Sudden Death Syndrome in soybeans remains an enigma. But scientists are beginning to find which management systems help fend off the disease. So far they lean toward two options: Find a variety with good resistance to the fungus that causes Sudden Death Syndrome and stagger soybean planting, leaning toward later dates.

The first thing to know about Sudden Death Syndrome (SDS) in soybeans is that the disease arrives--or doesn't--through a tangle of field-level variables. Research shows that early soybean planting followed by cool and wet weather gives Fusarium solani

(F. solani), the soil-borne fungus that causes the disease, a foothold in the soybean plant's root system. Later, the fungus produces toxins that move through the plant. It's only then that most growers realize their soybean stand suffers from the disease, seeing the stark contrast of leaves becoming bright yellow along their veins then eventually dying off.

"One thing that is confusing about SDS is that the symptoms show up later in the season when the plants are flowering and setting pods," said Laura Sweets, a plant pathologist for the Commercial Ag program at the University of Missouri. "But the infection actually occurs much earlier in the season, when the plants are in the young vegetative stages of growth."

Sweets said that cool, wet conditions may increase incidence of plant infection early in the soybean's life, but from what she has seen, moderate and wet conditions as the crop is flowering and setting pods really favor symptom development--the variegated yellow, curled and dying leaves as well as pod abortion. It's the onset of these symptoms during the disease's development that perpetuates the idea that a good year for soybean growth is a good year for SDS.

But what makes a favorable year for SDS is also influenced by variables. Sweets, who does field research on the disease near Boonville, Mo., said that a disease management prescription of late planting is misleading. "What is important are the conditions after you plant," she said. Warmer, drier weather post planting means a less likely chance of plant infection.

Sweets has had mixed success in her trial work. She's surely the only person planting soybeans in the area who hopes for conditions to favor SDS. But she doesn't always get them.

"You don't get those conditions every year, and that's a challenge," she said. There have been occasions in which she couldn't get plots planted in time for the cooler weather and was forced to plant later. In some of those cases, even in fields that have a known F. solani population lurking in the soil, the crop makes it through the season with little sign of damage from SDS. This anecdotal evidence, along with actual planting-date studies, provides mounting evidence to show that later planting dates--waiting for warmer, drier soil--is one way to minimize SDS.

That leads to one of the management options of the disease--staggered planting.

Sweets prefers to recommend "staggered" planting because even delayed planting can be followed by cool, wet conditions and high SDS damage. And obviously, waiting out spring's tumultuous weather for too long will short the soybean crop of yield by virtue of shortening the growing season.

Staggering planting, Sweets suggested, can be accomplished by keeping track of which fields have had heavy incidence of SDS in the past and saving them for last. Calendar-based planting, she said, is futile unless you're sure what the weather will do. And, of course, few growers will sit idle while conditions are favorable for planting.

Find the right variety
Variety selection is another obvious guidepost in avoiding yield loss from SDS. In her field trials, Sweets sees a number of varieties that offer good resistance to SDS, as well as some that succumb all too easily. The trick in selection, she said, is to evaluate your overall needs for a disease package, making sure to put the proper weight on SDS resistance if it is a problem you've had in the past.

"You don't want to focus on just one thing. You don't want to have tunnel vision, choosing the SDS reaction in exclusion to yield or maturity or phytophthora or soybean cyst nematode resistance," she said.

Her advice is to look at seed company brochures and literature to find the rating for SDS resistance as well as other disease resistance. Some literature will go so far as to suggest that the variety not be planted in fields with history of SDS because it is highly susceptible to the disease. Sweets said stopping by plots in your growing area is a valuable tool for future variety selection and that sitting down with a seed dealer to explain your needs can be enlightening.

Oval Myers, an emeritus professor of plant breeding and genetics at Southern Illinois University (SIU), has been involved in SDS research since 1985, when the disease first was given much scrutiny in the Midwest. He said that today's growers have better options for resistance packages, but as usual, caveat emptor.

"Most companies will make statements when it comes to SDS resistance. Generally speaking, I think the resistance levels are about twice as good as they were, say, 10 years ago. But if you look at a brochure and you don't see any comments about SDS, I'd be concerned," said Myers.

Companies don't have to back up claims they don't make, he said.

Asked about the coffee-shop indictment of Roundup Ready soybeans being more susceptible to SDS, Myers said that that might have been the case early in Roundup Ready's history.

"When we first saw the boom in Roundup Ready soybeans, it turns out, by chance, that the original [glyphosate resistance] insertion was made into a variety that was quite susceptible to SDS. That variety was good for other things like yield and SCN resistance, etc. But in the presence of SDS infection, it didn't fare very well."

Myers said that problem has diminished, however. Through the proliferation of Roundup Ready technology, more and more lines of soybeans have been bred, bringing with them new SDS resistance traits.

"Resistance is at least twice as good as it was 5 years ago," he said.

And it's got plenty of potential. Sweets said that her plot work shows existing standouts for SDS resistance, but there are more and better varieties in the breeding pipeline. She said that a multi-state consortium has focused on studying SDS and each year runs experimental variety tests coordinated by SIU.

"Some of that material, you're probably looking at 2 to 5 years before release," she said. "But the folks at SIU have been doing this for a long time, and I think you're seeing a shift. It's something we've made great progress on, but it's something that you can't give up on."

Even the best resistance isn't immunity--a qualifier Sweets was compelled to offer.

"Something that is resistant still can get the disease. And there are some years when disease pressure and the weather conditions are so conducive that even the best resistance won't do the job you wish it would."

Varied losses
Calculating yield loss from SDS can be difficult. Most growers know to look for the telltale circles or ovals of yellow and falling leaves. Sweets said in some years similar distinguished swaths of diseased plants will show up along drainage paths where the soil has remained wet. While these patches of damage look--and can be--alarmingly severe, they might represent a minor portion of the field's overall yield.

"In many cases, we see striking visual symptoms, but the yield loss may be less than 5 percent, maybe 5 to 10 percent," said Sweets.

Sweets cited a trial a few years back with varieties that ran the gamut from slight to severe damage all under the same disease pressure. In ballpark figures, yield loss ranged from 80 percent down to 5 percent--a clarion call for well-thought-out variety selection.

But, as you can count on with SDS, there's no "normal" damage.

"We'll see fields with the circular or oval damage. Or, we'll see fields with swipes running across it related to drainage. And in some years we'll see SDS damage across entire fields," Sweet said.

Rotate crops, improve drainage
Careful variety selection and managing planting dates are the easiest, year-to-year ways to guard against SDS. But there are other precautions to take in avoiding fungal disease.

Sweets and Myers both said that fields continuously planted with soybeans will build the levels of F. solani in the soil. Thus, rotating crops can decrease the chances of infection. However, Sweets said that given the typical rotation in this part of the Midwest, growers shouldn't count on dramatically reducing SDS pressure with a corn/soybean rotation.

"We've found that rotations have a larger impact on building populations than reducing them. The more frequently soybeans are in the rotation, the higher the [fungus] populations are likely to go and the more severe the chance for SDS," said Sweets.

And finally, one reason that SDS shows up in ovals, circles and along drainage ways is that water stays too long in parts of the field. Working to correct drainage problems might help lessen the impact of SDS. n