Insects and Diseases

Figure 3. White females (small arrows) of SCN on soybean roots are visible to the naked eye, but are still very small. Nitrogen-fixing nodules are much larger (large arrow).

Figure 4. The SCN cyst has been broken open to reveal the eggs inside. Eggs can remain viable for years, even in the absence of a suitable host. (photos courtesy of SCN Management Guide, 1999)

Go to the following pages to study:

Key plant symptoms of SCN

Management of SCN

Fact sheet on SCN

Soybean Cyst Nematode

The soybean cyst nematode (SCN), Heterodera glycines, is the single most damaging pest affecting soybean in the U.S. as well as in most of the other top soybean-producing countries of the world. Documenting the economic impact of SCN is difficult because many producers suffer declining yield for several years without knowing that they have SCN, but losses have been estimated at about $1.5 billion in the U.S. alone. This microscopic, soilborne worm is present in approximately 75% of the fields in Iowa, and can reduce soybean yield by 50% or more. Often there are no obvious aboveground symptoms of damage caused by SCN. Since there are rarely aboveground symptoms, many infestations go undiscovered for numerous years. On average, SCN cause producers in Iowa currently more than 50 million bushels a year.

Biology and Life Cycle

Figure 1. The life cycle of the nematode is shown above with the three developmental stages: egg, juvenile, and adult. (photo courtesy of SCN Management Guide, 1999)

Soybean cyst nematode is a microscopic roundworm that attacks the roots of soybean and a limited number of other host plants (Table 1). The developmental stages of the nematode include the egg, juvenile, and adult (Figure 1). The first juvenile molts to form a second stage juvenile (J2) within the egg. The J2 is the stage that enters the roots and actually infects the soybean plant. These juveniles penetrate soybean roots and cause the formation of specialized feeding cells in the root’s vascular system (Figure 2). If the juvenile becomes a male, it leaves the root after feeding for several days, moves through the soil, and does not damage the plant anymore. If the juvenile becomes a female, it becomes sedentary and molts three more times before becoming an adult. The adult female swells to a lemon-shape as it matures, eventually bursts, and this is what becomes visible on the roots surface (Figure 3).

Figure 2. Soybean cyst nematode juveniles (stained pink) inside a soybean root. The specialized feeding cells are visible as gray masses (arrows). (photo courtesy of SCN Management Guide, 1999)

Plant damage is due primarily to the feeding of these females and the indirect effects of such feeding. The specialized root cells, induced by SCN, will have a new physiological function supplying nutrients to the nematode instead of the plant. The second stage juvenile SCN, leaves the cyst and penetrates young primary roots or apical secondary root meristems. After penetration and migration to vascular tissues, the feeding process initiates by piercing its stylet into the pericycle, endodermis, or cortex cells withdrawing nutrients through a minute hole into the cytoplasm membrane at the stylet orifice. The penetration causes necrosis from the entrance to where the feeding site was established. Soybean cyst nematode can complete up to six generations during the growing season, depending on planting date, soil temperature, length of the growing season, host suitability, geographic locations, and maturity group of the soybean.

The female releases a pheromone to attract the male for mating. The female actually deposits a gelatinous mixture where they deposit a small proportion of the eggs they produce. The gelatin mixture is called an egg mass (Figure 4). Eggs from this mass hatch and infect soybean roots the same year they are produced. Many eggs still remain in the female’s body though. The females color changes white when they mature, when they die they change to brown. This is what is considered the cyst and what you will find when you soil sample your field. The cyst actually protects the eggs from being damaged. They are considered an overwintering structure for the eggs.

Without a host, SCN can not reproduce. The same conditions that favor soybean growth are also favorable for SCN development, and the worst yield loss from SCN is therefore often found in high yielding environments. Each cyst can contain up to 500 eggs, and the eggs can remain viable for many years, even in the absence of a suitable host. Plants infected with high numbers of SCN have poorly developed roots systems that can not utilize nutrients and water efficiently. The result may be stunted plants with chlorotic foliage. Soybean cyst nematode infection may also reduce the number of nodules formed by the beneficial nitrogen-fixing bacteria that are necessary for optimum soybean growth. However, yield loss of 40% can occur without aboveground visual symptoms. For the same reason, many research trials that include both resistant and susceptible soybean varieties show no consistent differences in plant growth measurements (visually). On the other hand, the yields of resistant varieties were consistently higher than those of the susceptible varieties mainly because of more pods per plant.

Table 1. Soybean cyst nematode hosts in Iowa.

More Information

ISU Nematology Lab web site.
Detailed information on SCN can be found on the Plant Health Initiative web site.
A revised classification scheme for genetically diverse populations of Heterodera glycines (pdf)

A fact sheet is availabe to be downloaded (pdf file) on SCN.