The soybean cyst nematode (SCN) causes greater annual yield losses in Kentucky than any other pathogen of soybean. Preliminary results from an on-going SCN survey initiated in 2019 show that approximately 80% of Kentucky fields are infested with SCN (Figure 1).
Risk of yield loss due to SCN can be estimated by the initial SCN egg count at the beginning of the season, where 0-499 eggs/100 cm3 soil is a low risk of yield loss, 500-1,999 eggs/100 cm3 soil is a moderate risk, 2,000-9,999 eggs/100 cm3 soil is a high risk, and at least 10,000 eggs/100 cm3 soil is a very high risk.
According to the ongoing survey (based on 360 samples representing 35 counties), nearly 40% of the fields surveyed have populations that will likely cause yield loss.
Although above-ground symptoms (stunting and yellowing) caused by SCN can occasionally be observed, affected soybean plants generally appear to be healthy. Unfortunately, “healthy-looking” soybean plants that are infected by SCN can still have up to a 30% yield reduction.
Management of SCN has gotten much more complex in the last few years, since SCN populations have adapted to the use of SCN-resistant soybean varieties. The primary source of SCN resistance used by commercial soybean breeding programs came from a soybean germplasm line known as “PI 88788.”
This source of resistance was highly effective in managing SCN for several years, but prolific use of soybean varieties with the PI 88788 background has selected for SCN populations that are able to overcome this source of resistance. In the 2006-2007 University of Kentucky SCN survey, the PI 88788 source of SCN resistance was not very effective against approximately 60% of the SCN populations in Kentucky, making management of this pathogen much more complex than before.
Figure 1. Preliminary results of an ongoing survey of soybean cyst nematode egg densities from Kentucky soybean fields. Eggs numbers are shown as number of eggs per 100 cm3 soil. These results represent 360 soil samples from 35 counties. Click Image to Enlarge
Managing SCN
As complex as it is, management of SCN is still doable, and is important for maintaining and increasing soybean yields. Below are the main steps for managing SCN:
- Test your fields to know the number of SCN eggs in your field. The best times to sample for SCN in your fields is in the fall or in the spring (before planting). A Fact Sheet on sampling for SCN is available here. Although the University of Kentucky does not currently have an active SCN Laboratory, samples can be sent to either the University of Illinois Plant Clinic or the University of Missouri SCN Diagnostics Lab. Similar to the past two seasons, the Kentucky Soybean Board is continuing to sponsor free SCN testing for Kentucky farmers. With this program, a limited number of samples for each county can be tested for free. Please check with your local County Extension Office for more information about the limited free SCN testing program.
- Rotate resistant varieties. If varieties are available that utilize sources of SCN resistance other than PI 88788 (such as Peking or Hartwig), then rotate the source of resistance every time you plant soybean in a field. Unfortunately, nearly all the soybean varieties adapted for planting in Kentucky utilize only the PI 88788 source of resistance. However, it is still important to rotate to different resistant soybean varieties, even though they are utilizing the same source of resistance. SCN is good at adaptation, so switching soybean varieties will help.
- Rotate to non-host crops. Rotating fields to a non-host crop, such as corn or grain sorghum, will help reduce SCN populations in fields. Wheat is another non-host crop that may help lower SCN populations by having it in the rotation. Several years ago, Dr. Don Hershman with the University of Kentucky evaluated the effect of wheat residue on SCN populations. His research found that planting soybeans into fields with standing wheat stubble reduced SCN populations at the end of the growing season. More information about that research can be found here.
- Consider using a nematode-protectant seed treatment. Several nematode-protectant seed treatment products are now available on the market. Although the effects of these seed treatments have not always been consistent in field research trials, they are additional tools that can be used along with resistant varieties and crop rotation to help manage this important pathogen.
A multi-state initiative funded by the Soybean Checkoff Program known as the SCN Coalition is helping to promote awareness of the damage caused by SCN and the importance in managing this pathogen. More information about the SCN Coalition is available on their website. Be on the lookout for information from the SCN Coalition about this important pathogen.
