By DARCY

DOUGHERTY

MAULSBY

Farm News staff writer

AMES – Sudden death syndrome is now among the top four yield-robbing diseases in soybeans, and its widespread presence in Iowa during the 2010 growing season has raised plenty of questions among farmers and researchers.

“The severe SDS that occurred in Iowa this year was not the result of any single factor,” said Alison Robertson, an Iowa State University assistant professor of plant pathology, who noted the disease was also widespread in Illinois, Indiana, South Dakota, Minnesota and Wisconsin. “There has been a gradual buildup of SDS in Iowa over the past 10 years, and the weather conditions in 2010 were ideal for severe SDS disease development.”

This year’s yield losses to SDS are expected to exceed 20 percent in some fields, added Robertson, who noted that SDS is caused by a fungus present in many Iowa soils that infects soybean roots and produces a toxin that moves up the plant and kills the leaves. Although the disease does not occur in corn, ISU research shows that fungus can survive on corn kernels and in other corn residue.

Wet weather and soil compaction favors SDS disease development, robertson said. Temperatures below 60 degrees at planting favor infection of soybean roots by the SDS fungus. However, greenhouse research has shown that infection can occur at temperatures up to 82 degrees. Moderate temperatures hovering around 80 degrees during the growing season lead to SDS symptoms developing on the leaves.

When SDS is present in a field, soybean roots will appear rotted, and plants will be easily pulled from the soil. The fungus that causes the disease may appear as blue fungal growth, or spore masses, on the main or tap root of the soybean plant. When split lengthwise with a knife, the internal tissue of the main or tap root will be gray to reddish brown, not a healthy white.

In addition, the areas between the leaf veins will turn bright yellow, then eventually brown. The dead, brown tissue between veins may fall out, leaving large ragged holes in leaves. While the leaf blades will fall off of the petioles – the thin “stems” that connect the leaf blades to the main stem, while the petioles remain attached to the stem.

SDS leaf symptoms look similar to symptoms of brown stem rot, Robertson said. To distinguish SDS from BSR, split a soybean stem lengthwise with a knife. The center of the stem, called the pith, will be brown with brown stem rot, but it will remain white with SDS.

Managing SDS

Currently there are no seed treatments or foliar sprays that can be applied to protect plants from SDS, said Robertson, who noted that an integrated, multifaceted approach is needed to manage the disease.

“The foundation of an SDS management program is use of resistant soybean varieties. Consult with (a) seed sales professional and agronomists for information on varieties that are resistant to SDS, and grow soybean varieties with the greatest resistance to SDS in the fields with the greatest history of SDS problems.”

Fields with a history of SDS should be planted later, rather than earlier in the spring, said Robertson, who added that it’s wise to take measures to avoid or reduce soil compaction. Also, consider improving soil drainage, if possible, in fields with recurring SDS problems.

“In addition, we know there’s a relationship between SDS and soybean cyst nematodes,” said Robertson, who noted that it’s important to take regular soil tests to monitor SCN populations. “SCN is much easier to manage at low levels.”

ISU’s plant pathologists, agronomists and soybean breeders continue to study various aspects of SDS and the fungus that causes the disease, said Robertson, who noted that these scientists are collaborating with scientists at other universities to find solutions for SDS management.

There is a bit of good news with SDS, she added. “While soybean plants that have been severely affected by SDS will produce smaller seeds, the fungus does not infect the soybean seed, so it does not cause any grain storage problems.”

Watching white mold

While SDS was the major disease affecting Iowa soybean fields this summer, other diseases like white mold could become a challenge in 2011 if weather conditions are favorable, Robertson said.

“White mold was quite severe in 2009, and a lot of growers will be planting those same fields to soybeans in 2011. Since the inoculum is out there, it will be important to plant soybean varieties with white mold resistance.”

While resistant varieties should be the cornerstone of any soybean disease management program, Robertson said, there are still many factors that can influence disease development. “Mother Nature still has the upper hand at the end of the day.”

You can contact Darcy Dougherty Maulsby at yettergirl@yahoo.com.