Soybean Cyst Nematode

-Written by 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.

Management

Genetic Resistance

Resistance to SCN has been identified and is available in many soybean varieties.  A rotation of resistance sources is recommended for SCN infested fields.  Anytime a field is identified to have SCN present the following rotation should be used:  non-host crop – resistant soybean – non-host crop – resistant soybean.   Additional years of the non-host crop will reduce the SCN population (number of eggs) further.  When different SCN resistance sources cannot be identified (PI88788 is the most common), a change in resistant soybean variety should be substituted.  All varieties with PI88788 are not the same as to their affect on SCN.

Cultural Practices

Yield loss due to SCN will be reduced by maintaining optimum growing conditions and avoiding plant stress.  Moisture stress is the most common factor which increases losses due to SCN.  Avoid spreading SCN by working and planting infested fields last.  Cleaning of equipment should be done when possible to minimize movement potential.  Anything which moves soil will spread SCN.

Chemical  / Biological Control

Nematicides are generally not recommended for managing this disease.

Links

Please refer to Neb Guide G99-1383-A, Soybean Cyst Nematode Biology and Management.