Soybean Cyst Nematode

Loren J. Giesler, Extension Plant Pathologist

Pathogen

The soybean cyst nematode (SCN) (Heterodera glycines) is a plant-parasitic roundworm. There are three main stages to the life cycle of the soybean cyst nematode. The cycle starts in the spring when temperature and moisture levels are adequate for egg hatch to release the juvenile nematode. Once a juvenile penetrates a soybean root, it moves through the root to the vascular tissue. In the vascular tissue the nematode establishes a feeding site. It then continues to feed and swell and eventually the females burst through the root tissue.

Eggs are produced mostly inside the female's body with some of eggs on the outside.  The eggs on the outside of the body hatch and juvenile nematodes re-infect soybean roots. The egg-filled body of the dead female is what is referred to as the cyst. Each cyst can contain up to 400 eggs.  There can be 3-4 generations of SCN in a single growing season.  Soybean cyst nematode is the most yield limiting disease of soybean in the U.S.

Disease Symptoms

Field Symptoms

The first indication of a problem is when soybean yields are lower than expected or are dropping when soybean are planted in the field. Lower yields will usually be associated with dryer growing seasons. Low levels of SCN may not produce visible aboveground symptoms, yet yields may be reduced. High SCN levels can cause plant stunting and yellowing. Aboveground symptoms can be confused with damage from compaction, nutrient deficiencies, drought stress, low-lying wet areas, herbicide injury, and other plant diseases. Circular to oval areas of stunted, yellowed plants can be observed. Areas of SCN injury are typically elongated in the direction of tillage practices, since the cysts are spread by tillage equipment.

No Symptom Field	Field with Symptoms

Plant Symptoms

Symptoms include stunted roots with fewer nitrogen-fixing nodules. SCN infestation may increase susceptibility to soil-borne fungal infections, such as Rhizoctonia. The only unique symptom or "sign" is the presence of the adult females and cysts on the roots. Adult females appear as extremely tiny lemon-shaped bodies on the roots and are initially cream-colored. They later turn yellow and finally tan to brown as they mature to form the cyst. These are seen with the unaided eye and are much smaller than nitrogen nodules. Observation of adult females and cysts on the roots is one way to confirm SCN infestations in a field. In most SCN-infested fields in Nebraska, females are evident on roots by July 4 or after; however, the absence of cysts on the roots does not mean a field is free of SCN. In fields with a low population, very few cysts may be found on the roots and they may be easy to miss by visual observation. The only way to know a field is free of SCN is with a soil test.

Signs	Lemon Bodies
Cysts on Roots	More Cysts

Favorable Environmental Conditions

Weather conditions which favor maximum soybean yields are those which favor maximum SCN reproduction. In years with dryer conditions, especially in sandy soils, yield losses are higher. In a regional survey, higher SCN populations have been associated with sandier, well drained soils. Fields in no-till with high clay content soils tend to have lower SCN populations. High soil pH is also associated with high SCN populations. In Nebraska, SCN is not found in approximately 10% of our production fields.

Management

Cultural Practices

Links

• UNL Nebraska Extension NebGuide Soybean Cyst Nematode Biology and Management (G1383) 
• North Central Soybean Research and Education Initiative: Soybean Cyst Nematode Guide (5th Edition)
• North Central Soybean Research and Education Initiative: Fact Sheet on Resistance and SCN Populations