Soybean cyst nematode (Heterodera glycines; SCN) is the most economically important soybean pest in the United States, resulting in an estimated $783.8 million in yield losses in 2002 ( Wrather et al. 2003 ). Since its discovery in the United States in 1954 in North Carolina, SCN has been detected in most soybean production states and is especially common in Indiana where it currently infests at least 82 of 92 counties (Faghihi et al. 2006). Yield losses within fields infested with SCN can range from slight to near-total depending upon severity of infestation, soil type, soybean cultivar, weather conditions, and presence of other soybean pests (e.g., weeds, insects, and fungal pathogens).

SCN Biology

Soybean cyst nematode (SCN; Heterodera glycines ) is a microscopic roundworm that parasitizes a number of plants including soybean. The lifecycle of SCN begins with the egg where the first-stage juvenile undergoes an initial molt. When environmental conditions are suitable, the second-stage juvenile (J-2) emerges from the egg and maneuvers through the soil in search of the root of a suitable host plant. Exhudates from soybean roots can stimulate egg hatch and can also attract SCN to its roots. If a suitable host is not located within a few weeks, the juvenile will die.

Once inside the root, the juvenile moves through the cells of the cortex and uses its stylet to pierce the cells adjacent to the vascular cylinder. These cells enlarge to become a group of specialized feeding cells called syncytium from which the juvenile draws nutrients and materials from the plant root for its growth and development. If the nematode is unable to stimulate syncytium formation, development will cease. The juvenile undergoes three more molts before adulthood is reached. The syncytia of the male degenerates, signaling an end to the feeding period, and the wormlike adult becomes mobilized to search for females to fertilize. The female, on the other hand, remains immobile and the syncytia continues to supply food until egg production is complete. The mature female is white and lemon-shaped but turns pale yellow when egg production begins then light brown with death. The dead female body forms a cyst and can be filled with as many as 600 eggs. Cysts serve to protect the eggs of SCN from potential hazards in the soil until the next growing season or until favorable conditions arrive for the eggs to hatch.

The total number of SCN eggs produced in a season is a reflection of the compatibility of the host and both the length and environmental conditions of the soybean growth period. Soybean varieties differ greatly in their ability to sustain SCN reproduction. Cultivars vary in the total number of cysts that can develop as well as in the number of eggs contained within each cyst. The length of the season is controlled by the maturity group of the soybean variety where longer season varieties result in more SCN reproduction. In addition, hot and cold weather can also inhibit SCN development. Soil temperatures greater than 30°C or less than 10°C generally cause reproduction to cease. Under optimal conditions (moist soils with moderate temperatures) SCN completes its reproductive cycle in three to four weeks.

SCN Symptoms

The presence of SCN in soybean roots inhibits normal water and nutrient transport. Visible foliar symptoms can range from no injury to complete plant death depending on SCN population density and environmental conditions. Symptoms of SCN infection often include stunted, yellow plants, especially when the plant is experiencing moisture stress. SCN impacts on yield vary depending on the level of SCN damage. Foliar symptoms of nematode damage are difficult to distinguish from other soybean production problems such as nutrient deficiency, drought, herbicide injury, other soybean diseases, and compaction. Roots of SCN infected plants are stunted, necrotic, and may possess fewer rhizobium nodules than healthy roots. White females can be seen on roots of infected, flowering soybean plants but brown cysts (older, dead females) are not visible on roots or in the soil. The best means of confirming an SCN infestation is through cyst extraction from soil samples.

Other SCN Resources:

Soybean Cyst Nematode (J. Faghihi and V. Ferris - E-210-W)

Soybean Cyst Nematode Management Guide (NCSRP)

Illinois Soybean Cyst Nematode Coalition

Literature Cited

Faghihi, J., and V. R. Ferris.. 2006. Soybean Cyst Nematode. Department of Entomology. Purdue University : http://www.entm.purdue.edu/Entomology/ext/targets/e-series/EseriesPDF/E-210.pdf. Date accessed: November 2006.
Wrather , J. A., S.R. Koenning , and T.R. Anderson . 2003. Effect of diseases on soybean yields in the United States and Ontario (1999-2002). Online. Plant Health Progress doi:10.1094/PHP-2003-0325-01-RV.