This post originally appeared on the Soybean Research and Information Network website.

By Carol Brown

Photo – Soybean Research & Information Network

For decades, farmers have been trying to overcome soybean cyst nematode, or SCN, in their fields. There are two major sources of SCN resistance in PI 88788 and Peking on the market today, and 95% of soybean cultivars contain resistance from the PI 88788 source, according to the SCN Coalition. But researchers and farmers are finding that after years of their use, the resistance is waning.

A team of researchers has been collaborating on a complex project to enhance SCN resistance with long-term, strategic SCN management. The project, supported by the North Central Soybean Research Program, holds great potential for soybean farmers.

Leading the project is Andrew Scaboo, assistant professor in plant science and technology at the University of Missouri and an expert in soybean breeding. The team is approaching the project from two directions: exploring the genetic makeup of the nematode and developing soybeans with the ability to resist their attack. The two come together in the greenhouse and small research plots to test SCN virulence levels in different soybean varieties.

“We want to understand SCN and how it adapts to reproduce on resistant soybean varieties,” says Scaboo. “We are also testing soybean variety rotations, as well as developing germplasm and varieties with new combinations of resistance genes.”

The term “virulence” describes the ability of nematodes to overcome resistance in the soybean plant. Scientists measure virulence levels with an HG type test, or Heterodera glycines, the scientific name for SCN. The number of female nematodes developing on a resistant plant are counted and compared to the number of females that have formed cysts on a susceptible variety. When the number of females is above a 10% threshold on the resistant plant, it indicates the SCN population is overcoming the resistance. The more females, the more virulent the SCN population is on the resistant plant.

The complete sequencing of the SCN genome — its entire genetic blueprint — was a major outcome from this project in 2018. Because of this, scientists can now better identify genes in the nematode that grant it the ability to overcome resistance in the soybean plant.

Since this milestone for soybean nematode research was completed, Melissa Mitchum at the University of Georgia, Thomas Baum at Iowa State University, and Matt Hudson at the University of Illinois, continue to conduct groundbreaking work on nematode virulence.

“Drs. Mitchum, Baum and Hudson are currently sequencing the genome of several different SCN populations, spanning different HG types. This helps to construct large assemblies of every gene and chromosome in the nematode and make comparisons. These expert scientists are working to understand the underlying mechanism that allows the nematodes to overcome the resistance that we deploy in our soybean varieties,” Scaboo explains.

“With these breakthroughs in genome sequencing, we have applied population genetic tools to precisely map candidate virulence genes in the SCN genome,” Mitchum adds. “Once we have validated the function of these genes in virulence, we may be able to design faster methods to determine the HG type of a population within a farmer’s field and prescribe the most effective type of resistance.”

Meanwhile, new soybean resistance genes are being discovered and incorporated into breeding programs in a concerted effort to bring new modes of resistance to the market. Scaboo’s soybean breeding team, in collaboration with Mitchum’s group, have recently discovered a gene in PI 90763 and PI 437654 that is missing in the Peking source of resistance. This will help to combat the most virulent SCN that are overcoming the Peking variety, and they hope it will be incorporated into soybean cultivars to provide new tools for farmers in the fight against SCN.

Out of the Lab and Into the Soil

The other component of the research is a field-based microplot study comprised of experimental trials with soybean varieties initially infected with SCN. The team is evaluating the changes in nematode populations over time, including their density and virulence, in the presence of specific soybean varieties with varying genetic modes of resistance.

Scaboo, along with Greg Tylka and Eliana Monteverde, have university research microplots in Missouri, Iowa and Illinois. Tylka is a nematologist at Iowa State University and a leader of The SCN Coalition. Monteverde, a soybean breeder, is a newcomer to the team. She assumed the role at the University of Illinois that Brian Diers held on this project, who recently retired.

The team is rotating PI 88788- and Peking-type varieties with new experimental lines that carry novel genetic mechanisms of resistance including traits from exotic plant introductions as well as the wild soybean, Glycine soja. 

“In the microplots, we measured nematode virulence over four years while rotating the different forms of genetic resistance in the plants,” says Scaboo. “We found a significant reduction in SCN population density and a lesser virulence when the genetic modes of action were rotated, as compared to when the same resistance varieties were used over time.”

Ultimately, the team is in pursuit of developing new rotation strategies and new cultivars that are SCN resistant so farmers have more weapons in the fight. The team has accomplished a great deal since the project was first launched, including developing several soybean varieties that farmers are now growing through Scaboo’s and Dier’s breeding programs.

“With these new varieties put forth in the public and private sector, it has improved the genetic base of SCN resistance for the entire industry,” remarks Scaboo. “Another highlight includes the value of our public institutions’ extension programs. There is great value in bringing this knowledge to farmers. Also, Dr. Tylka has evaluated nematode resistance in the many varieties available to farmers, which was also supported by this project.”

The team continues to work for farmers and their productivity by improving the quality of soybeans at the genetic level.

Additional Resources

Soybean Cyst Nematode – SRIN information page

An Integrated Approach to Enhance Durability of SCN Resistance for Long-term Strategic SCN Management – SRIN article

Making Advancements for Soybean Cyst Nematode Through Plant Breeding – SRIN article

NCSRP Research Team Unravels Genome of the Soybean Cyst Nematode, Revealing Genes Involved in Parasitism – SRIN article

Soybean Breeders Focused on Sustainability – The SCN Coalition YouTube video

What is a Virulent Nematode? – The SCN Coalition YouTube video