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February 1, 2006
Pg. 26 ISSN: 0892-8312
Tougher beans
By Karen Simon
STEVE HENRY, who farms near Nevada, IA, is trying to learn everything he can about
Sudden Death Syndrome (SDS). However, he's frustrated with the lack of hard data
about the disease.
He began noticing yield loss from SDS in 2003. That year, a 50-acre field with the
disease averaged 32 bu./acre while his whole farm average was 52 bu./acre. "When
SDS hits, it can be very devastating," he says. "And I've noticed that some soybean
varieties are less tolerant to the disease than others."
Henry has learned a few things to help manage SDS, but he'd like more information.
"There's not a lot of data available to help us accurately manage this. We need to figure
it out," he says. "We aren't even sure which varieties are more tolerant to SDS. The seed
companies have no standardized way to rate SDS tolerance. We're lacking good
information.
"I don't want to discount the impact or seriousness of Asian soybean rust, but we have
diseases that are robbing our yields right now," Henry says. "I think more research
dollars should be spent toward formulating effective management plans to combat the
diseases we have right here in Iowa."
SDS and soybean cyst nematode (SCN) account for as much as 60 to 70% of total yield
loss in Iowa alone - close to 70 million bushels, according to Palle Pedersen, soybean
extension agronomist at Iowa State University. "SDS and SCN have been confirmed
in more than 75% of Iowa fields," he says.
Pedersen says SDS is caused by a pathogen called Fusarium solani, which can be
extracted out of almost every field in North America. But certain environmental
conditions need to be in place for the disease to be expressed. SCN "opens the gate" for
SDS.
Terry Niblack, an extension specialist at the University of Illinois, explains that generally
SDS is expressed when soybeans are exposed to a cool, wet spring, which allows the
fungus to grow. The disease is often triggered by hot, dry conditions in July or August.
"This isn't always the case," she explains. "It seems to be the pattern, but it's hard to
prove in an experiment."
Niblack says SDS may or may not affect yield. Potential yield loss depends greatly on
the timing of infection and weather conditions during the growing season, and if SCN is
present.
Identification
Farmers may be able to identify the disease by splitting the plant's stem near the first
couple of nodes, Niblack says. While another disease, brown stem rot, will cause the
pith to be brown, SDS causes the tissue surrounding the pith to be discolored. Farmers
can send samples to a university plant diagnostic clinic to determine whether the
disease is present in a particular field.
"We think the SDS yield loss moves with the nematodes, so if you get high nematode
pressure, you start seeing more SDS symptoms," Pedersen says. "SDS by itself might
not cause a lot of yield loss, but the combination of SDS and SCN can cause a
tremendous loss."
This year, Pedersen says, the disease was prevalent in Illinois and the eastern twothirds of Iowa. "There is no doubt that we have seen a lot of SDS in Iowa over the last
three years," he says.
Resistant varieties
One of the best management tools is to plant a variety that's resistant to both diseases.
"Sometimes farmers think they're doing the right thing by selecting an SCN-resistant
variety. However, that variety might be susceptible to SDS. A farmer in central Iowa can't
just select a good SCN-resistant variety. SDS resistance is also needed and that is now
available," Pedersen says.
The researcher urges growers to use variety trial data and then do their own on-farm
testing to see how the varieties will perform on their farms. Seed companies have
different rating systems and it can sometimes be a challenge to gauge effectiveness.
"Every farmer, whether he has 50 or 50,000 acres, should have a small experiment on
his farm with replicated strips across the field. The data he collects can be used with
other trial information to help choose effective varieties," Pedersen explains.
In the past, there was some yield drag with SCN-resistant varieties, but Pedersen says
that's no longer the case. In addition, the SCN-resistant seed costs the same as nonresistant varieties.
However, as farmers manage for more and more diseases, it becomes more challenging
to find varieties that will be effective. Growers with iron chlorosis in their fields, for
example, will find it challenging to find a variety that will take care of iron chlorosis, SDS
and SCN.
"Controlling the nematode will go a long way toward making it less likely that SDS will
show up," Niblack says. "If you can find a good variety that's resistant to both diseases, if
you have them, that's a good choice."
To access SDS variety ratings, visit http://www.vipsoybeans.org, or request a hard copy
of the ratings from the Illinois Soybean Association (http://www.ilsoy.org).
"We can't control Mother Nature, but we can build a platform for healthy plants that can
better withstand disease pressure," Pedersen says. "Plant and root health is critical for
high soybean yields. SCN and SDS should be on all farmers' radar since they together
account for more than 50% of the yield loss from pathogens in most fields. And this is
not just in Iowa. SCN is the most yield-limiting pathogen in most soybean-growing
states."
This story first appeared in the Mid-February issue of The Corn and Soybean Digest.