Soybean cyst nematode management in the corn belt
This is the first installment of a four-part series on soybean cyst nematode management. This article focuses specifically on the Corn Belt. Subsequent articles will address the Western Corn Belt, the South, and the Mid-Atlantic States.
Don't let out of sight be out of mind. Soybean cyst nematode (SCN) can silently rob 30 to 50% of soybean yield, according to Iowa State University nematologist and SCN expert Greg Tylka. Put another way, “SCN-resistant variety yields are often more than double the yields of non-resistant varieties,” he says.
These invisible worms can feed off soybean roots for months or years before any aboveground symptoms appear. They penetrate vascular root tissue, blocking nutrient uptake.
“My biggest fear is that 20 years from now we stop growing soybeans in the Midwest because there's no effective resistance to SCN,” Tylka says.
Preventing SCN in the heart of the Corn Belt is probably not possible anymore, he says. “We simply don't have anywhere near enough SCN-resistant varieties with the Peking source of resistance, which has become more effective than PI 88788 in many fields. If there is not a drastic change, I see yield losses of 15 to 30 bu/ac because SCN has adapted really well to the widely used PI 88788 resistance. This happened over the last 20 years due to the same phenomenon that led to development of glyphosate-resistant weeds—nature adapts to our management tools unless we change them over time.”
Choosing an effective resistant variety can make a huge difference in your yields and your SCN numbers, according to annual Iowa State University SCN-resistant variety trials. In 2019, there were 69 SCN-resistant soybean varieties and three susceptible varieties representing 18 different brands studied and compared in three SCN-resistant variety trial experiments across northern, central, and southern Iowa.
In an experiment in southeast Iowa where SCN reproduction was high, varieties with PI 88788 resistance yielded on average 51 bu/ac compared with 72 bu/ac on average for varieties with Peking SCN resistance. Also, SCN numbers decreased by 80% in plots where varieties with Peking SCN resistance was grown but increased up to 900% on individual varieties with PI 88788 resistance (see https://bit.ly/IowaSCNSTudy for further details). Put another way, SCN populations ranged from greater than 10% reproduction to 71% reproduction on PI 88788 (HG Type 2).
HG Types and SCN Races Explained
By Greg Tylka, Iowa State University nematologist
When fully effective, SCN-resistant soybean varieties will allow no more than 10% reproduction of the nematode relative to a susceptible (non-resistant) variety. There are several breeding lines or sources of resistance (e.g., Peking, PI 88788, etc.) used to develop SCN-resistant varieties. When these varieties were first grown in the U.S. and Canada, all of the breeding lines were fully effective against most all SCN populations. But over the decades, SCN populations developed higher levels of reproduction on PI 88788, a widely available and grown resistance breeding line.
The race and HG type of an SCN population shows how well an SCN population can reproduce on different sources of resistance. Stated another way, SCN race and HG type tell you which source(s) of resistance in varieties will best control the SCN population that is present in a field. More than 10% reproduction on a source of resistance is considered “elevated.”
The original SCN race test was established in 1970 and used four resistance sources to differentiate the races. The more recent HG type test was established in 2002 and uses seven sources of resistance, including three of the four sources of resistance used in the race test. There are more resistance sources with HG types than with SCN races because new resistance sources were discovered after the race test was established. Even though there are seven sources of resistance in the HG type test, it is much easier for farmers and crop advisers to understand and use HG types than races, as explained below.
With the SCN race test, there are 16 possible combinations of + (which means >10% reproduction) or – (<10% reproduction) for the four sources of resistance used in the test, so there are 16 possible races (see graphic). To know which resistance source(s) an SCN race will have elevated (>10%) reproduction on, one must consult (or memorize) the SCN race chart. Almost all (95%) SCN-resistant soybean varieties currently have resistance from the PI 88788 source of resistance, so it would be helpful to farmers and crop advisers to know if an SCN field population can reproduce well on a resistant variety with PI 88788 SCN resistance.
There are eight SCN races with elevated reproduction on PI 88788: races 1, 2, 4, 5, 7, 11, 15, and 16 (see graphic). So, be on the lookout for these races because most SCN-resistant varieties have resistance genes from PI 88788. Seven of the eight races with elevated reproduction on PI 88788 also have elevated reproduction on one or more other sources of SCN resistance (see graphic).
