Stop Soybean Rust News, KS
04-17-06
Seven questions (and answers) about Asian soybean rust
By Gil Gullickson, Successful Farming Crops Technology Editor
Asian soybean rust isn't generating as many headlines as it did last year at this
time.
However, it's important not to drop your guard against this devastating disease.
It's an opportunist waiting to strike if conditions warrant.
[In this slideshow,] we've rounded up seven common questions and answers
about Asian rust and fungicides that farmers are asking these days.
1. Why didn't rust strike in 2005?
It did surface in the United States last year, heading as far north as Kentucky.
Even so, infestations were few and light due to the following reasons:
A colder-than-normal 2004 and 2005 winter contributed to more dieback of kudzu
than normal. Kudzu is a host for rust. With less kudzu, there was less green
tissue to harbor the spore. "Once green tissue dies, rust cannot survive," says
Daren Mueller, an Iowa State University plant pathologist.
Light wind speeds. "This probably suppressed spore lifting," says Mueller.
Fewer southern winds than normal, which possibly reduced spore transport to
the central United States.
Dry conditions west of the Mississippi River. "If spores were pushed out of
Florida, they weren't landing in an environment conducive for disease," says
Mueller.
Low spore viability.
2. Where's rust been found this year?
Asian soybean rust has not yet been found on new planted soybeans in 2006. It
has, however, surfaced in kudzu in five counties in Alabama, eleven counties in
Florida and four counties in Georgia. It was also found on soybeans in Hidalgo
County, Texas, as they were being harvested in February 2006. (These beans
were planted very late in 2005.)
3. If you have spores in your field, will you have rust?
Not necessarily. Mueller notes that Asian-rust like spores were found as far
north as North Dakota.
"Soybean rust spores can travel long distances," he says. "Most regions had
positive samples, including the Midwest and Northeast."
However, these samples gave no indication about spore viability. Nor was
soybean rust observed in the vicinity of these samples.
"Weather conditions near the sampling sites play a dominant role in determining
whether soybean rust develops or not," says Mueller.
"In areas of Iowa that were dry, spores could have landed in soybean fields
where conditions were not conducive for the disease to develop."
4. What will happen this year in the Midwest?
Much depends on if rust overwinters in Texas. If that's the case, prevailing wind
patterns could make it more likely for spores to land in the Midwest.
As last year showed, though, spore arrival doesn't mean rust will occur. For rust
to develop, spores need to land on soybeans at the right growth stage during
periods of extended leaf wetness and moderate temperatures. Asian rust
outbreaks have occurred when temperatures are between 60 to 80 degrees and
leaves are wet 6 to 8 hours each day. Economic damage occurs between the R1
(beginning bloom) and R6 (full seed) stages.
Mueller advises growers to monitor the media and Web sites for the spread of
rust. There is a first detector system in place in Iowa and other states in which
agricultural professionals are specially trained to diagnose Asian soybean rust.
5. Why didn't storms like Hurricane Katrina spread rust last year?
After all, rust likely arrived in the United States in 2004 in the aftermath of
Hurricane Ivan. But there's a lot more to the spread of rust than hurricanes.
Mueller says for disease to develop, you need a susceptible host (soybeans), a
pathogen (the fungus) and a conducive environment.
Even if hurricanes transfer spores, they still need to land on soybean plants at
the right growth stage with periods of extended leaf wetness and moderate
temperatures.
6. How can I head off rust before it hits?
Timely applications of preventive fungicides are the best way to head off rust.
Fortunately, growers have a wide selection of fungicide from which to select to
control rust. Fungicides with Section 3 full label approval can be used to treat
Asian rust and other soybean fungal diseases, while those with Section 18
emergency exemption approval are labeled in soybeans for just treatment of
Asian rust.
In Iowa, for example, Section 3 fungicides include Quadris, Headline, Echo and
Bravo. Fungicides with Section 18 emergency exemption approval for Asian rust
include ones like Laredo, Folicur, Domark, Quilt, Stratego and Headline SBR.
7. What about spraying fungicides for "plant health"?
"Plant health" is the name of the program BASF is using to promote promoting
the use of its Section 3 fungicide Headline. The premise is that this strobilurin
fungicide can boost yields by curbing late-season diseases and triggering
positive changes in plant physiology. Syngenta is also recommending another
strobilurin fungicide, Quadris, for control of late-season diseases and positively
impacting plant physiology.
A concern growers have regarding preventive fungicide applications is that they
will be for naught if Asian rust doesn't strike. Preliminary evidence indicates
strobilurin fungicides can positively impact yields even in the absence of Asian
rust. However, yield responses don't always occur.
In 2005, Marty Draper, South Dakota State University Extension plant
pathologist, compiled 65 university fungicide trials from 13 Midwestern and
Northern states. Trials occurred in the absence of Asian rust. He figured a 4bushel an acre yield bump would be needed to pay for the application and
fungicide cost.
One third of the time, strobilurin fungicide applications boosted yields above this
level. Yields rose another one third of the time, but not enough to cover costs.
Yields actually decreased in the remaining one third of the trials.
Results regarding these types of applications have been more consistent in
Southern states with higher late-season disease pressure. "In general, drier
conditions not conducive to disease occurred in the Midwest," says Mueller.
However, plant pathologists acknowledge something is occurring in the plant in
the absence of Asian rust due to strobilurin fungicide applications. The challenge
now is to find how to make responses more consistent, says Mueller.