Over the years, continued planting of soybean varieties with PI 88788 resistance has caused soybean cyst nematode to develop resistance to SCN-resistant soybean varieties. There are hundreds of SCN resistant varieties available, but almost all of them have resistance genes from the PI 88788 breeding line. Fewer than 5% of SCN-resistant soybean varieties available for farmers to plant have the alternative source of resistance named Peking.
There are yield consequences to this situation, similar to using the same herbicide on weeds continually for a number of years. Over time, the weeds develop resistance and are more difficult to control. For decades, farmers have planted SCN-resistant soybean varieties to manage SCN using the PI 88788 trait each time.
Yield trial comparisons
Iowa State University’s annual soybean cyst nematode yield trials show what’s happening. ISU Extension nematologist Greg Tylka and his team conduct these tests, supported by soybean checkoff funds from the Iowa Soybean Association. The trials are conducted at nine different locations in Iowa. Results of the 2019 tests at the southeast Iowa site were especially dramatic and alarming. The data illustrate what likely could occur in SCN-infested fields throughout the state in future years.
There were 67 varieties with PI 88788 and two varieties with the alternative, hard-to-find Peking resistance in the southeast trial. This trial also had three SCN susceptible varieties for comparison. The susceptible soybean varieties in this test had an average yield of 41.8 bushels per acre. Yields of varieties with PI 88788 SCN resistance ranged from 41.0 to 58.7 bushels per acre and averaged 51.2 bushels. The highest-yielding variety with the PI 88788 trait produced 58.7 bushels per acre. The two varieties with Peking resistance yielded 71.3 and 73.5, averaging 72.4 bushels per acre.
Full results from this trial and the eight similar trials conducted in 2019 are available online here.
The farmer who hosted the SCN variety trial in southeast Iowa grew two different soybean varieties with PI 88788 SCN resistance in the remainder of the field surrounding the experiment, and his two varieties yielded 50 bushels per acre. If one of the varieties with Peking SCN resistance in the trial had been grown in the field, the farmer would have earned nearly $200 per acre more (based on 22 bushels per acre yield difference between the Peking varieties and the PI 88788 varieties grown by the host farmer and $9 per bushel soybeans).
“These results were very consistent and statistically significant,” Tylka says. “And they illustrate the staggering amount of yield loss that can be caused by SCN when resistance from PI 88788 is overcome.”
The soil in the area of this southeast Iowa field where the test plots were located had a moderate SCN egg count at the beginning of the 2019 season. The number of eggs in the soil at the end of the growing season for the three susceptible soybean varieties averaged three times more. The end-of-season SCN populations for the PI 88788 varieties were all higher than at the beginning. In contrast, there were a lot fewer SCN eggs in the soil at the end of the growing season with the two Peking SCN-resistant soybean varieties.
Evaluating the results of all nine of the SCN soybean variety trials around the state in 2019, Tylka says, “What we are seeing every year, more and more, is the soybean varieties with Peking resistance are yielding higher and higher in the overall rankings relative to the varieties with PI 88788 resistance. That has as much or more to do with the nematode becoming resistant to the PI 88788 SCN resistance as it does with the genetics of the soybean varieties.”
Problem must be addressed
Many fields in Iowa are similar to this southeast Iowa field, Tylka says. The only difference between the field where the yield trial was conducted and most SCN-infested fields in Iowa is that this southeast Iowa field had well-drained, sandy soil. And the dry soil conditions that occur in this texture of soil favor SCN reproduction and yield reduction.
Very large differences in yields of soybean varieties with PI 88788 and Peking SCN resistance may not commonly occur in fields with medium- and heavy-textured soils in Iowa — at least not yet. But results like this that occurred in the 2019 trial are possible in any SCN-infested field with any soil texture, especially under dry growing conditions.
Most soybean varieties have PI 88788 SCN resistance, and it’s inevitable that yields of these varieties will continue to decrease in fields infested with SCN because reproduction of nematode populations on PI 88788 will steadily increase. The trend will continue until soybean varieties with Peking resistance are commonly and widely grown in rotation with PI 88788 resistance in SCN-infested fields, which is not possible to do currently because very few soybean varieties with Peking resistance are available.
Farmers must seek out soybean varieties with Peking SCN resistance to grow in rotation with varieties that have PI 88788 SCN resistance. Tylka advises farmers to persistently request soybean varieties with Peking resistance from seed providers to grow in rotation with PI 88788 resistance.
“New soybean varieties with Peking resistance likely will not become plentiful until farmers express the need,” he adds. “I don’t think a seed company is going to react to my message, but hopefully they will react to their customers.”
McGrath is the on-farm research and Extension coordinator for the Iowa Soybean Research Center at ISU.