Sudden death syndrome is ravaging many soybean fields in Iowa this year. The disease has become an important yield-limiting factor in soybean production, says David Wright, the Iowa Soybean Association's director of contract research.

"This year's yield losses to SDS are expected to exceed 20% in some fields."

Dean Coleman, a farmer from Humboldt in north-central Iowa, says, "In our area of the state we are seeing widespread SDS for the first time. We have spots in fields varying from small areas of the field to more than two-thirds of some fields being infested with SDS."

In southeast Iowa, the region of the state where this disease has been a problem in past years, John Heisdorffer of Keota has seen areas of SDS grow larger and larger. "Five years ago we saw a few small areas of SDS the size of a room or smaller in our fields. Those spots have become almost entire fields this year."

This is not a leaf disease, it is a fungus disease that enters roots

The cause of the disease is a soil fungus that is widespread throughout Iowa and the Midwest. It infects the soybean root early in the plant's development. Later, the pathogen rapidly kills the plant during pod set and pod filling. Infection of the root is worsened by cool weather as well as soil compaction and poor drainage, says Wright. The severity of SDS this year is the result of early planting and extreme weather events which led to excess soil moisture after planting.

Five years ago ISA farmer-leaders identified the increasing threat posed by SDS and committed additional checkoff dollars to expand Iowa State University research of soilborne pathogens. ISU's SDS research team includes plant pathologists, agronomists and soybean plant breeders. Together they have generated much of the knowledge about SDS that is currently used throughout the seed industry and by soybean growers.

ISU researchers offer several recommendations for SDS management

* Select tolerant soybean varieties. Make note of varieties showing good tolerance to SDS in production fields and in variety test plots this year. Choose bean varieties to plant in the future that have the best combination of high yield and tolerance to SDS.

* Plant varieties strategically. Plant soybean varieties with the best tolerance to SDS in those fields where the disease has historically been severe. Also, plant fields with a history of SDS last, allowing excess water to drain and soil to warm.

* Improve soil drainage. Install tile in the slow-drying areas of fields. Excess water and soil compaction are key factors that increase the severity of SDS.

* Manage soybean cyst nematode. SDS is often more severe in areas that are also infested with soybean cyst nematode (SCN). Therefore, you should plant soybean varieties that have good resistance to SCN and test soils frequently to monitor the nematode populations in your fields.

Soybean varieties with improved resistance to SDS are needed

For the past decade, ISU agronomist Silvia Cianzio has been conducting an intensive breeding program to improve SDS-resistance in soybean varieties adapted to Iowa. Her work includes development of soybean breeding lines in maturity group II, with significantly improved field tolerance to SDS.

"It's not just about improving resistance to SDS," Cianzio says. "Anything we release also has to be high yielding with good resistance to SCN and iron deficiency chlorosis or brown stem rot. We've had great success developing breeding lines the industry wants."

To enable plant breeders to efficiently screen varieties for resistance to SDS, ISU developed a field screening method now commonly used in the industry. It helps seed companies provide farmers soybean varieties with the best combination of yield and SDS resistance.

Should you plant beans early? Or wait until soils are warmer?

ISU research also discovered that soybean seedlings are most susceptible to infection the first few days after planting and that the disease is less severe when infection occurs on older plants. In warm soil, roots grow faster and are susceptible for a shorter period of time.

Farmers in recent years have been urged by ISU agronomists to plant beans in late April and early May to try to maximize yield. Now, farmers are wondering if they would have been better off waiting to plant beans during the last half of May or around the first of June when soils are warmer—to try to avoid SDS problems. 

ISA's David Wright is an advocate of planting soybeans early to try to improve the yield, but he recommends that farmers evaluate the seedbed to ensure they're not creating areas of soil compaction and excess soil water which will restrict root growth and extend the period during which roots are susceptible.

New fact sheet on SDS offers management guidelines for farmers

ISU research by plant pathologist X.B. Yang has discovered that corn kernels left on the ground from harvest harbor the SDS fungus. Doing a good job of harvesting corn and reducing the amount of kernels lost during harvest may help reduce severity of SDS when beans are planted in that field the following year.

"While there are currently no chemical seed treatments or foliar fungicides available for control of SDS, we at ISA are providing the checkoff funding and the researchers at ISU are continuing this extensive research effort toward understanding and controlling this pathogen," says Wright.

An SDS publication and a new SDS fact sheet can be found in the Production Research section of ISA's website www.iasoybeans.com.