The only people who grow soybeans in the U.S. not worried about soybean cyst nematode farm in West Virginia or Brown County, Ind. SCN is documented everywhere else. Even those growers might keep one eye open!
“Soybean cyst nematode continues to spread, with yield losses increasing every year,” explains Greg Tylka, an Extension specialist and researcher at Iowa State University.
Thirty-one new counties confirmed SCN across the country from 2020 through 2023. “SCN sometimes flies under the radar because infested fields don’t always show stunting or yellowing,” Tylka says. “The only symptom may be lower yields, often blamed on other things.”
Truth about SCN
Here are key facts about SCN:
Without food, nematodes die. Rotating to a nonhost crop, usually corn, helps because when eggs hatch into juveniles and find corn roots, they starve and die, Tylka says. “We see a 5% to 50% decrease in egg numbers after one year in corn,” he notes. “That decrease should mean lower yield loss in future years.”
Decreases are much less for second- or third-year corn. Some eggs remain dormant until soybeans come back. No nonhost crop is better at reducing egg numbers than any other nonhost crop.
Resistant soybeans perform two key functions. Soybean varieties with SCN resistance help mitigate potential yield loss and reduce SCN egg counts. “Reducing egg counts helps avoid yield losses in future years,” Tylka explains.
Not all SCN-resistant varieties are equal. Not being equal happens two ways — in not producing the same amount of yield protection, and in not reducing SCN population numbers as much. Both factors are important, Tylka says.
Two main types of resistance exist. There are some 900 soybean varieties available in maturity groups 0, 1,2 and 3, and 700 have PI 88788 resistance. About 200 use Peking resistance. At one location in 2019, Tylka compared two Peking varieties, 67 PI 88788 varieties and three susceptible varieties. Peking outyielded PI 88788 on average by 21.2 bushels per acre. At the same time, SCN egg counts at the end of the season for PI 88788, Peking and susceptible varieties, per 100 cubic centimeters of soil, were 14,431; 800; and 13,600, respectively.
Peking doesn’t always yield more. Consider results from the Lauren, Iowa, site in 2023. Seven of the eight top-yielding varieties used Peking vs. PI 88788. Three of the bottom dozen varieties were also Peking. However, every Peking variety showed almost no SCN reproduction on roots. Average reproduction on PI 88788 varieties was 60%.
Top 5 management tips
Here are key management tips Tylka recommends for minimizing SCN yield losses:
1. Include Peking varieties. Some 200 Peking varieties are available now. Begin growing them if you have not before, Tylka says.
2. Never use Peking resistance two years in a row. Tylka and others fear that if growers abuse Peking’s effectiveness, resistance will develop quickly. “We anticipate that SCN could develop resistance much faster to Peking than it did with PI 88788,” Tylka says.
3. Alternate Peking and PI 88788 resistance. Switch back and forth in years when the field is in soybeans.
4. Rotate with corn. A corn-soybean rotation helps keep SCN numbers down.
5. Use seed treatments. Tylka’s trials over time with Ilevo and Saltro show additional benefits when using seed treatments effective against SCN.