Managing SDS? Plan For High Yield First

As harvest began, the U.S. Department of Agriculture’s (USDA’s) September crop report estimated total soybean production at 3.483 billion bushels. The report came as a surprise to Iowa farmers looking at large areas affected by the yield-robbing disease, sudden death syndrome (SDS).

“SDS showed up in 2010 on a scale not seen since it hit Illinois fields in the early 1990s,” says ISA Director of Contract Research David Wright, who anticipated yield losses this year could exceed 20 percent in some fields.

“In north central Iowa we saw widespread SDS for the first time,” says Dean Coleman, soybean farmer from Humboldt, Iowa. “We had spots varying from small areas to more than two-thirds of fields.”

John Heisdorffer of Keota, Iowa, says, “Five years ago we saw a few small areas of SDS the size of a room or smaller. Those spots became almost entire fields this year.”

Meanwhile, Illinois was not as severely impacted by SDS in 2010.

“SDS was severe in pockets of Illinois, but not over the whole state,” says Glen Hartman, research soybean pathologist with the USDA-Agricultural Research Service and professor at the University of Illinois.

The soilborne fungus which causes SDS infects the soybean root early in the plant’s development. Through the growing season the fungus colonizes more and more of the roots, and during pod set and filling a toxin produced in the roots moves to the foliage of the plant, causing early senescence of the plant. High soil moisture favors infection by the fungus. Rainy weather throughout the first half of the growing season and cool soil temperatures, as well as soil compaction and poor drainage, increase the risk of infection.

According to Iowa State University (ISU) Plant Pathologists Leonor Leandro and Alison Robertson, a “perfect storm” of conditions created the ideal circumstances for SDS to develop at near epidemic scale in Iowa this year. Leandro says a combination of early planting into wet soils and a dip in soil temperatures to 50°F for the entire second week of May, when about 50 percent of soybeans had been planted, likely favored infection by the fungus. Wright adds that this coincided with soil compaction from last fall’s wet harvest, which may have also contributed to infection.

USDA figures show 13 percent of Iowa soybeans were planted by April 26, compared with 2 percent in 2009 and zero in 2008. All in all, Iowa soybean planting was more in line with 2005 and 2006, but ahead of any year since 2000.

ISU Agricultural Meteorologist Elwynn Taylor says, “When we look at areas of Iowa severely affected by SDS, we see unfavorably cool and moist conditions during much of the planting season were replaced by a summer with precipitation averaging 240 percent of normal during June and August in much of the state.”

In Illinois, Hartman says, “Those who had wetter conditions in general may have had more SDS. When it turned dryer about mid-summer, it probably avoided a very severe SDS situation throughout much of the state.”

Another factor that may have made SDS less severe in Illinois is that soybean varieties with greater field tolerance to SDS have been developed for southern regions while farmers in northern Illinois and Iowa plant maturity groups I and II, which generally have lower levels of tolerance to the disease.

Farmers impacted by SDS this year are considering their options for the 2011 soybean crop.

“It is unlikely that we will again see, with any frequency, the severity of SDS that farmers experienced this year,” Wright says. “While the fungus has been and will remain in the soil, there will always need to be a particular combination of factors for SDS to be problematic.”

Leandro and Robertson agree. “The disease will not develop if weather conditions are not favorable,” Leandro says.

However, Wright cautions, “Farmers will see SDS more frequently in low-lying areas that are compacted and poorly drained.”

Some farmers have questioned recommendations to plant soybeans in late April-early May.

Wright advocates early planted soybeans because it maximizes yield, despite the risk. Although early planted soybeans can be at more risk of SDS, yield gains from early planting outweigh the risk if farmers avoid the soil conditions that promote disease development.

So, what should farmers do to minimize their risk of SDS?

Based on soybean checkoff-funded research, plant pathologists, agronomists and soybean plant breeders recommend an integrated approach to managing SDS, including the following strategies:

• Choose high yielding, tolerant varieties. Variety selection is the number one management tool. Choose varieties with the best combination of high yield and agronomics targeted to each field, such as tolerance to SDS.
• Plant varieties strategically. Plant varieties with the best tolerance to SDS in fields where the disease has been most problematic. Also, plant fields with a history of SDS last, allowing excess water to drain and soil to warm, but do not delay planting to the point of compromising yield potential.
• Reduce soil compaction. Compaction provides a wet seed zone environment and puts stress on the plant. Deep ripping may break up compaction zones.
• Tillage. Wright says the impact of tillage on SDS remains unclear and farmers should not use tillage as a primary SDS management tool. “Although research suggests incorporating residue can reduce disease severity, no-till has its advantages. Mature no-till fields tend to drain excess water more quickly, providing a less saturated seed zone,” he says.
• Improve soil drainage. Install tile in slow-drying areas because excess water and soil compaction are key factors increasing severity of the disease.
• Manage SCN. “It’s quite apparent that you find SDS in close association with SCN,” says Jason Bond, Southern Illinois University plant pathologist. “Therefore, plant varieties with good resistance to SCN and test soils frequently to monitor nematode populations.”
• Minimize harvest losses in corn. Because dropped corn kernels may harbor the pathogen causing SDS, ISU Plant Pathologist X.B. Yang says a clean corn harvest should help reduce the risk of SDS.
• Understand limits of crop rotation. Wright says, “University of Illinois researchers report finding SDS in continuously grown soybeans, soybeans following one or two years of corn, and in fields that had been out of soybean production for many years. They reported that the fungus causing SDS can persist in soil for many years, even without a host crop.”

Meanwhile, checkoff-funded researchers throughout the Midwest are working feverishly to develop breeding lines with improved tolerance to SDS, which will be the long-term solution to the threat of this disease.