Take the Test to Control SCN

Call it farmer's intuition, a hunch, a gut feeling. Zach Grossman knew something was going on in one of his soybean fields, even if he couldn't see it.

"It just didn't seem like I was getting the same performance in that portion of the field as I was on the whole field," says the farmer from Tina, Missouri. "Everything's the same -- soil type, fertility, pH -- so that's why I chose to test that area for the nematodes."

In 2023, Grossman, along with Chandra and Mike Langseth, of Barney, North Dakota, participated in DTN/Progressive Farmer's "View From the Cab" series, providing a weekly perspective from their farms throughout the growing season. After harvest, they agreed to sample a field for soybean cyst nematode (SCN) and share their results as part of a project sponsored by The SCN Coalition, a public/private/checkoff partnership formed to encourage growers to actively manage a pest estimated to cost farmers $1.5 billion annually.

For years, the coalition has encouraged growers to "know their numbers," taking soil samples to determine the number of SCN eggs present in a particular field.

"But, egg counts don't tell the whole story," says Horacio Lopez-Nicora, a soybean pathologist and nematologist at Ohio State University. "Yes, farmers should know their egg count numbers, but without knowing the type of soybean cyst nematode in the field, you're missing the information you really need to manage the issue."

SCN 'TYPE' CASTING

While the egg count test estimates the SCN population density in a particular field, it doesn't tell a grower how vulnerable a particular soybean variety might be to yield-robbing damage. For that, an HG type test is required. The "HG" stands for "Heterodera glycines," the scientific name for SCN.

"The HG type test measures the ability of a specific nematode population to reproduce on a set of soybean lines containing different genes or combinations of genes conferring resistance to SCN," says Jeff Barizon, senior research specialist at SCN Diagnostics at the University of Missouri. "These lines, or plant introductions, include PI 88788, which is the most common source of resistance found in today's varieties."

Performing the HG type test requires at least 40,000 SCN eggs, so a grower is asked to submit a 1-gallon soil sample. If an insufficient number of SCN eggs is present in the sample, the SCN population is increased by one or more life cycles on susceptible soybeans in the greenhouse. A life cycle requires about 30 days.

Once enough SCN eggs are available to begin, soybeans are planted in separate containers of sterilized sandy soil. In the HG type test conducted by SCN Diagnostics, the soybeans are inoculated with 1,000 eggs. Four seedlings represent Lee74, a soybean variety susceptible to SCN. Four seedlings also represent four other plant introductions with SCN resistance found in commercial soybeans. These include PI 88788, PI 548402 (Peking), PI 90763 and PI 437654.

The containers of inoculated soybeans are suspended in greenhouse tables filled with water and set to about 80°F. This maintains the optimum soil temperature for SCN growth. After 30 to 35 days, the plants are removed from their containers, and the SCN females (the cysts) are removed from the roots of each plant and counted.

"Using these numbers, we calculate the Female Index (FI)," Barizon explains. "We divide the average SCN reproduction on each soybean line by the average from Lee74, the susceptible line, then multiply by 100 to get a percentage. The higher the FI, the better that SCN population reproduces on that source of resistance. Anything above 10%, and that population is considered able to reproduce."

SCN exceeding the 10% threshold for a source of resistance receives a number designation. For example, SCN with an HG Type 1.2 can reproduce above 10% on Peking and PI 88788. An HG Type 2 is only above 10% on PI 88788.

ROTATE RESISTANCE

Repeated use of the same genetic sources of resistance exerts selection pressure on SCN populations, resulting in survival of individuals who can then reproduce on that source. Over time, a once-effective tool no longer controls the pest. Lopez-Nicora compares the breakdown of SCN resistance sources to the development of herbicide resistance in weeds.

"If I ask growers, 'What happens when we use herbicides with the same active ingredient year after year to control weeds?', they know the answer," he says. "The same thing is happening with SCN resistance in soybeans. We've been using PI 88788 so much for 30-plus years that it's now ineffective at controlling many nematode populations."

The HG type test results from the samples from the Grossman and Langseth farms confirmed the nematologist's concern. In Missouri, Grossman's test revealed that while his SCN egg count was considered low at 100 eggs per 100 cubic centimeters of soil, the nematodes present in the field had an FI of 80.7% on PI 88788, meaning that SCN control was less than 20%. For every five nematodes that developed on a susceptible soybean, four of them could also develop on Grossman's soybeans.

"It made me feel good that the overall eggs were low, but the variety I planted had PI 88788, so we've definitely got to be proactive on it," he says. "I'll be honest. SCN wasn't something that I had considered at all when selecting seed varieties.

"I've tried to pick my varieties kind of the same way that I pick out bulls," Grossman adds. "You want a balanced bull, and you want a balanced bean that's good across the board. Moving forward, SCN will need to be part of that."

In North Dakota, the Langseths also have planted soybeans with PI 88788 exclusively since the early 2000s. While their egg count was moderate at 800 eggs per 100 cubic centimeters of soil, the HG type test revealed an FI of 67.8% on PI 88788.

"I guess it shouldn't be surprising," Chandra Langseth says. "It explains a little bit of that yield drag we've seen during the past few years. I ran the numbers through The SCN Coalition's Profit Checker, and it estimated we lost more than $90 an acre because of SCN. It's definitely a wakeup call."

Both Grossman and the Langseths say that moving forward, they'll be looking for varieties with a different SCN resistance source, namely Peking. While the number of commercial varieties with Peking is still relatively small when compared to PI 88788, Lopez-Nicora says growers should be wary of planting it exclusively.

"If they do, we'll just shift the nematode virulence again," he says. "We need to think very wisely about how to use these tools and protect them. Rotation is key. Rotate from soybean to a non-host crop. Rotate soybean varieties. Rotate the source of SCN resistance. Don't feed these animals the same genetic material.

"Rotate. Rotate. Rotate."

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-- Take the test to find your number at The SCN Coalition: https://www.thescncoalition.com/…

-- Follow Jason on social platform X @JasonJenkinsDTN

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