Each of seven resistance sources used in the HG type test is assigned a number (1 = Peking, 2 = PI 88788, 3 = PI 90763, 4 = PI 437654, etc.)—see the graphic. When an SCN population has elevated (>10%) reproduction on one or more sources of resistance, the number(s) (1 through 7) of the source(s) of resistance on which reproduction is elevated becomes part of the HG type designation. For example, if an SCN population has elevated reproduction only on the Peking source of resistance, the SCN population is HG type 1 because the index number for Peking is 1. If an SCN population has elevated reproduction on both Peking and PI 88788 sources of resistance, the SCN population is an HG type 1.2 because Peking is indicator line 1, and PI 88788 is indicator line 2. The number 1 always represents elevated reproduction on Peking, and the number 2 always represents elevated reproduction on PI 88788, no matter what other numbers are in the HG type designation. HG type 1.3.5 has >10% reproduction on Peking (represented by the 1 in the type) but not PI 88788 (there's no 2 in the HG type), and HG type 2.5.7 has >10% reproduction on PI 88788 (represented by the 2) but not Peking (no 1 in the HG type).
Since virtually all commercially available SCN-resistant soybean varieties throughout the U.S. and Canada were developed with SCN resistance from either Peking or PI 88788, you need only remember the numbers 1 and 2 as important in an HG type test. The 1 in an HG type always means elevated reproduction on Peking resistance, and 2 always represents elevated reproduction on PI 88788 resistance.
Finally, if the SCN population has low (<10%) reproduction on all seven sources of resistance, it is an HG type 0.
Know Your Number
Be sure to sample your soil before every third soybean crop to monitor your SCN population, Tylka recommends. Your test results will fall into one of four categories:
- A zero SCN count doesn't necessarily mean there is no SCN in the field. SCN may be present but not widespread, and thus missed in the soil sample that was collected, Tylka says.
- A low egg count (up to 2,000 eggs per 100 cm3 or half cup of soil heading into a soybean crop) could mean that SCN population densities are relatively low or that the nematode may be present in high numbers, but only in field patches and not widespread.
- A moderate egg count (2,001 to 12,000 eggs per 100 cm3 of soil) means that SCN numbers are likely both significant and widespread in the field.
- A high egg count would be one of 12,000 or 14,000 eggs per half cup of soil in which case, “You might consider not growing soybeans for another season or two until population densities drop,” Tylka says.
For recommended management options for the above categories, see Table 1. Note that the numbers considered low, moderate, and high vary by state. See Table 2 for how prevention and management approaches change based on region.
Table 1. Soybean cyst nematode management recommendations based on infestation category (Source: Iowa State University)
| Infestation category | Soybean not next crop to be grown | Soybean next crop to be grown | Management recommendation |
|---|---|---|---|
| egg count | |||
| No SCN eggs detected | 0 | 0 | No management strategies are necessary. However, not finding SCN in a soil sample does not prove that it is not present in the field. Follow-up sampling is recommended to check for SCN infestations in future years. |
| Lowa | 1–4,000 | 1–2,000 | If this is first discovery of SCN, follow six-year rotation described in the “Battle Plan” section of this article starting with Year 1 the next time soybeans are to be grown. If Years 1–4 of rotation already have been completed, continue with Year 5 of the rotation. |
| Moderatea | 4,001–16,000 | 2,001–12,000 | Begin Year 1 of rotation described in the “Battle Plan” section of this article the next time soybeans are to be grown. |
| Higha | > 16,000 | > 12,000 | Grow several years of a nonhost crop and sample field again every fall to monitor decrease in SCN population densities |
Table 2. Soybean cyst nematode prevention and management based on region (source: adapted from information provided by Greg Tylka, Iowa State University nematologist)
| Region | Action | Percent contribution |
|---|---|---|
| MN, IA, northern MO, northern IL, IN, OH, WI, MI | Growing resistant varieties (including proper variety selection) | 40% |
| Growing nonhost crops | 40% | |
| Sampling fields for SCN to determine presence and current numbers | 10% | |
| Using seed treatments (results vary among products and among fields in which most individual products are used) | 10% | |
| Southern MO, southern IL, AR, LA, MS, AL, GA, SC, and NCa | Sampling fields for SCN to determine presence and current no.'s | 40% |
| Growing nonhost crops | 35% | |
| Growing resistant varieties (selection may be limited) | 20% | |
| Using seed treatments (results vary among products and among fields in which most individual products are used) | 5% | |
| ND, SD, NE, KS, OK, NY, PA, VA, KY, TN, DE, MD, and NJ | Growing resistant varieties (including proper variety selection) | 40% |
| Growing nonhost crops | 40% | |
| Sampling fields for SCN to determine presence and current no.'s | 10% | |
| Using seed treatments (results vary among products and among fields in which most individual products are used) | 10% |
A new generation of SCN is born every 24 days during summer. “What might have started out as one in a million SCN worms being resistant (meaning being able to reproduce on PI 88788 resistance) now becomes one in four or one in three as PI 88788 resistance is grown continually over decades. That's the situation we face across the Midwest and Canada,” Tylka says. “Even if you have an extremely low SCN population, like 200 eggs per half cup of soil, and 98% don't survive to adulthood, you still can end up with 10,000 eggs per half cup of soil in a single growing season.”
