SDS meets match in rotations, sets scene for research mystery

By LAURA MILLER, Newsletter editor

A cool, wet summer created flooding problems for many Iowa growers along with another crop headache: an outbreak of soybean Sudden Death Syndrome (SDS), the worst in many years.

Severe defoliation caused by SDS in the two-year rotation plots about five feet away.
SDS is caused by a soil fungus that infects soybean roots soon after planting, producing a toxin that later moves up the plant, damaging leaves and eventually causing them to die and drop. In mid-August, soybean fields infected with the fungus began to turn brown and drop leaves, leaving nearly denuded plants and empty pods in some places. If the disease gets ahead of soybean maturation, yield losses can be as high as 80 to 100 percent, and growers have no effective treatments on the shelf to rescue the crop.

Iowa State agronomist Matt Liebman saw some of the same effects of SDS in his soybean plots at the Marsden Farm west of Ames with one surprising exception: crops in three-year rotations with corn, oat and red clover, and in four-year rotations with corn, oat and alfalfa, remained green into early September. Soybean plants in the two-year corn-soybean rotation suffered markedly from SDS, even though they were one row away from the healthier soybean plants.

“We have had SDS in these plots, but it’s just so dramatic this year,” Liebman said. “It’s definitely a rotation effect because everything else is the same, we just don’t know why.”

All soybeans in the experiment follow corn. All rotations have Roundup Ready™ and non-Roundup Ready™ soybean varieties. All are planted under similar conditions, on similar days, with similar rainfall. The longer rotations have fewer external inputs, relying on red clover, alfalfa and cattle manure for weed control and nutrients.

Liebman has been studying the longer rotations the past eight years, thanks to an initial competitive grant from the Leopold Center and continued with support from the Leopold Center and the U.S. Department of Agriculture.

ISU plant pathologist Leonor Leandro visited the plots in early September and was eager to look at what might be causing the differences. “We don’t know the exact mechanism,” she said, “but the differences in SDS severity are impressive.” ISU research shows that SDS fungus can survive in corn kernels and stalks, but they have not looked at oats, wheat or alfalfa. Other microbes or bacteria in the soil could be suppressing the pathogen, or allowing roots to grow with fewer problems.

Liebman said he would like to add plant pathologists and microbiologists to the team researching these plots.

Ninety percent of the plants were infected by SDS in the two-year rotation while less than 10 percent were infected in the longer rotations. Of the plants that were infected, severity was considerably greater in the two-year rotation (90 percent) compared to the longer rotations (60 percent and 35 percent).

The plots had not been harvested as of press time, so no yield data were available.