Tylka Nematology Lab

SOYBEAN CYST NEMATODE - BIOLOGY AND MANAGEMENT

Gregory L. Tylka
Plant Nematologist
Associate Professor, Department of Plant Pathology
Iowa State University

Soybean cyst nematode, Heterodera glycines, is a microscopic, unsegmented plant-parasitic roundworm that attacks the roots of soybeans. Many plant-parasitic nematodes are believed to be native to the United States, but soybean cyst nematode likely was introduced into the United States from Japan. Soybean cyst nematode was first discovered in the United States in 1954 in North Carolina (Winstead et al., 1955) and since has spread to 26 additional states in the Southeast and Midwest (Figure 1) (Noel, 1992). The nematode was first discovered in Iowa in Winnebago County in 1978. Currently, soybean cyst nematode is known to exist in 90 Iowa counties and is suspected to be present in many other counties as well. More than 70% of 400 randomly selected Iowa fields sampled in 1995 and 1996 were infested with the nematode (Workneh et al., 1998).

Figure 1. Known distribution of SCN in the United States in 1999 (known infested counties are shaded in gray).

Symptoms

Symptoms of soybean cyst nematode damage can be classified into two categories, aboveground and belowground. Numerous infestations in Iowa go unnoticed each year because aboveground symptoms of soybean cyst nematode damage do not appear consistently and may be absent for several years following introduction of the nematode into a field. Yield-suppressing infestations of soybean cyst nematode often exist for years in fields without ever causing noticeable aboveground symptoms.

The aboveground symptoms of soybean cyst nematode, when present, may appear in circular or oblong patterns which vary in size or may be more generalized across much or all of the field. When symptoms appear in a localized spot, the most severe damage usually occurs in the center of the spot. Aboveground symptoms initially may appear near a gate or entrance to a field where farm machinery enters or along a fence line where wind-blown soil tends to accumulate.

Aboveground Symptoms

When aboveground symptoms appear, they are not unique and can be mistaken for damage due to compaction, iron deficiency chlorosis, other nutrient deficiencies, drought stress, herbicide injury, or other plant diseases. Often, soybean injury and yield loss due to soybean cyst nematode probably have gone undetected for numerous years because of the absence of aboveground symptoms or because the nondescript symptoms were attributed to some other soybean production problem.

The first obvious symptom of soybean cyst nematode injury to soybeans may be the appearance of stunted, yellowed, less vigorous plants. Additionally, rows of soybeans grown in infested fields often are slow to "close" or fill in with foliage. Stunted and yellowed plants that appear in dry years often will exhibit a dramatic growth resurgence following rainfall, although plants growing in heavily infested soils may remain stunted throughout the growing season. Yield loss continues to occur, even if aboveground symptoms lessen at times throughout the growing season.

Yellowing of soybeans due to soybean cyst nematode damage often is confused with iron deficiency chlorosis, particularly in geographical areas with high pH soils where iron deficiency is a problem. There are differences between the symptoms of the two problems. Iron deficiency chlorosis symptoms will appear early in the growing season, usually in early June, whereas yellowing due to soybean cyst nematode damage will occur later in the season, usually in July and August. Furthermore, yellowing due to iron deficiency chlorosis primarily affects the areas between the veins of the upper leaves while soybean cyst nematode yellowing usually begins at the leaf margins of leaves over the entire plant. Iron deficiency chlorosis and soybean cyst nematode may occur in the same field with symptoms of both occurring on the same plant.

As mentioned earlier, aboveground symptoms of soybean cyst nematode damage do not always occur consistently. Symptoms can range from nonexistent to severe. The age and vigor of the soybean plants, the nematode population density in the soil, soil fertility and moisture levels, and other environmental conditions influence the intensity of the symptoms. Soybean cyst nematode damage usually is more severe in light, sandy soils but will occur readily in all types of soil.

Belowground Symptoms

Most belowground symptoms of soybean cyst nematode injury are not unique. Roots infected with the nematode are dwarfed or stunted. Soybean cyst nematode also decreases the number of nitrogen-fixing nodules on the roots, and infection of roots by soybean cyst nematode may make the roots more susceptible to infection by other soil-borne plant pathogens. Often, it is difficult to recognize roots as being stunted and having fewer nodules unless some uninfected soybean roots are available for side-by-side comparison.

