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Managing a nightmare soybean disease
It sounds like the plot for a cheap horror movie: For
months, a monster lives buried in the soil, only to reach up and kill healthy
looking soybeans, sometimes in broad swaths across the landscape.
That monster is a fungus, Fusariaum Virguliforme, the
cause of Sudden Death Syndrome, a nightmare confronting growers across the
Midwest this year. The disease starts in the roots and sends a toxin to the
leaves that kills the plant. Foliar fungicides are useless against the toxin.
Cathy Schmidt, a plant pathologist at Southern Illinois
University, understands the frustration of watching helplessly as SDS takes its
"Sudden Death Syndrome is a disease of high yield. It
appears when you have good conditions for growth," she tells Agriculture.com.
Schmidt should know. She coordinates the largest trial in
the country of commercial soybean varieties for tolerance to SDS. The Illinois Soybean Association has been funding this trial for 25 years. At this
point, there is no complete resistance to SDS, but you can sift through the thousands of varieties she has tested at this website. Generally, about 7% to 12% of commercial varieties are moderately resistant to highly resistant, which could still result in yield loss up to about
Seed selection is just part of the management options for
anyone trying to avoid a sequel to the nightmare.
As Steve Butzen, agronomy information manager for Pioneer
sums it up: "Management practices for SDS include selecting tolerant varieties,
planting problematic fields last, managing SCN (soybean cyst nematode),
improving field drainage, reducing compaction, evaluating tillage systems and
reducing other stresses on the crop."
After a 2009 season that left some growers finishing
harvest just before 2010 planting, this year brought a series of perfect storms
for SDS. A cool, wet spring
favored growth of the fungus, with warmer weather later in the summer helping
the toxin build up in leaves, says Aaron Robinson, technology development
manager for Monsanto.
The fungus does especially well in poorly drained,
compacted soils, a condition not helped by last year's wet harvest and this
year's wet spring.
"Farmers just haven't had the opportunity to get out and
take care of compaction," Robinson says.
X. B. Yang, a plant pathologist at Iowa State University
agrees that improving drainage will help reduce SDS. "This only works in a
normal year, not a year like this summer," he says. He, too, recommends
delaying planting on problem fields. "This works all the time," he says.
He also believes that certain soil types may be a
better environment for the disease. "No work has been done to prove it," he
says. "It is my observation."
The disease survives for several years in the soil,
making crop rotations an imperfect prevention.
"It also survives well in corn residues," he adds. "It
survives better on corn residues than on soybean residues. We have had two
years data to prove this. Better corn harvest is important for SDS management."
All this means that seed variety selection is important
for fields hit hard with SDS. In that case, Schmidt would look for those
varieties she labels as R or MR, for resistant and moderately resistant.
Schmidt rates varieties based on a disease index. That's
a combination of the disease incidence, measured by the percentage of plants
with symptoms, and a disease severity score. The score is based on a 1-100 scale,
with 1 being mild chlorosis, 50 equaling severe leaf scorch and 100 meaning the
plant died prematurely.
Any variety with a relative disease index of 0 to 20 gets the R
In one trial at Havana, Illinois last year, out of 554
varieties tested, 39, or 7%, had the R rating. More than half, 53%, were rated
as susceptible to the disease (with a relative disease index of 61 or worse.)
That squares roughly with Aaron Robinson's observations
"I would say that as far as those that have a high level
of tolerance go, probably less than 50% of the varieties in the marketplace
have that high level of tolerance," he says.
This year, conditions were so bad in some areas that even
those beans showed some loss.
"Even some of the highest levels of tolerance we have in
commercial varieties, we're seeing it in those fields," he says.
Pioneer also uses a one through nine rating system for
soybean seed tolerance. Here's how their seed tolerance measures up, according
to Don Schafer, senior marketing manager.
"There are no soybean products that have complete
resistance to SDS, nor are there known sources of complete resistance to SDS.
There are only levels of tolerance. At Pioneer, our goal is to work with
growers on a field-by-field basis to provide products that are best suited for
all agronomic and defensive needs for those acres. Included in that overall
package is tolerance to SDS," he says.
"Given that, let me provide you these numbers. We rank
SDS tolerance on a 9 to 1 scale with 9 being the highest tolerance. In recent
years, between 60 to 70% of our soybean units have a score of 6 or better. During
the same time, between 70 to 80% of our soybeans have a score of 5 or better.
In 2011, we expect to have nearly 70% with a 6 score or better and more than
80% with a score of 5 or better. That's the best way I can describe our level
of tolerant products. Through such programs as Accelerated Yield Technology and
marker assisted selection, Pioneer's soybean breeding program is continuing to
raise the level of all disease traits to provide growers with the best possible
combination of agronomic and defensive traits while still continuing to improve
Monsanto, too, is using marker-assisted breeding to find
historical lines with SDS tolerance, says Robinson. "That's definitely
something we have a high level of awareness and effort around."
Robinson advises keeping good maps of yield levels and
SDS infestation for buying seed for the 2012 crop, when soybeans in a typical
corn-bean rotation will be going back into problem fields.
That's where it makes the most sense to plant the high
SDS tolerant beans.
At Southern Illinois University in Carbondale, Illinois,
Schmidt agrees with the advice of her scientific peers. But she's also been
researching SDS for almost 20 years, long enough to know that a lot of the
advice is far from fool proof.
Planting late reduces the odds of exposing young beans to
the fungus, but she has seen SDS in soybeans double cropped after wheat. And
she has seen SDS in a field not planted to soybeans for ten years, testament to
the staying power of the fungus.
Still, Schmidt believes in rotating soybeans, for a lot
of agronomic reasons. For one thing, it helps reduce soybean cyst nematode
damage. That stress on soybean roots is thought to give SDS a head start, too.
Right now, there is no seed treatment against SDS, but
Schmidt has hopes for that.
"That could change. I'm sure it will, because we've been
beating our heads against this for 20 years," she says.
Meanwhile, if it's any comfort, Schmidt says that when
she tries to set up a field trial that will stress soybean varieties with SDS,
she usually has to plant plots in three different locations in Illinois to get
one trial that has much of the disease.
And she always plants early.
"When I try to make a field sick, I work very hard to get
sick soybeans, so I will plant when corn is going in the ground," she