‘We are losing our grip on SCN’

September 23, 2021 | Kriss Nelson

Producers have let their guard down when it comes to combating Soybean Cyst Nematode (SCN).

“I don’t know how many farmers out there have tested their fields at all in the last 10 or 20 years because resistant varieties were working and now they are not,” said Greg Tylka, Morrill Professor and Director of the Iowa Soybean Research Center at Iowa State University.

About SCN

Tylka said there are two main soil factors that appear to universally affect SCN.

“SCN reproduces much, much better in high pH soils and when we get those Clarion-Nicollet-Webster soils, particularly, there can be some high pH and SCN just loves it,” Tylka said.

The second factor attributed to helping SCN thrive is hot and dry soils.

“Obviously, we can associate the words hot and dry soil with the 2021 growing season. I am curious and nervous about what the end of the season SCN numbers are going to be in farmers’ fields and our research plots,” he said.

Soil sample for SCN

Tylka said he is hopeful farmers can get back into the mindset of soil sampling for SCN.

“We are losing our grip on SCN because resistant varieties are starting to fail,” he said. “I don’t sense that many fields have been sampled in recent years. We just need farmers to get those fields sampled to know what the situation is.”

That time is now.

“Fall, after harvest, is the absolute perfect time to do it,” Tylka added.

Farmers are encouraged to not only soil sample fields where soybeans were recently harvested, but to test soils after corn in those acres where soybeans will be planted in 2022.

“If they sample fields that had soybeans this growing season, they can look back and see if they got 45-bushel beans when they were expecting 60, and if they have 10,000 nematodes per half a cup of soil, that might explain it,” said Tylka.

That method allows producers to look back on the growing season. Sampling corn fields will allow them to look forward.

“I would like every farmer in the state to get into the habit of sampling for SCN in the fall every third time a field is going to soybeans,” he said.

These fall soil sample results will give producers plenty of time to research and discuss some potential strategies before deciding if they should even plant soybeans next spring.

“There is a level at which Iowa State recommends to not even raise soybeans because resistant soybeans are still going to get hit hard if the egg counts are high,” said Tylka. That number is 12,000 eggs per half-cup or 100 cc of soil.

How to sample

To sample, probe six to eight inches deep into the soil, zig-zagging your way through a field, gathering 15 to 20 cores for every 20 acres. The soil cores should be combined in a bucket, mixed up and put into a soil sample bag.

Soil samples can be sent to the Iowa State University Plant and Insect Diagnostic Clinic to be processed. The address to deliver or send samples to is Plant and Insect Diagnostic Clinic, Iowa State University, 2445 ATRB, 2213 Pammel Drive, Ames, Iowa 50011-1101. For more information, call 515-294-0581 or visit http://clinic.ipm.iastate.edu.

In addition to the university’s lab, there are private soil labs that can conduct tests for SCN on soil samples as well. A map of private soil testing laboratories is available on the SCN Coalition webpage.

How to manage SCN

A free publication is available to help farmers and agronomists interpret their SCN soil sample results and can be downloaded at https://store.extension.iastate.edu/Product/ipm61-pdf

Once a producer receives their results and discovers they have an SCN problem, what can they do to help combat the disease?

Tylka said if the numbers are drastically high (>12,000 eggs per half-cup of soil), a producer may have to opt out of growing soybeans for a few years. This could help drop SCN numbers anywhere from 5% to 50% in the first year of corn, with additional years on those acres dropping it further.

Keep in mind, growing continuous corn not only comes with its own risks of potential soil fertility and pest problems; dormant SCN eggs will live in the soil for several years.

“That is one of the reasons SCN is such a tough critter to deal with,” said Tylka. “Not only does it reproduce quickly at 250 eggs per female and several generations of females produced every growing season, the problem with those eggs is that up to half of them will be dormant and will not hatch for up to eight years down the road.”

If a farmer decides to go ahead and plant soybeans, there are two types of resistant varieties available: varieties with Peking resistance, which Tylka said has been performing better than those with PI 88788 resistance. Besides those two, farmers are left with little options.

“There is no other widely available source of resistance,” he said. “It is really just those two as it has been for 20 years.”

Seed treatments are an option with several different products that are available providing different modes of action.

Thanks to funding the Iowa Soybean Association (ISA), field research through Iowa State has been done on the effectiveness of seed treatments on SCN. The results so far, however, have been inconsistent.

“We see some seed treatments increase yields in some SCN-infested fields but then not in other fields,” Tylka said. “We test several different seed treatments every year in nine different experimental locations and have not had consistent results.”

Tylka encourages farmers to use the resistant varieties and experiment with seed treatments on their own.

“I can’t possibly do enough experiments to generate information that is applicable to all of the farmers in the state, but the farmers can do some experimenting themselves – with yield monitors. Just try some things out and be observant,” he said. “Most farmers I know would make good scientists because they are keen observers.”

Effects of SCN

SCN has evolved over time to be in sync with the soybean crop.

“The nematode population grows with the crop. It builds up numbers during the season as the crop is growing,” said Tylka. “It is in the soil hanging tight not doing anything until the soil temperatures are about at the level farmers are going to start planting soybeans.”

This adaptation allows SCN to thrive.

“Imagine if you are this nematode worm that lives in the soil and your only source of food is the roots. If too many of you feed on the roots and kill the plant in a matter of four to six weeks, you have just lost your food source for the rest of the growing season,” he said.

This is the reason maximum numbers of SCN will be detected in the soil at the end of the season, making it the ideal time to soil sample.

Economic impact of SCN

Every year extension plant pathologists from across the United States and parts of Canada provide yield loss estimates from soybean diseases. For the last 20 years, SCN has been the No. 1 yield-reducing pest from that study.

According to data provided by Tylka, recent analyses by scientists at Penn State University using multiple statistical approaches estimated yield losses caused by SCN in the United States to total nearly $32 billion from 1996 through 2016, which is more than $1.5 billion annually.

SCN Coalition

A nation-wide effort to help educate farmers and agronomists better manage SCN has been implemented.

The SCN Coalition, which is funded by the Soybean Checkoff can be found at www.thescncoalition.com  The coalition is encouraging growers to “Know Your Number” and actively manage SCN, with a goal to decrease SCN populations and increase yield potential.

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