Soybeans: Keep SCN Numbers in Mind When Selecting Seed – DTN

    Soybean cyst nematodes under microscope magnification. Photo: University of Arkansas

    Soybean cyst nematode is a hidden soybean killer. The parasitic pest can strip soybean yield with nary an above ground symptom.

    The good news is farmers who have long relied on soybean cyst nematode (SCN) resistance called PI 88788 have alternative options. More seed companies are integrating Peking resistance into their lineups and PI 89772 varieties from Syngenta became available on limited acres in 2021. Even better, the yield penalty once associated with newer alternatives has disappeared, according to Iowa State University research here.

    Just as you switch up herbicides or insecticides to avoid overuse, using different sources of SCN resistance keeps nematodes guessing. It may sound simple, but University of Georgia molecular nematologist Melissa Mitchum said there are differences between SCN resistance varieties that should be considered as farmers develop strategies for 2022 and beyond. Understanding those differences is also important to keeping all available tools working.

    “Most soybean growers are familiar with PI 88788 and Peking, the two most widely used sources of resistance to soybean cyst nematode,” Mitchum said in a news release from The SCN Coalition. “What might be news to growers is these different sources of SCN resistance have different resistance genes — also known as different modes of action.”

    Resistance from the PI 88788 line contains one gene, Rhg1. Resistance from Peking contains two resistance genes, Rhg1 and Rhg4. “There are also different flavors — aka alleles — of the Rhg1 genes, which is where the A and B designations come into play,” Mitchum added. “PI 88788 has Rhg1b, and Peking has Rhg1a plus Rhg4. With Peking you need both genes to confer resistance.”


    If that alphabet soup isn’t enough, there’s another wrinkle when it comes to resistance genes: the number of times that gene is repeated in an SCN-resistant soybean variety. “Not only do you have different alleles at Rhg1, but there are also differences in the copy number of the genes,” Mitchum explained.

    In other words, there could be varieties from PI 88788 that have slight differences in copy number. Therefore, if growers can only get soybeans with PI 88788 resistance, The SCN Coalition recommends rotating different varieties of soybeans with PI 88788 because not all PI 88788 varieties are the same.

    While copy numbers are part of the equation, Mitchum said how the genes interact is also important. “For example, we’ve shown that Rhg4 from Peking doesn’t function on its own. Rhg4 also doesn’t talk to Rhg1b from PI 88788. Combining them doesn’t do anything,” she said. “But pyramiding two resistance genes from wild soybean (Glycine soja) onto Rhg1b does have an effect on combating virulent nematodes. Virulence is the ability of SCN to reproduce on a resistant soybean plant.

    “Ideally, we’d like growers to rotate among several different modes of action of SCN resistance,” she adds. “That’s what we’re working to provide.”


    There are additional resistance genes that haven’t been used yet in commercial soybean varieties, Mitchum noted. “We’ve only utilized Rhg1 and Rhg4, and that’s what you see in growers’ fields, but we can breed with other sources to introduce other resistance genes.”

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    Mitchum is working closely with soybean breeders to investigate other sources of resistance. “We’ve already found new genes and gene combinations that are different from Rhg1b in PI 88788 and the Rhg1a/Rhg4 combination in Peking to help fight back against virulent SCN.”

    For several years, checkoff-funded researchers have been working to identify novel types of nematode resistance. Advances in technology, such as the ability to clone resistance genes and the development of precise molecular markers, have allowed university soybean breeders to speed up the process of getting resistant cultivars out to commercial breeders.

    “Today, we can quickly test more soybean germplasm for nematode resistance,” Mitchum explained. “We’re also looking at which genes we should combine, which genes we shouldn’t, and the best rotation strategies for different modes of action.”

    The goal is to get more SCN-resistant modes of action on the market for farmers and protect existing SCN resistance sources. “We want to keep PI 88788 in the toolbox and offer growers other options to protect their soybean yields,” Mitchum said.

    To learn more about the checkoff-funded research that’s focused on bringing new tools to soybean growers in the fight against parasitic nematodes, watch “Let’s Talk Todes” videos here.

    For more from DTN on this subject:

    Pamela Smith can be reached at

    Follow her on Twitter @PamSmithDTN

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