This crazy math explains how rapid SCN numbers can increase and also how it manages to continually spread. Between 2017 and 2020, SCN was found in new counties in 11 U.S. states—namely Kansas, Kentucky, Michigan, Minnesota, North Carolina, North Dakota, Nebraska, New York, South Dakota, Virginia, and Wisconsin—and in the Canadian provinces of Manitoba, Ontario, and Quebec.
Battle Plan
In in most cases where SCN is well established, such as in Wisconsin, the eastern Dakotas, Iowa, and the eastern Corn Belt, total SCN prevention probably is not possible, Tylka says. Use all of these tactics to reduce SCN populations:
Sample your soil before every third soybean crop to know your (SCN) number. Track changes in these numbers.
Plant an SCN-resistant variety with Peking SCN resistance and/or with PI 88788 SCN resistance that shows good nematode control in industry and university testing programs (like www.isuscntrials.info), Tylka says. Only about 5% of resistant varieties are based on Peking (often the more effective source), he adds, so do your homework. One look at yield trials clearly shows how much more effective the Peking source is, but translating seed variety sales materials into the source of the resistance is not always straightforward. “PI 88788 resistance is failing,” Tylka says, and “not all PI 88788-based resistant varieties have the same amount of nematode control. In fact, a few varieties described as resistant don't appear to be resistant at all when tested in the greenhouse.”
Rotate with a non-host crop, such as corn, wheat, sorghum, oats, alfalfa, or sunflowers, to reduce SCN population densities in your field. And, rotate among various resistance sources to reduce chances of more resistance developing to these genetics. Here is an example of a six-year rotation sequence to minimize SCN populations:
- Season 1: Plant a PI 88788-resistant soybean variety that is high yielding and known to provide good SCN control.
- Season 2: Plant a non-host crop such as corn, oats, wheat, sorghum, sunflowers, or alfalfa.
- Season 3: Plant a Peking-resistant variety or a different PI 88788-resistant variety than grown in Season 1.
- Season 4: Plant a non-host crop.
- Season 5: Plant a variety different than was grown in Seasons 1 and 3.
- Season 6: Plant a non-host crop.
It might take 10 years for SCN to be driven down to undetectable levels if non-host crop crops are grown continuously, Tylka says.
Try new seed treatments for added nematode protection but don't consider them a substitute for resistance, Tylka says. “Seed treatments may give 1 to 5 bu/ac yield increase, but ineffective resistance (PI 88788) may result in a 10 or 15 bu/ac yield loss. Seed treatments can vary tremendously in their yield effects, and none can replace good resistant varieties.”
Where SCN is relatively new—such as in parts of North Dakota, South Dakota, Nebraska, Kansas, and some Mid-Atlantic states—farmers might be able to slow the spread of SCN by reducing field-to-field soil movement on equipment, Iowa's Tylka says, but wind and water erosion will still move soil on the most diligently managed farms.
How SCN Works
Soybean cyst nematode reproduces best in hot, dry soils, but it occurs and can thrive almost anywhere. Besides sapping root vigor, SCN can reduce nodulation and nitrogen fixation. In fields with both SDS and SCN, SDS symptoms will occur earlier in the season and will develop to more severe levels, causing more yield loss from SDS. “Through Soybean Checkoff-funded research, we now know that the SDS fungus doesn't enter soybean roots through wounds created by SCN juveniles infecting the roots,” Tylka says. “But we can't rule out the SDS fungus entering roots through wounds created by SCN females rupturing through the root epidermis once they become adults.”
Where SCN is new to an uninfected field or region, it often “goes crazy,” likely because there are no natural SCN soil enemies yet,” Tylka says.
