The good news is that you can buy cyst nematode-resistant soybeans if you need them. The bad news is that the primary source of resistance used today isn’t as effective as it once was. Soybean cyst nematodes are breaking this form of resistance.
Jamal Faghihi, the leading researcher on SCN at Purdue University, says this is not an easy subject to understand, but it’s worth spending time to master the basics. How well your soybeans yield this year may depend on what type of varieties you place where.
The major source of SCN resistance today is PI 88788, Faghihi says. He estimates that 96% of SCN-resistant soybeans use this type of resistance. The other primary source of SCN resistance is called Peking.
For the past several years, Faghihi and others have noticed that some varieties containing PI 88788 resistance are no longer as effective at controlling SCN. Yields may still be better than if you planted a susceptible variety, but relatively speaking, yields may not be as good as they once were. Nematodes are getting a foothold, and when weather conditions favor SCN, yield potential is affected.
There is still a wide variation in how effective varieties with this source of resistance are today, Faghihi says. Nematologists can rate them for effectiveness, but there’s no foolproof system.
One option is to switch to varieties with Peking resistance, he notes. Obviously, since it’s not used extensively, there’s a much narrower selection of varieties.
What you can do
If you suspect that resistant soybean varieties are not as effective as they used to be, the first step is to investigate further. You have two options: You can take a soil sample and have it analyzed for common races of SCN. Or you can wait until midseason, dig roots on varieties that are supposed to be resistant, and check for new white cysts.
If you soil sample, the test to determine if nematodes are present is inexpensive, at $10 per sample; however, the test to determine which races are present is costly. That’s because the only way to determine races today is to grow soybeans in the soil sample and judge results based on soybean growth and cyst development. It can take weeks to run, and each test cost $100.
“If you’re going to do it, it might be a test you only do once in a field,” Faghihi says. “Once you know what races are present, you have information that can help you make better management decisions.”
The other option is to dig up plants midseason. “You’ll be able to see cysts, which are sacs of eggs,” Faghihi says. "They’re lemon-shaped. Don’t confuse them with beneficial nitrogen nodules. If you dig and find a considerable number of cysts on a variety that’s supposed to be resistant, you know resistance is breaking down.”
If you opt for a variety with Peking resistance, Faghihi suggests coming back with PI 88788 resistance the next time the field is in soybeans.