowa Secretary of Agriculture.
“This is an important find for Iowa soybean growers,” added Curt Sindergard,
president of the Iowa Soybean Association. “It confirms that soybean rust can
develop in Iowa. We are fortunate that this disease was found at a time when it
will have little economic impact for soybean producers.”
Officials were called to the Dallas County field after a report that a soybean leaf
with rust-like pustules had been found. Iowa State University Extension plant
pathologist X.B. Yang collected hundreds of leaf samples from the area.
Subsequent testing in the ISU Plant and Insect Diagnostic Clinic confirmed the
presence of soybean rust.
While experts say soybean rust could cause damage to Iowa’s late-planted
soybeans, those at less risk are plants at the R6 (full-seed developmental)
growth stage or later.
Yang said soybean rust will not reduce soybean yields if infections occur after the
crop has reached the R6 stage.
‘‘Applying foliar fungicides for control of soybean rust after the crop has reached
the R6 stage likely will not provide an economic benefit in terms of yield
protection,’’ said Yang. ‘‘The seeds have fully formed at this stage and need only
to mature before harvest.’’
The Iowa Soybean Rust Team - which includes representatives of Iowa State
University, Iowa Department of Agriculture and Land Stewardship, the Iowa
Soybean Association and the USDA Animal and Plant Health Inspection Service
- have recommended not spraying soybeans with fungicide once the crop has
reached the full-seed stage.
In fact, application of foliar fungicides this late in the growing season may be
illegal as it exceeds the minimum pre-harvest interval, the time between the last
application of a pesticide to a crop and when the crop may be harvested.
Dr. David Wright, Iowa Soybean Association director of contract research,
recommends that soybean growers scout fields with soybeans at the R4 to R6
growth stages to determine their overall risk.
‘‘Most Iowa soybean fields are likely mature enough that producers need not
spray this season,’’ said Wright. ‘‘Scout late-planted fields first, particularly those
that retain moisture from morning dew. First determine soybean growth stage.
Then look for infected leaves in the lower canopy. Look at the underside of lower
leaves for small volcano-like structures. The opening of these structures will be
round, not linear. A 20-30X hand lens is recommended.’’
According to experts, the fungus and spores of soybean rust cannot survive an
Iowa winter, plus they require green leaf tissue to sustain themselves. This,
according to Iowa State University Extension, is why any treatment of lateseason soybean rust infections in Iowa would be unnecessary and not impact the
overwinter survival of the fungus.
Recent cases of rust infection have been discovered in Illinois, Kansas and
Missouri. The disease is spread from southern states by wind and air currents. Its
discovery this late in the season fits within air current models, according to Iowa
State University Extension.
This find marks the 144th U.S. county or parish in which soybean rust has been
confirmed this year. Iowa is also the 14th and northernmost state to have rust
found during the growing season in the United States. In 2006, soybean rust
moved as far north as Tippecanoe County, Ind.