Soybean Cyst Nematode (SCN) is a growing concern for many farmers because it is one of the most damaging pests to soybean production.1 Yield loss from SCN varies with infestation level, but up to 30 percent yield loss has been documented in some places. So, if your soybeans yields were lower than expected, have you considered whether you might have SCN in your fields?
What is the soybean cyst nematode?
Soybean cyst nematode was first positively identified in a few county’s across four states in 1957, but today it has been found to be widespread in 30 states. SCN are quite prolific and every one has the potential to produce 35,000 to 40,000 more nematodes by seasons end—fortunately, SCN larvae have a high mortality rate.
How to identify the soybean cyst nematode:
Look for small SCN cysts on the lateral roots. Immature cysts can be white or cream-colored before turning brown at maturity.
SCN is a small roundworm, but one of the largest nematode species. The cyst is actually the mature female nematode. It is a lemon shaped cyst, brown in color when mature and attached to the outside of the soybean root. The cysts are much smaller than the nitrogen-fixing nodules that are commonly found on roots of soybeans, and can be seen with the naked eye.
A mature cyst can contain up to 200 eggs. They hatch and the small nematode larva chew and enter the roots of host plants (certain weed species and common cover crop plants) and soybean roots. SCN larva continue to feed until the female becomes so large that the root bursts open. The female nematode then attaches to the outside of the root. The eggs are fertilized by the male SCN, and when the eggs are mature, the female SCN dies to release her eggs. The eggs then hatch to begin the second generation of SCN. With warm soils a generation requires only 40-45 days to complete a cycle, and there can be six generations per year.
How does SCN damage crops?
Yield loss can occur without any symptoms evident above ground. You’ll have to examine the soybean roots of suspect fields by carefully digging and crumbling or washing the soil away from soybean roots. Above-ground symptoms usually mimic other problems, such as soil compaction, stunted plants, herbicide injury, nitrogen shortage, iron chlorosis and drought.
Yield loss can be caused by:
- Reduced nitrogen-fixation efficiency.
- Damage to vascular tissues in the roots, which limits nutrient and water usage.
- Root stunting and root damage can then create secondary pathogens and provide insects with an entry point—SCN feeding on the roots also provides an entry point for soil-borne fungus, like soybean sudden death syndrome, or SDS, to enter the plant. (SDS and SCN are often associated with each other in the field, and managing SCN has show to be an effective way to also manage SDS.)
How does SCN spread?
SCN can be spread in many ways—wind, rain, animals and birds, as well as humans from soil on shoes, and mechanically, so be sure to protect your fields from SCN spreading into soils not yet infested.
How do I know if I have a problem with SCN?
The most reliable way to determine if SCN are present in your soil is through soil samples. Collect samples immediately following harvest when you have easier access to the field, and sample your fields each year at approximately the same date to get an indicator if SCN populations are increasing or declining.
How to take a soil sample to identify SCN:
- Use a common soil probe and collect 20 random cores to a depth of 8 inches for every 20 acres of suspect fields.
- Mix the cores well and put them into soil sample bag.
- Keep the samples cool and deliver them to a laboratory as soon as possible for analysis.
What should I do about SCN?
Once you have determined that you have SCN in a field, there is no way to totally eliminate the nematode. But there are a few management practices that can help your soybean crop:
- Overall plant health, such as keeping fertility levels high, weed control that eliminates host plants and adequate water intake.
- SCN can be managed well by variety selection—some soybean varieties contain a PI number as a resistance source, which provide good control of SCN, up to 90 percent.2 There is no advantage when planting GMO over non-GMO or conventional soybeans—SCN doesn’t discriminate and will attack GMO and conventional soybeans equally. Rotating PI sources of resistance helps control SCN populations. (Think of it as rotating modes of action when applying herbicides to reduce the possibility of resistance.)
- If your yields do not increase with these varieties, an HG Type test3 may be needed to determine the specific type of SCN in your soil.
- Crop rotation is also an effective way to aid in control of SCN.
- Unfortunately most cover crop plants are good hosts for SCN. If planning a cover crop on fields where SCN was identified be sure to check a host plant list. (Your local ag extension office should be able to help with this.)
Are there any good treatments for SCN?
There are a few nematicide seed treatments on the market to aid in SCN control at planting, but seed treatments do not always control SCN below damaging levels. Always read and follow label use instructions.