AMES, Iowa – Greg Tylka, Iowa State University nematologist, has found that soybean cyst nematode (SCN) resistant soybean varieties are becoming less effective.
“This is an alarming trend and sets the stage for even greater yield loss from SCN in the future,” Tylka said.
Resistant varieties have helped farmers manage the nematodes for decades. Almost all SCN-resistant soybean varieties possess the same resistance genes, from a soybean breeding line called PI88788.
Recently, Iowa State researchers analyzed 25 years of data, from tens of thousands of four-row variety evaluation research plots, to look for long-term trends. The results, published in an article titled “Increase in Soybean Cyst Nematode Virulence and Reproduction on Resistant Soybean Varieties in Iowa from 2001 to 2015 and the Effects on Soybean Yields” in the scientific journal Plant Health Progress, showed a breakdown of resistance in SCN-resistant varieties. In recent years, the work has been supported in part by soybean checkoff funds from the Iowa Soybean Association.
“In the 1990s, SCN was well controlled with PI88788 resistance. Starting in 2001, we saw a steady decrease in SCN control of varieties with PI88788 resistance,” Tylka said.
The analysis indicates increasing damage from SCN to soybean resistant varieties. In some instances, resistant varieties allowed as much SCN feeding as susceptible varieties with no resistance. This trend will result in an increase of SCN, which means lower yields.
The findings are similar to current trends in weeds that have developed resistance to herbicides.
“The buildup of SCN on resistant soybean varieties is much like weeds developing resistance to glyphosate due to prolonged use of that single herbicide's active ingredient,” Tylka said.
Tylka and his co-authors concluded that SCN numbers will continue to increase and soybean yields will continue to decrease if PI88788 is the only resistance farmers have available to use. One solution is to develop soybean varieties with resistance genes from other breeding lines.
“This is a serious situation,” Tylka said, “because SCN has infested almost 70 percent of Iowa’s fields.”
The article concludes that long-term management of SCN requires a multi-faceted and integrated approach. Iowa State is continuing to study use of resistant soybean varieties, seed treatments and whether cover crops can reduce SCN numbers.
Those interested in learning more about the breakdown of SCN resistance and also insecticide-resistant soybean aphids can attend an all-day workshop at Iowa State’s Field Extension Education Lab on Aug. 17.