Soybean growers are losing ground to a nematode nemesis as traditional sources of resistance
Every season, Missouri’s soybean producers are prepared to combat pests and diseases. From planting through harvest, they monitor their fields diligently. Should they see an issue, growers take action to limit losses to yield or seed quality.
But this strategy only works when they can see their adversary. And soybean cyst nematode (SCN) is different. Like a thief in the night, this pathogen that feeds on the roots of soybeans and other legumes can seemingly slip into a field and go undetected. Though no above-ground symptoms may be visible, SCN eventually announces its presence on the yield monitor, where losses of 15% to 30% or more can be realized.
Today, SCN is the most economically damaging pathogen for soybeans in North America. It’s estimated that SCN causes more than $1.5 billion in damage annually, according to the SCN Coalition, a group of public universities, checkoff organizations and corporate partners working to raise awareness and combat the pest.
Traditionally, soybean growers have battled this subterranean foe by planting resistant cultivars that reduce SCN’s ability to develop and complete its life cycle. But after decades of using the same source of resistance, the system is breaking down.
“I think a real argument can be made that we currently do not have sufficient SCN resistance in modern soybean cultivars,” says Andrew Scaboo, an assistant professor at the University of Missouri who leads the Northern Missouri Soybean Breeding Program.
Battle Plan Breakdown
It’s been nearly 70 years since SCN was first detected in the United States. The parasitic roundworm was found in North Carolina in 1954; its presence in Missouri was noted two years later.
“Generally, where you have soybean in the U.S., you have SCN present,” says Mandy Bish, interim director of SCN Diagnostics at the University of Missouri. “It’s found in all soybean producing counties in Missouri.”
Following SCN’s discovery, soybean breeders went to work developing resistant cultivars. In 1968, the first set of SCN-resistant soybeans were released, offering a weapon in the war against this potentially devastating pathogen. More than 95% of commercial soybean varieties today derive their SCN resistance from a single breeding line called PI 88788.
But a multi-state survey in 2016 found that 100% of SCN populations in Missouri show elevated reproduction on soybeans with PI 88788-type resistance.
“As we’ve learned from herbicide resistant weeds, if you use the same method of control over and over again, the pathogen is going to develop resistance, and that’s what we’re seeing now,” Bish says. “It’s providing some protection, but we’re losing ground on that protection.”
Other resistant lines have been identified, including one known as Peking. However, few commercial varieties using Peking-type resistance have been bred for soybean maturity groups suitable in most of Missouri.
As the leader of the Northern Missouri Soybean Breeding Program, Scaboo and his team are working to identify novel genes that control SCN resistance and introduce them into cultivars for Missouri growers. It’s been known that one specific gene, called rhg1-b, confers SCN resistance in PI 88788, and that two other genes, rhg1-a and Rhg4, do the same for Peking. Recently, it was discovered that a third gene, Rhg2, in Peking provides even greater protection against the most virulent nematodes.
“True Peking-type resistance is a three-gene model, not a two-gene model,” Scaboo says. “It’s the interaction of these genes that gives you that resistance to the types of virulent nematodes that are found in Missouri.
“We hope to have at least one three-gene Peking-type variety released in the next one to two years,” he adds. “Five years from now, we will predominantly release varieties that have these two-and three-gene combos.”
Five years from now, we will predominantly release
varieties that have these two- and three-gene
While new varieties with stronger resistance are in development, growers need to actively manage their fields for SCN. That process begins with a soil test, Bish says.
“It’s best to take samples in the fall right after soybean harvest because that is when egg counts will be highest,” she says. “When sampling a larger field, break it into 10- to 20-acre blocks. In each block, use a soil probe to take 15 to 20 subsamples that are 1 inch in diameter and 8 inches deep. More is always better. Combine them to create one pint-sized composite.”
Sampling in a zig-zag pattern helps ensure that the soil collected is representative of the block. Currently, thanks to support from soybean checkoff funds, the diagnostic lab is able to offer SCN egg counts on four soil samples at no cost to Missouri growers.
“Just note ‘free farmer sample’ on your submission form,” Bish adds.
By themselves, the microscopic nematodes can only move inches through the soil. They are more likely to be spread by anything that moves soil, including wind, water and machinery. Bish recommends that when feasible, fields with known SCN populations should be worked last to prevent spreading the nematodes to other fields. Power-washing planting and tillage equipment between fields also reduces the risk of transferring infected soil.
Rotating the SCN-resistant soybean variety that’s planted in a field can also help keep nematodes in check. Though most commercial varieties use PI 88788 for resistance, they do vary in the number of copies of the rhg1-b gene they contain.
Another consideration is the use of a seed treatment such as Saltro or ILEVO. In a Missouri Strip Trial study, for example, soybeans treated with ILEVO averaged 2 bushels per acre more than untreated soybeans. At locations where sudden death syndrome was observed, the advantage increased to 3.5 bushels per acre. While the cysts that contain the nematode’s eggs can persist in the soil for years, SCN is parasitic and requires a host. Planting a non-host crop in a field helps break up the SCN life cycle and reduce egg counts.
“A good rotation for Missouri would be to follow soybean with corn, then follow the corn with a non-host cover crop such as cereal rye before coming back to soybean,” Bish says. “You’re not going to eradicate SCN from that field, but you an reduce them.”
Missouri soybean growers interested in receiving free SCN egg counts on four soil samples can download the sample form at www.scndiagnostics.com or call 573-884-9118 for more information.