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This Pest Costs Soybean Farmers $1 Billion Annually

That’s how much soybean cyst nematode costs soybean farmers.

Soybean farmers stymied because they can’t push beyond a certain yield level may find the answer literally under their feet. Soybean cyst nematode (SCN) that infests the roots of their soybeans may be the yield-robbing culprit. 

“Oftentimes, we are not aware of how big of a problem soybean cyst nematode is for our growers ” says Melissa Mitchum, a University of Missouri nematologist. “Soybean cyst nematode causes around $1 billion in yield losses annually.” 

SCN-resistant varieties have been a great tool to manage SCN. One catch, though. Over 95% of SCN-resistant varieties share the same source of resistance—PI88788. Repeated use of the same control measure time after time—whether for weeds, insects, or SCN—sets the stage for resistance. Over time, the continuous planting of soybeans with the PI88788 resistance source has caused SCN to resist SCN-resistant varieties.

“There are yield consequences to this,” says Mitchum. She cites research done by Greg Tylka, Iowa State University (ISU) Extension nematologist, showing that SCN populations can increase in aggressiveness with repeated plantings of SCN-resistant soybeans with the PI88788 source. 

“Farmers who are planting PI88788 (soybeans) are selecting for resistant nematodes, and consequently are losing yields despite the fact they are planting (SCN) resistant soybeans,” she says. 

The United Soybean Board (USB), the North Central Soybean Research Program (NCSRP), and the SCN Coalition aim to change that. These groups briefed members of the agricultural media at this week’s Commodity Classic in Orlando, Florida, on the National Soybean Nematode Strategic Plan. The plan was formed by scientists (including Mitchum and Tylka) across the U.S. to guide current and future nematode research and support complementary projects and programs for SCN solutions. It also aims at controlling other types of nematodes that impact crop production. This plan is broken into six goals.

1. Develop genomic and genetic tools, resources, and data.  

“One component of this is a better understanding SCN’s genetic blueprint,” says Mitchum. “As researchers, we can then pinpoint which one of those genes are points of vulnerability in the parasitic life cycle (of SCN),” she says. This will help researchers identify chemical, biological, genetic, or bioengineering approaches in developing new management strategies for farmers, she says.

One breakthrough in this area has been the SCN reference genome. “It allows us to know of all the genes in the nematode,” she says. Besides understanding how SCN’s genes function, it also gives scientists insight how to inhibit it and make it a less effective parasite. 

2. Discover, leverage, and enhance native nematode resistance in soybeans.

This goal aims to expand resistance beyond the PI88788 sources. “Native genetic resistance in soybeans is the primary management strategy for soybean cyst nematode and other types of nematodes,” she says. 

3. Engineer resistance using molecular tools to generate or improve nematode resistance in soybeans.

This aims at using transgenic and gene-editing technologies to develop more effective engineered resistance. 

4. Assess the impact of management practices on nematode populations.  

This emphasizes agronomic strategies including:

  • Rotating with nonhost crops like corn. Although not broadly available, soybean varieties exist with the Peking resistance source that enable farmers to diversify resistance sources without sacrificing yield. 
  • Managing other soybean diseases. 
  • Assessing the impact that cover crops have on SCN.
  • Identifying soil, root, and rhizosphere microbes that naturally control nematodes.
  • Evaluating products and practices that manage nematodes. These can include varietal performance trials and products.
  • Rotating with different sources of resistance.

“There are a number of seed treatments are now out on the market for nematode control,” adds Mitchum. “It’s very important that university researchers are focused on providing unbiased assessments of the treatments and look at their efficacy for our farmers.”

5. Conduct nematode surveys to improve diagnostics and economic impact. 

“We need to know what types of populations are out there so that we can more effectively deploy genetic resistance and other types of resistance as they become available,” says Mitchum.

This goal also aims to standardize the SCN soil sampling process in order to provide more accurate results for farmers. 

6. Foster Extension education and outreach.

"We want to make sure as researchers and Extension that we're fostering attention, education, and outreach, making sure that our farmers are aware of the nematode issues where they are growing soybeans,” she says. This information will help farmers know their nematode population levels, so management recommendations can be customized. 

This goal would also give specific nematode-management recommendations for farmers and advisers in different geographic areas that include:

  • Newly emerging SCN regions in the Dakotas, northeastern U.S., and Canada
  • Established SCN regions (north-central states and Ontario)
  • Mixed nematode regions in the southern U.S.

Breakthroughs

Researchers have identified the major genes that control the majority of SCN resistance in major soybean cultivars: Rhg1 and Rhg4. 

“Finding those genes and knowing what they are has opened up a huge amount of information that we didn't know and understand,” says Mitchum. “We know what the genes are doing; we are starting to understand how they are functioning, and it’s allowed us to place markers on those genes that we've been able to get to our breeders so they can streamline the breeding process.” 

University of Illinois researchers also have also identified two genes from wild soybean—Glycine soja—that they have stacked with the PI88788 resistance source into high-yielding varieties. 

“This is a major breakthrough,” she says. “It not only is identifying novel resistance, but also is bringing it in and stacking it with current sources of resistance.”

These findings can be used by seed companies to incorporate new resistance sources into their soybean varieties to get to farmers. 

“It’s now in the hands of the seed companies,” says Tylka. “They are the folks who deliver new sources of resistance and new varieties to farmers. We hope that happens and it happens quickly.”

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