Tylka Nematology Lab

SOYBEAN CYST NEMATODE: PICKING YOUR POCKETS

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

{Paper presented at the 5th Annual Southwest Agricultural Conference, Ridgetown College, Ridgetown, Ontario, January 7-8, 1998}

The 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 North America, but the soybean cyst nematode likely was introduced from Japan. The 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 (Noel, 1992). In Canada, the soybean cyst nematode first was discovered in 1987 in Kent County and since has spread to Elgin, Essex, Haldimand-Norfolk, Lambton, Perth, and Middlesex Counties in Ontario (A. Tenuta, personal communication). It is likely that the soybean cyst nematode was introduced into Canada on vegetable transplants obtained from the southeastern United States (A. Tenuta, personal communication).

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 southern Canada. 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 migrate through the root until they come in contact with the plant’s 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 which 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 300 eggs contained within. Soybean cyst nematode eggs survive within the cyst until conditions become proper for hatching.

Symptomatology & Field Identification

Numerous infestations in the north central United States and in Canada go unnoticed each year because above-ground 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. Damaging infestations of soybean cyst nematode often exist in fields without ever causing noticeable above-ground symptoms, in essence "picking your pockets" of profit-producing soybean yield.

The above-ground symptoms of soybean cyst nematode damage, 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. Above-ground 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. When above-ground symptoms appear, they are not unique and can be mistaken for damage due to compaction, iron deficiency chlorosis, other nutrient deficiencies, lightning damage, drought stress, herbicide injury, or other plant diseases. Often, soybean injury and yield loss due to the soybean cyst nematode probably have gone undetected for numerous years because of the absence of above-ground 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 symptoms lessen at times throughout the growing season. The intensity of the symptoms is influenced by the age and vigor of the soybean plants, the nematode population density in the soil, soil fertility and moisture levels, and other environmental conditions. Soybean cyst nematode damage usually is more severe in light, sandy soils but will occur readily in all types of soil.

One can not rely upon above-ground symptoms for identification of soybean cyst nematode infestations. If the soybean yield obtained in a particular field has leveled off or decreased for no apparent reason or if the soybean cyst nematode has been confirmed on nearby land, more thorough examination of plants for below-ground symptoms and a soil analysis are needed.

Management

For all practical purposes, the soybean cyst nematode can never be eliminated from soil 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. Practices recommended in Iowa for managing the soybean cyst nematode fall into five categories.

1. Maintain plant health

Plants that have adequate moisture and fertility are better able to endure infection by the soybean cyst nematode. Consequently, maintaining proper soil fertility and pH levels in land infested with the soybean cyst nematode is more critical to maximizing yield than when land is noninfested. Also, it is important to control other plant diseases as well as insect and weed pests that weaken the plants and make them more susceptible to the yield-loss from the nematode.

2. Manage the movement of soil

Common sense sanitation practices can be very effective in preventing or delaying the spread of the soybean cyst nematode to noninfested land. If only certain fields on a farm are infested, planting and cultivating of infested land should be done only after noninfested fields have been worked. Equipment should be cleaned thoroughly with high pressure water or steam, if available, after working in infested fields. Also, one should not use seed grown on infested land for planting unless the seed has been properly cleaned; the soybean cyst nematode may be spread in seed-sized soil peds associated with the seed.

3. Grow resistant soybean varieties

Resistant soybean varieties are the most effective tool available for management of the soybean cyst nematode. Some resistant varieties may yield slightly less than susceptible varieties in noninfested fields, but will yield significantly better in fields infested with the soybean cyst nematode (Tables 1&2). The performance of resistant varieties on noninfested land is constantly being improved. In addition to providing increased soybean yields, growing resistant soybeans in infested soil suppresses reproduction of the nematode (Table 2). 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.

Table 1. Agronomic performance of soybean varieties in soybean cyst nematode-infested fields in Ontario from 1995 to 1997 (Ontario Oil & Protein Seed Crop Committee, 1997).

(1) Yield index = (individual variety mean yield, mean yield of all recommended varieties grown in area) x 100.
(2) Mean of three varieties with above-average yield potential.

In the past, there were few resistant varieties available for Canada, but in recent years many public and private varieties have been released. Soybean cyst nematode-resistant soybean varieties available in the United States get their resistance from one of two soybean breeding lines, 'Peking' or 'PI88788'. Virtually all soybean cyst nematode-resistant varieties available in Canada contain genes from 'PI88788'. Soybean varieties with resistance from 'Peking', or some other sources of soybean cyst nematode resistance, likely will be available in Canada in the near future.

Table 2. Soybean cyst nematode (SCN) reproduction and agronomic performance of maturity group I and II soybean varieties in an infested field and a noninfested field in north central Iowa in 1997 (Tylka and Souhrada, 1997).

Values presented in table are means; each variety was grown in four replicate plots in each field.
(1) Final SCN egg population density (eggs per 100 cc soil).
(2) Susceptible check variety.
(3) Least significant difference: values are from Fisher's least-significant-difference test.

Although use of resistant varieties is the most effective strategy for managing the 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.

Current management recommendations in Ontario consist of growing resistant and susceptible soybean varieties alternating with nonhost crops in "low risk" situations; two years of nonhost crops separating alternating years of resistant and susceptible soybean varieties is recommended for "high risk" situations (A. Tenuta, personal communication).

In the United States, growers are encouraged to alternate use of soybean varieties with different sources of soybean cyst nematode resistance. Additionally, like in Canada, it is recommended that a susceptible soybean variety be grown periodically 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 widely available soybean resistance in conjunction with susceptible soybean varieties and nonhost crops. Growers should consult seed company representatives for information on resistant soybean varieties and their sources of resistance.

* 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

4. Grow nonhost crops

The 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.

5. Use nematicides

There are no nematicides registered for use with soybeans in Canada, and it is extremely unlikely that any new compounds will be introduced in the future. In the United States, there are a few nematicides which are labeled for use against the soybean cyst nematode. However, these materials often do not give season long control. When applied at planting, the effect of the nematicides may last long enough to provide an economic yield benefit. By the end of the growing season, though, soybean cyst nematode numbers may be as high or higher than they were at planting. No nematicide will kill all soybean cyst nematodes in the soil. The performance of the nematicide will depend on soil conditions, temperatures, and rainfall. A yield benefit is not guaranteed, and nematicides are expensive. Consequently, growers in the United States are advised to consider economic, environmental, and personal health factors before utilizing nematicides for management of the soybean cyst nematode.

Summary

The soybean cyst nematode always will be a major threat to soybean production in Canada 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.

Ontario Oil & Protein Seed Crop Committee. 1997. Ontario Soybean Variety Trials 1998 Report.

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

Tylka, G.L., and S.K. Souhrada. 1997. Evaluation of resistant soybean varieties for management of soybean cyst nematode in Iowa. Publication IPM-52, 12 pp.

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