Careful planning is key to managing this soybean disease.

Symptoms of sudden death syndrome on a soybean leaf.

By LeAnn Strother and Karen Simon

Sudden death syndrome (SDS) can be disarming because plants suddenly turn yellow and die. However, scientists have discovered that while the plant may exhibit symptoms suddenly, the disease may actually be harbored in the plant for a long time, perhaps infecting the plant just days after germination.

“The best way to manage this disease is by planning,” says Jason Bond, plant pathologist at Southern Illinois University. “You can’t do much in August, but in October, you can plan for seed selection. Most cultural practices do little to control SDS; however, when soil is compacted, sub-soiling or chisel plowing can reduce it, though it won’t eliminate it.”

Bond’s checkoff-funded research by the North Central Soybean Research Program (NCSRP) has shown early planting increases the chance of SDS,

“We hesitate to tell farmers to delay planting, but we encourage them to plant fields with history of SDS last,” Bond says.

Rainfall also increases the chance of SDS. “Ironically, the conditions that drive high yield also contribute to greater SDS,” Bond says.

X.B. Yang, Iowa State University plant pathologist, says that management of SDS should start when you harvest corn fields. His group compared the survival of SDS fungus in different crop residues (corn and soybean) and found that corn kernels consistently had the highest SDS fungus population. This corresponded with observations that severe outbreaks of SDS can occur after a few years of continued corn production. Yang concludes that a high amount of harvest loss in corn increases the risk of SDS in the following soybean crop; minimizing harvest loss of corn may help reduce the risk of SDS.

Additional NCSRP-funded laboratory research reinforces what fields have shown. Leonor Leandro, Iowa State University plant pathologist, and her colleagues have learned that, although symptoms appear at flowering, soybean roots can become infected with the SDS fungus as young seedlings. As roots mature, they are less susceptible to infections.

“If the fungus remains only on the outside layer of the root, the toxin can’t move up to the leaves,” Leandro says. “The fungus must make its way into the center of the root. If infection is delayed, roots can resist having the fungus get to the center so leaves will stay healthy.”

Leandro’s research also shows that in warmer conditions young plants are susceptible to infection for only four days or less, while in cool temperatures roots are susceptible up to two weeks.

Farmers can apply this knowledge by planting SDS-infected fields last to reduce the chance of infection. If planting while soil temperatures remain cool, they should try to avoid planting when it is wet.

Leandro’s group is now focused on learning whether there’s another susceptible time for infection, perhaps in the plant’s reproductive phase.

Meanwhile, using lines with greater SDS resistance, plant breeders are now using genetic markers to breed resistance genes into high yielding varieties.

“Breeding for SDS resistance is difficult because it is controlled by many genes with each having a small effect on resistance,” says Brian Diers, soybean breeder at the University of Illinois.

Another challenge is that many sources of SDS resistance genes are agronomically poor, making it difficult to breed resistance into high yielding varieties.

“We can’t yet say we’ve solved the problem,” Diers says. “However, we have found some genes that should increase SDS resistance and reduce grower losses.”

From this research, breeding lines with greatly improved resistance to SDS have been released for Maturity Groups I, II and III.  Because this research is checkoff-funded, results are made available to both private and public breeders. Seed should be available to producers in three to seven years.

Identifying SDS

SDS is now one of the most important pathogens of soybean in the United States and Canada. It robs U.S. producers of an estimated 28 million bushels annually. Yield loss from SDS is the result of reduced pod and seed fill, or in severe cases, flower and pod abortion. It continues to move northward and is now not only found in Iowa and Illinois, but in Minnesota, South Dakota and Wisconsin as well.

Fields with high yield potential, including those with high fertility levels, tend to show more severe symptoms (interveinal chlorosis). Also, fields higher in soil moisture, such as poorly drained and no-till fields are more symptomatic. Symptoms are most likely to appear first in low-lying areas with excessive moisture and in areas of compaction.

Identifying SDS is the first step to managing it. Knowing which fields have SDS will allow soybean growers to implement proper management practices the next time soybeans are planted. Because the foliar symptoms of SDS are similar to brown stem rot, the two diseases are often misdiagnosed.

To accurately diagnose SDS, cut open the stem and tap root of the symptomatic plants. SDS infected roots will be gray or brownish in color, but the center of the stem will be white. In contrast, the center of the stems of brown stem infected plants will be chocolate brown.

Managing SDS

SDS is caused by a soil fungus that can survive in the soil for many years. Effective management of the fungus requires an integrated approach.

If SDS is causing defoliation in your fields, do not plant highly susceptible varieties. Ask your seed dealer for information on varieties with partial resistance or tolerance to SDS. Managing soybean cyst nematode is also very important for managing SDS because the nematode can infect the soybean root with the fungus while feeding. Don’t forget to choose varieties first for high yield potential based on multi-location data, then choose the agronomic package you need for each field.

Other management considerations include:

• Plant fields with a history of SDS last
• Reduce harvest loss in corn
• Make sure fields are well drained
• Break up compaction zones

*Content Funded through the Soybean Checkoff