AMES, Iowa — An Iowa State University plant pathologist says Midwest farmers are slowly losing the battle against soybean cyst nematodes (SCN) with a common tool used to manage the destructive soybean pest it’s been increasingly resisting for nearly two decades.

“I have observed a slow but steady decrease in effectiveness of PI 88788 resistance against the soybean cyst nematode in Iowa over the past 15 years, and it is becoming a serious concern,” said Greg Tylka, ISU professor of plant pathology and microbiology, an internationally-recognized SCN expert.

According to Tylka, PI 88788 is a common source of genetic resistance to the SCN that was introduced into many soybean varieties to counter the pest, but now is costing U.S. soybean producers an estimated $1 billion-plus each year in lost yields.

“Midwest soybean farmers desperately need another type or source of resistance against this widespread pest,” he added.

Clarke McGrath, a former ISU extension field agronomist who was recently named on-farm research and extension coordinator at its new Iowa Soybean Research Center, said Iowa has been fighting the SCN for more than 35 years.

Widely considered to be the most damaging pathogen of soybeans in Iowa, he said surveys of the state – funded by the soybean checkoff and conducted in the mid-1990s and again in the mid-2000s – indicate SCN is likely present in 75 percent or more of Iowa fields. McGrath added even if Midwest farmers plant SCN-resistant soybeans, they should stay vigilant.

“Unfortunately, much like other pests we fight, SCN is adapting quickly and the effectiveness of our favorite SCN-resistance gene is eroding,” he explained. “For several decades, the PI 88788 source of SCN resistance has produced soybean varieties with greater yields than varieties with SCN-resistance genes from other sources, such as Peking.

“Many soybean varieties with PI 88788 SCN resistance have been developed over the years, and these varieties have allowed Iowa soybean farmers to continue to produce soybeans profitably in the state for 20-plus years.”

But McGrath said repeatedly using the SCN resistance genes from PI 88788 for two decades or more has resulted in selection of SCN populations with increased reproduction on plants with PI 88788 resistance.

“And elevated SCN reproduction on resistant soybean varieties leads to SCN numbers ‘creeping up,’ and soybean yields heading on a slow and steady decline,” he said. “It is estimated currently that more than half of the SCN populations in Iowa have an increased level of reproduction on soybean varieties with PI 88788 SCN resistance.”

McGrath said if it looks like PI 88788 SCN resistance is not effective, he encouraged farmers not to “give up” on soybeans. “Iowa State University has the nation’s largest and most comprehensive field evaluation program for SCN-resistant soybean varieties,” he said. “Farmers battling SCN should continue to use resistant soybean varieties with PI 88788 resistance.

“Many of these varieties still offer good SCN management and profitable soybean yields. But not all SCN-resistant soybean varieties are equally effective. So, some effort may be needed to select effective SCN-resistant soybean varieties.”

Tylka was one of a group of nematologists from eight Midwest states and Ontario that discussed SCN at an early July meeting of the North Central Committee on Practical Management of Nematodes on Corn, Soybeans and Other Crops of Regional Importance (NC1197) in Chaska, Minn. He said the committee reviews and coordinates ongoing research on managing nematode parasites of crops, with special emphasis on corn and soybeans.

Tylka said the committee concluded a coordinated approach using multiple management options – such as alternating soybeans with non-host crops, planting SCN-resistant soybean varieties and using nematode-protectant seed treatments – provide the greatest likelihood of sustained success for producing soybeans profitably in SCN-infested fields.

Tylka added the committee also planned coordinated research projects for upcoming years, including work on nematode-resistant varieties, non-host crops, seed treatments, new nematode detection methods and soil health. What’s more, McGrath said seed companies can also be good sources of information about which SCN-resistant varieties offer the greatest nematode protection, as well as yield potential.

“In addition to selecting good SCN-resistant varieties to grow, farmers battling SCN should seek out resistant varieties with sources of SCN resistance other than PI 88788,” he said. “Finding these varieties might be easier said than done, but try to plant some, if at all possible.”

He said growing corn is another SCN management strategy to keep SCN in check. “SCN numbers may drop as much as 50 percent in a single growing season with corn, but there is much less of a decline in SCN numbers in second-year corn and the effect becomes even less effective after that.”

“Finally, there are several seed-applied SCN management products that are available for soybeans now,” he added, “and my bet is that more are in the pipeline.”