By DOUG SCHMITZ
Iowa Correspondent

URBANDALE, Iowa — The Iowa Soybean Assoc. (ISA) late last month reported the recurrence of Sudden Death Syndrome (SDS), a lethal soybean disease with remnants that have actually been lingering around some Iowa soybean fields for the last 15 years.

“SDS has been in Iowa for many years, observed most frequently in eastern Iowa counties,” said David Wright, director of contract research and strategic initiatives for the ISA’s North Central Soybean Research Program. “It was observed by Palle Pedersen, (Iowa State University) extension agronomist, as early as June 2008.”

First discovered in Washington County in southeastern Iowa in 1993, SDS is caused by a fungal pathogen that enters the root within four days of germination. The ISA report added the pathogen remains in the root and crown area until the soybean plant enters the reproductive stage, when it releases a toxin that moves throughout the plant, quickly killing the leaf tissue.

“The disease shows up most in areas where soil drainage is poor or where there is soil compaction, so tile drainage and reducing compaction can help reduce the disease,” said Jim Fawcett, extension field agronomist in Iowa City. “Infection occurs in the spring when soils are wet and cool.”

While he didn’t specifically identify which counties or farms are involved in last month’s recent discovery, Wright said SDS is often be found in early-planted soybean fields. But Wright said this year, it seems to be appearing in fields planted through late May.
“It has spread from eastern Iowa to west central and northwest Iowa over the past three to five years,” Wright said. “It is being found for the first time in several fields throughout west central and northwest Iowa, it is not limited to just one or two new counties.
“Agronomists have been noting its presence in those new fields for about two weeks.”

Fawcett said SDS symptoms usually don’t appear until late July and early August.

“Sudden death occurs in all of the counties I cover,” he said. “Last year, SDS was more widespread than I have ever seen in the area. The fungus is already widespread. Its most likely method to spread is with the soil.

“I could not find any fields that did not have some SDS in them last summer, although in some fields it was just in small pockets.”
Now, Fawcett said, SDS has also been found in 11 counties in Minnesota.

“It was first observed in Arkansas in 1971, and was already fairly widespread in Illinois and Indiana before it was discovered in Iowa 15 years ago,” he said. “As far as I know, it is found across the Midwest now.”

The report stated SDS tends to be most severe on well-managed soybeans with high-yield potential, and can also be found in fields infested with the soybean cyst nematode (SCN), and is generally more severe in those fields. Recent checkoff-funded research has shown that SCN cysts can carry the SDS pathogen.

Plants infected with SDS will show browning of the outer vascular stem tissue and signs of root rot; in contrast, Brown Stem Rot infects and browns the inner vascular tissue and shows no root rot.
“The best way to manage this disease is to choose varieties with greater genetic resistance,” Wright said. “It is import for soybean producers and crop advisors to be able to accurately diagnose this disease. You must know what disease you are dealing with before you can effectively manage it.”

For now, management options include planting varieties highly resistant to SCN and with greater resistance to SDS.

“I think in the future, variety selection will likely be an important way to manage the disease, but there are not many varieties yet adapted to this area with good tolerance. Preventing soil compaction and improving soil drainage are very important,” Fawcett said.

“It was easy to see areas in fields where there was more traffic resulting in more compaction and thus, more Sudden Death.”
Since the disease cannot be managed with fungicides, U.S. soybean producers should also identify which soybean varieties are more impacted by SDS, rotating away from those varieties and their sister varieties next year.

“We’re looking at the possibility of reducing yield losses from SDS by improving our current management practices,” said Pedersen. “It is too early to tell right now, but we should know for sure in a year or two.”

While planting later can also reduce the severity of SDS, Fawcett said it isn’t an option most growers are willing to adopt.
“I do encourage producers to try to plant fields with a history of sudden death last, even if this means early May versus late April,” he said. “It’s also important to manage soybean cyst nematode, since most SDS fields also have nematodes. Planting varieties resistant to SCN can often help reduce SDS.”