Ankeny, Iowa, USA
August 27, 2010

Sudden death syndrome (SDS) is ravaging many Iowa soybean fields. David Wright, Iowa Soybean Association’s (ISA’s) director of contract research, says the disease has become an important yield-limiting factor in soybean production. “This year’s yield losses to SDS are expected to exceed 20 percent in some fields,” Wright says.

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

John Heisdorffer of Keota has seen areas of SDS grow in his area. “Five years ago we saw a few small areas of SDS the size of a room or smaller. Those spots have become almost entire fields this year.”

The cause of the disease is a soil fungus that is widespread throughout Iowa and the Midwest. It infects the soybean root early in plant development; later, the pathogen rapidly kills the plant during pod set and pod filling.

Infection of the soybean root is worsened by cool weather as well as soil compaction and poor drainage. Wright says the severity of SDS this year is the result of early planting combined with extreme weather events which led to excess soil moisture after planting.

Five years ago ISA’s farmer leaders identified the increasing threat posed by SDS and committed additional checkoff investment to expand Iowa State University (ISU) 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 currently used throughout the industry.

ISU researchers offer several recommendations for SDS management:

• Select tolerant varieties. Make note of varieties showing good tolerance to SDS in production fields and variety test plots. Choose varieties with the best combination of high yield and tolerance to SDS.
• Plant varieties strategically. Plant varieties with the best tolerance to SDS in 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 slow-drying areas because excess water and soil compaction are key factors increasing severity of the disease.
• Manage SCN. SDS is often more severe in areas also infested with soybean cyst nematode (SCN). Therefore, plant varieties with good resistance to SCN and test soils frequently to monitor nematode populations.

The ISA-ISU partnership includes several areas of ongoing SDS research.

For the past decade, ISU agronomist Silvia Cianzio has been engaged in an intensive program to improve SDS-resistance in soybeans adapted to Iowa. Her work includes the 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.

In 2006 ISA worked with ISU to hire Leonor Leandro, a plant pathologist specializing in soilborne fungal pathogens. Leandro was first to discover 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.

ISA’s Wright is an advocate of planting soybeans early but 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 roots are susceptible.

Soybean checkoff-funded research also led to the discovery that exposed corn residue, including dropped kernels, harbors the SDS fungus. Reducing kernel loss during harvest may help reduce the severity of SDS.

While there are currently no chemical seed treatments or foliar fungicides available for control of SDS, ISA and ISU continue their extensive research efforts toward understanding and controlling this pathogen.

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

The Iowa Soybean Association develops policies and programs that help farmers expand profit opportunities while promoting environmentally sensitive production using the soybean checkoff and other resources. The Association is governed by an elected volunteer board of 21 farmers.
 

Website: http://www.iastate.edu

Published: August 27, 2010