The only unique sign of soybean cyst nematode infection is the presence of adult female nematodes and cysts on the soybean roots. Females and cysts appear as tiny, lemon-shaped objects which are white initially but turn yellow, then tan to brown as they mature. The females and cysts can be seen on infected roots with the unaided eye, although observation with a magnifying glass usually is much easier. The females and cysts are about the size of a period at the end of a sentence and are much smaller than nitrogen-fixing nodules. Roots should be carefully dug, not pulled, from the soil to observe the nematodes on the roots, otherwise many of the females and cysts may become dislodged. Observation of the nematodes on the roots of infected soybean plants is the ONLY accurate way to diagnose soybean cyst nematode infestations in the field. In most years, such diagnoses can be performed beginning four to six weeks after planting and continuing through September in Iowa.

Life Cycle of Soybean Cyst Nematode

The soybean cyst nematode life cycle has three major stages: egg, juvenile, and adult. The life cycle can be completed in 24 to 30 days under optimum conditions in the summer. Consequently, two to four generations per growing season are possible in the Midwest. Worm-shaped soybean cyst nematode juveniles hatch from eggs in the soil when adequate temperature and moisture levels occur in the spring (Schmitt and Riggs, 1989). These juveniles are the only life stage of the nematode capable of infecting soybean roots. Hatched juveniles that do not penetrate host roots and begin feeding will die from starvation, predation, or parasitism within several days to a few weeks.

After penetrating the soybean roots, juveniles move through the root until they contact the vascular tissue. There they cease moving, lose most of the muscles in their bodies, and begin to feed. In order to feed, the nematodes inject secretions that modify root cells and transform them into specialized feeding sites called syncytia.

As the nematodes feed, they swell. Eventually the female nematodes become so swollen that they break out through the root tissue and are exposed on the surface of the root. Male nematodes, which are not swollen as adults, migrate out of the roots into the soil and fertilize the lemon-shaped adult females on the roots. After fertilization, males eventually die whereas females remain attached to the roots and continue to feed. The swollen females begin to produce eggs, initially in a mass or egg sac outside the body and later within the body cavity of the female. The entire body cavity of the adult female eventually becomes filled with eggs, and the female dies. It is the egg-filled body of the dead female that is referred to as the cyst.

Cysts eventually will dislodge from the roots and become free in the soil. The walls of the cyst become very tough and provide excellent protection for the 200 to 400 eggs contained within. Soybean cyst nematode eggs survive within the cyst until conditions become proper for hatching. Although many of the eggs may hatch within the first year, many also will survive within the cysts for many years.

Spread of Soybean Cyst Nematode

Soybean cyst nematode can move through the soil only a few inches per year on its own power. However, it can be spread great distances in a variety of ways. Generally, anything that moves even small amounts of soil is capable of disseminating soybean cyst nematode. Spread can occur by soil moved by farm machinery, vehicles and tools, wind, water, animals, and farm workers. Seed-sized clumps of soil, called soil peds, often contaminate seed harvested from plants grown on infested land. Soybean cyst nematode can be spread if this seed is planted in noninfested fields. There even is evidence that birds can spread cysts of soybean cyst nematode. Obviously, only some avenues of spread of this nematode can be prevented. Recent flooding in the Midwest undoubtedly has expanded the distribution of soybean cyst nematode throughout the region.

Management of Soybean Cyst Nematode

For all practical purposes, soybean cyst nematode never can be eliminated from a field once it is present. However, there are things that can be done to manage the nematode in order to maximize yields and minimize reproduction of the nematode. Management of soybean cyst nematode in Iowa is a three-prong approach.

1. Early detection

Because aboveground symptoms are not a reliable indicator of soybean cyst nematode damage, you cannot tell if you have soybean cyst nematode just by looking at your fields. The lack of symptom occurrence and subsequent missed diagnosis are unfortunate because the key to effective management of the soybean cyst nematode is early detection, before large nematode population densities develop.

The only reliable way to know if a field is infested with soybean cyst nematode is to either dig roots of soybean plants during the season and look for adult nematode females or take a soil sample and have it analyzed by a qualified laboratory. If soybean yields obtained in a particular field have leveled off or decreased for no apparent reason or if soybean cyst nematode has been confirmed on nearby land, one should begin to actively scout for the nematode.

