As the 2019 corn and soybean harvest is in full swing, it’s not too early to start thinking about and planning for the 2020 growing season. For soybean farmers in Iowa, a critical fall activity that can pay large dividends is soil sampling to determine soybean cyst nematode population densities in fields in which soybeans will be grown in 2020.
Also, some farmers might consider sampling fields for SCN in which soybeans were grown in 2019 that produced disappointing yields for no apparent reason. Often, SCN infestations are found in areas of fields in which soybean yields were inexplicably low. The tiny wormlike pest works underground, out of sight, feeding on soybean roots and robbing yield.
“It’s important to know your fields’ SCN numbers,” says Greg Tylka, Iowa State University Extension plant pathologist who specializes in nematode management. Although SCN had been kept pretty much “in check” for decades with SCN-resistant soybean varieties, almost all those varieties had the same source of SCN resistance genes — from a breeding line called PI 88788. And using the same resistance genes for 20-plus years has led to the buildup of SCN populations with increased reproduction on resistant soybean varieties.
“The SCN populations in a majority of fields infested with the nematode in Iowa have relatively high levels of reproduction on soybean varieties with the common PI 88788 SCN resistance,” Tylka says. “Populations of SCN that used to reproduce at a level of 5% to 10% now are reproducing at levels of 50% or 60% or more. Put simply, many if not most SCN-resistant soybean varieties aren’t as effective as a management tool as they used to be.”
Know your numbers
Now more than ever, farmers need to know if their fields are infested with SCN and what the numbers are, he adds. The higher the number of SCN eggs in the soil, the greater the yield loss even if you plant resistant soybean varieties.
SCN is a consistent soybean yield reducer every year. It’s not a “hit or miss” issue depending on the weather, as is the case with many other pathogens and pests. The nematode survives very well in the soil, even surviving through a few years of nonhost corn. SCN will reduce yields every year that soybeans are grown in infested fields, regardless of weather.
“Fall is an opportune time to sample for SCN,” Tylka notes. “It’s relatively easy to determine SCN numbers in fields. All it takes is collecting soil samples and having them tested for the nematode. And fall is a prime time to collect samples from fields.”
He offers sampling guidelines below to get a representative sample:
- Use a soil probe, not a spade, to collect soil cores, at about 8 inches deep.
- Collect more soil cores from a smaller area to get more accurate the results — 15 to 20 cores every 20 acres is recommended.
- Collect soil cores from agronomically logical areas; see field map.
- Combine all cores representing a sampling area in a bucket and mix well; place mixture into a sample bag or other water-resistant bag.
- Submit samples to ISU’s Plant and Insect Diagnostic Clinic, Room 2445, Advanced Teaching & Research Building, 2213 Pammel Drive, Ames, IA 50011.
SAMPLING PATTERN: This is an example of how to collect soil samples when testing for SCN in a field with different management zones. Each “X” represents the location from which a soil core was collected.
If you have questions, call 515-294-0581 or email firstname.lastname@example.org. More information is available online here. Many private soil-testing labs also can process soil samples and test for SCN. Information about private SCN labs in Iowa or any other state is available online here.
Meaning behind numbers
The egg counts that correspond to risks of low, medium and high yield loss in Iowa based on SCN numbers from the ISU Plant and Insect Diagnostic Clinic are shown in the table. The numbers are estimates; actual severity of yield loss is greatly affected by in-season temperatures and rainfall, Tylka says.
Also, there is no one standard extraction method used to process soil samples for SCN; a few different methods are used throughout the U.S. and Canada. “Different extraction methods can be more or less effective at recovering SCN cysts and eggs from the soil,” he says. “So, categories of egg counts corresponding to yield loss severity likely will vary among different laboratories.”