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Plant breeders looking for better ways to improve SCN resistance


BLOOMINGTON, Ill. — Soybean breeders have been working to improve soybean cyst nematode (SCN) resistance, especially since it causes the most yield loss of any soybean pathogen in North America, with economic impact in excess of $1 billion per year, according to the SCN Coalition.

“Damage from soybean cyst nematode (SCN) is an annual issue,” Chad Kalaher, Beck’s Hybrids field agronomist for east-central Illinois, told Farm World. “Yield reductions from SCN are often difficult to quantify, since most soybean fields are not tested for SCN.”

Kalaher said when he has encountered SCN issues in the past, he usually asks for another opinion from local university experts for positive identification. “This usually involves submitting plant and soil samples to confirm. After confirming SCN is the cause of aboveground plant symptoms, I typically have a conversation with the farmer about sources of SCN resistance.

“It’s usually a process that describes how the most commonly used gene, PI88788, is in most soybean varieties planted today and that the resistance is failing to control SCN,” he added.

That’s because, like herbicide-resistant weeds, the SCN organism adapts to eventually overcome the same source of genetic resistance deployed in a field year after year. Consequently, constant use of a single source of resistance (such as the PI 88788 source) will eventually wear thin, if not improved upon or rotated with other unique sources.

Expanding the sources of SCN resistance hasn’t always been easy. Public soybean breeders have spent years working with SCN resistance breeding lines other than PI 88788, which is the source of resistance used in 95 percent of commercially available SCN-resistant varieties.

Unfortunately, breeding resistance genes from those other sources – such as PI 548402 (Peking, which involves two genes, Rhg1 and Rhg4), PI 90763, and PI 437654 (Hartwig, which was released in 1992) – into elite varieties has been challenging.

“With these unique resistance sources, you start with low-yielding backgrounds,” said Brian Diers, plant breeder at the University of Illinois Urbana-Champaign. “It takes time to breed resistance from these new sources into elite genetic backgrounds.”

He said that’s partly why the PI 88788 source of resistance has been over-used.

“It’s worked really well, and breeders have been successful at incorporating it into high-yielding varieties,” he said. “It’s been more difficult and taken longer to get yield parity with other sources of resistance, but we’re solving that problem.”

Virgil Schmitt, Iowa State University field agronomist for southeastern Iowa, said funding for improved SCN resistance “is desperately needed and is great news.

“Rotating sources of resistance to SCN is as important as changing up herbicide sites of action or insecticide or fungicide modes of action,” he added. “We are starting to see that PI 88788 isn’t doing the job as well as it used to because of its overuse.

“If PI 88788 continues to be overused, its performance will continue to decline as SCN adapts. PI 548402 (Peking), has not been popular with breeders because of the difficulty in breeding that trait, and the joke about PI 437654 (Hartwig) was that it had excellent resistance to both SNC and yield.”

For nearly 30 years, Greg Tylka has been studying SCN at ISU. Since 2001, he and his research team have been documenting steady increases in SCN reproduction on soybean varieties with PI 88788 SCN resistance in Iowa farm fields and accompanying yield decreases.

“University researchers across the country report a similar trend of increased SCN reproduction on PI 88788,” said Tylka, a professor of plant pathology and microbiology, and a nematologist who leads the SCN Coalition.

That’s why the development of these new resistant soybean lines with unique and stacked SCN resistance genes could not have come at a better time, he said. “Soybean cyst nematode populations throughout Iowa and the Midwest continue to develop resistance to the commonly-used SCN resistance genes” from PI 88788.

“Almost all SCN-resistant soybean varieties sold by seed companies possess resistance genes from PI 88788,” he added.

He said the soybean lines Diers and his colleagues are developing is novel and quite promising, “but they will help soybean farmers only if seed companies incorporate the unique resistance genes into commercial soybean varieties that will be sold to farmers.”

That’s why soybean breeders like Diers and Tylka are funded by the soybean checkoff (including United Soybean Board and the North Central Soybean Research Program): To improve and add to current genetic sources of SCN resistance and breeding them into high-yielding backgrounds.

Diers said plant breeders at several universities have been developing cultivars with the PI 437654 resistance source (Hartwig). Moreover, plant breeders have continued to breed with resistance from this source and are now obtaining good yields.

“Our program has released two high-yielding lines with PI 437654 resistance, which were commercialized by companies through licenses from the university,” he said. “George Graef, a plant breeder at the University of Nebraska-Lincoln, has recently developed top-yielding lines with PI 437654 resistance.”

He said soybean growers, particularly in the Midwest, should be able to find more soybean varieties than ever with the Peking source of resistance.

“If you can, rotate the sources of resistance you use,” he said. “We have a large amount of evidence showing that this reduces selection pressure on SCN populations to continually adapt.”

But Kalaher said once SCN is identified in the field, there is nothing a farmer can do in the current growing season to address an SCN issue. However, in some instances, the farmer may have planted a bean variety without a SCN resistance gene.

“Either way, I often recommend rotating to a non-host crop, such as corn, the following year,” he said. “Since SCN cysts can survive in the soil for up to eight years, if a soil test confirms a very high SCN count, I have recommended in the past that soybeans not be grown for several years in a field, in an attempt to help reduce SCN pressure.

“In the past, I have recommended farmers plant varieties with SCN resistance, in addition to a seed treatment labeled for control of SCN, to help reduce yield losses,” he added.

For 2020, he said Beck’s will offer soybean growers the option of adding Nemasect to their Escalate Yield Enhancement Seed Treatment system. “Nemasect is a nematicide that is safe to handle, safe for the environment, and kills soybean cyst nematodes within 24-48 hours of exposure.”