2. Use of soybean cyst nematode-resistant soybean varieties

Resistant soybean varieties are the most effective tool available for management of soybean cyst nematode. By planting resistant soybeans in infested soil, reproduction of the nematode is suppressed. Most soybean cyst nematode juveniles will be unable to feed and reproduce on the roots of resistant varieties, but a few nematodes will survive and reproduce. Some resistant varieties may yield slightly less than susceptible varieties in noninfested fields, but will yield significantly better in fields infested with soybean cyst nematode. The performance of resistant varieties on noninfested land is constantly being improved. In the past, there were few resistant varieties available for Iowa, but in recent years many public and private varieties have been released. Almost all soybean cyst nematode-resistant soybean varieties available in Iowa obtained resistance from one of two soybean breeding lines, 'Peking' or 'PI88788'.

Although use of resistant varieties is the most effective management strategy for soybean cyst nematode, RESISTANT VARIETIES SHOULD NOT BE GROWN YEAR AFTER YEAR. If resistant varieties are grown for several years in a row, eventually a population (or race) of soybean cyst nematode may develop that is capable of reproducing on the resistant varieties. Growers are encouraged to alternate use of soybean varieties with different sources of soybean cyst nematode resistance. Furthermore, it is recommended that a susceptible soybean variety be grown once after all types of available resistance have been used to offset the effect of growing the resistant soybean varieties. Following is a recommended six-year rotation scheme using the two types of soybean resistance widely available in Iowa in conjunction with susceptible soybean varieties and nonhost crops for integrated management of soybean cyst nematode. Growers should consult county extension personnel and seed company representatives for information on suitable resistant soybean varieties and their sources of SCN resistance or to further discuss other aspects of effective crop rotation schemes.

1st year - Nonhost crop
2nd year - 'PI88788' Resistant soybean
3rd year - Nonhost crop
4th year - 'Peking' Resistant soybean
5th year - Nonhost crop
6th year - High-yielding, Susceptible soybean

If an appropriate Peking soybean cyst nematode-resistant soybean variety is not available for use in the 4th year, one should grow a soybean variety with resistance derived from PI 88788 that is different from the PI 88788 resistant soybean variety that was grown in the 2nd year. As a last resort, one could grow the exact same PI 88788 soybean cyst nematode-resistant soybean variety in the 2nd and 4th years but only if no other Peking or PI 88788 resistant soybean varieties are available.

3. Use of nonhost crops

Soybean cyst nematode is an obligate parasite. The nematode is unable to mature and reproduce in the absence of host roots. Consequently, soybean cyst nematode population densities decline during any year that nonhost crops are grown. Alfalfa, corn, and oats are common nonhost crops grown in Iowa, and soybean cyst nematode densities decline similarly when these three crops are grown in infested fields. Soybean cyst nematode population densities generally decline from 10 to 50% during a year that a nonhost crop is grown, but the magnitude of decline is unpredictable because the change in population density varies from year to year and is greatly influenced by environmental conditions.

Summary

Soybean cyst nematode always will be a major threat to soybean production in Iowa because the nematode survives for many years in infested soils, causes significant reductions in soybean yields at relatively low population densities, and reproduces to high population densities very quickly. However, an integrated management program can effectively prevent increases in soybean cyst nematode population densities and maintain profitable soybean yields in fields infested with the pest. Such a management program must include scouting for early detection of soybean cyst nematode infestations followed by rapid implementation of a thorough crop rotation program including nonhost crops and resistant soybean varieties.

Literature Cited

Noel, G.R. 1992. History, distribution, and economics. Pp. 1-13 in R.D. Riggs and J.A. Wrather, eds. Biology and management of the soybean cyst nematode. St. Paul, APS Press.

Schmitt, D.P., and R.D. Riggs. 1989. Population dynamics and management of Heterodera glycines. Agricultural Zoology Reviews 3:253-269.

Winstead, N.N., C.B. Skotland, and J.N. Sasser. 1955. Soybean-cyst nematode in North Carolina. Plant Disease Reporter 39:9-11.

Workneh, F., G.L. Tylka, X.B. Yang, J. Faghihi, and J.M. Ferris. 1999. Assessment of soybean brown stem rot, Phytophthora sojae, and Heterodera glycines in the northcentral United States using area-frame sampling: prevalence and effects of tillage. Phytopathology 89:204-211.