Winter annuals that are host to SCN, like purple deadnettle and henbit, contribute to the problem, says Jason Bond, Southern Illinois University plant pathologist and Director of the Illinois Soybean Center.
Sampling Tips
Soybean cyst nematode can be in a field for many years without the tops of the plants appearing yellow or stunted, Tylka says. “So include healthy looking plants when checking roots or soil sampling. Areas with a higher SCN risk include near a field entrance, previously flooded areas, high-pH soils (above 7), weedy areas, low-yielding areas, and along fence lines where wind-blown soil accumulates.
“Dig soybean plants no sooner than 6 weeks after planting and no later than the end of August,” Tylka says. “Earlier is better because the females and cysts occur on new roots that can be easily dug from the soil surrounding the base of the plant stem. July and August are good times also.
“Later in the season, adult SCN females and cysts appear on new roots deeper down in the soil and farther laterally from the stem of the plant. Dig at least 6 to 8 inches away from the plant stem to avoid cutting off much of the root system.
“Never try to pull up a plant to see SCN; you'll lose too many fine roots and strip off the nematode females as you pull the root system through the soil,” Tylka says. “Gently shake excess soil from the root system.
“Adult SCN females are about the size of the period at the end of a sentence—a lot smaller than the nitrogen-fixing nodules that occur on healthy soybean roots. The nematode females will be white or light yellow and lemon shaped. A magnifying glass or hand lens might help you see the females.
“If a resistant variety is effectively controlling the nematode, there should be no more than about 50 SCN females seen on roots in a spade-full of soil and roots dug up from underneath the plant,” Tylka says.
Try to send your soil samples to the same lab each year, Bond says. “Each lab has its own protocols and methodologies, and you want to compare apples to apples each year.”
SCN Resistance Update
For many SCN populations, Peking offers much more SCN resistance than PI 88788, university trials show (see https://bit.ly/ISU_2019_SCN_trials). Yet 95% of commercially available resistant varieties are based upon PI 88788, says Iowa State University nematologist Greg Tylka.
“CCAs may not know that not all PI 88788 varieties have the same amount of SCN control. And, some ‘SCN-resistant’ varieties might not be resistant at all. There's no legal definition of SCN resistance, and no standard testing to verify or certify resistance.”
Jason Bond, Southern Illinois University plant pathologist, agrees. “Southern Illinois has seen much higher SCN populations the past three to four years. Resistance to PI 88788 is a big part of it,” he says. Also, with newer seed treatments and other tools to manage other pathogens like seedling diseases and SDS, “We are raising very healthy host plants to support higher cyst populations. We have a ton of 40,000 per 100 cm3 of soil cyst populations where you still don't see aboveground symptoms; that obviously steals a lot of yield,” Bond says.
Here are a few things you may not know about SCN genetic resistance:
“The ‘yield drag’ associated with Peking SCN resistance (i.e., lower yields when grown in non-SCN fields) has diminished greatly over the years due to continued improvement of the yield potential of Peking varieties through plant breeding,” Tylka says. “Now, SCN very often ‘drags down’ yields of PI 88788 varieties more than any remaining ‘yield drag’ associated with Peking.”
“It is hard to find or to understand the source of SCN resistance in seed literature, and the varieties change so frequently,” Bond says. “By the time you also consider weed traits and regional problems like Phytophthora, SCN resistance can get lost in variety selection priorities. And, we don't have a lot of Group 4 Peking varieties. Yield hits of 30% from SCN are pretty typical here.”
Syngenta is about to release an SCN-resistant soybean variety under the Northrup King and Golden Harvest brands that was developed using the breeding line PI 89772, a source of resistance first published in 1994, Tylka says. “We don't know how different it is from Peking in terms of specific resistance genes. PI 89772 has never been used in any resistant variety available for farmers to grow, as far as I know.”
“By definition, an SCN-resistant variety should allow less than 10% reproduction of the nematode. In other words, a resistant variety should stop 90% of a field's SCN from reproducing. Varieties with PI 88788 resistance aren't hitting the mark. In some fields, one out of every two nematodes (50%) can reproduce on PI 88788.
“If they can find them, farmers should grow varieties with Peking SCN resistance alternating with high-yielding PI 88788 SCN-resistant varieties that allow only low SCN reproduction,” Tylka says.
Tylka publishes an annual list (https://bit.ly/ISU_2019_SCN_trials) of SCN-resistant varieties in Maturity Groups 0 to 3.9.
Text © . The authors. CC BY-NC-ND 4.0. Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.
