Soybean Cyst Nematode

(SCN)

by Berlin D. Nelson and Carl A. Bradley,

Professor of Plant Pathology and Extension Plant Pathologist, Dept. Plant Pathology, NDSU

Visit the NCSRP Plant Health web site for more information

Funding for the development of this information was provided by the North Dakota Soybean Council

To find information on other diseases see the menu to the right—

HISTORY

The soybean cyst nematode (SCN), Heterodera glycines is the most important soybean pathogen in the United States. In 2002, the estimated losses from SCN in the USA were 133 million bushels. The nematode attacks the roots causing major damage to the plant. This nematode was first reported in Japan in 1915, and was found in North Carolina in 1954. It probably was imported along with soybean seed from Asia. The nematode has spread throughout the soybean growing areas of the Midwest, southeast, eastern seaboard and into Canada. It was first reported in Minnesota in 1978 and in South Dakota in 1995. It now occurs in western Minnesota counties and in the northern counties of South Dakota bordering ND. In August, 2003, SCN was found in eastern North Dakota, but the number of infested soybean acres has not yet been determined.

SYMPTOMS

Identification of this disease may be difficult because yield loss can occur with no obvious above ground symptoms. Declining soybean yields may be the indication of an SCN problem. When growing conditions are optimal, infected plants may appear nearly normal. When above ground symptoms appear, it usually indicates a long term problem that is just being recognized. SCN populations can require many years from initial infestation to reach damaging levels. Although above ground symptoms of SCN damage are common, especially at high populations of nematodes in the soil, they are highly varied and can be confused with other causes such as iron deficiency, high pH, soil compaction, nutrient deficiency, chemical injury, or root rots. Yellowing of the foliage (chlorosis) in July or August, stunting of plants, thin stands and slow closing of rows are observed. A wavy growth pattern may be seen down the rows due to stunted and/or poorly growing plants. Symptoms may first occur near field entrances, vehicle and storage areas in the field, compacted headlands, along fence rows where windblown soil accumulates, on the tops of knolls or in low or flooded areas. Leaf yellowing may not be observed at low to moderate levels of SCN, but often appear on infected plants stressed by drought. SCN can cause various types of chlorosis that can be confused with nutrient deficiency. Disease often occurs more pronounced in certain parts of a field as circular or oval shaped areas, and the pattern of symptoms will follow the direction of tillage. Weeds may infest the affected areas. This patchy pattern is very evident when viewed from the air. In highly infested fields, plants can be severely stunted with few pods developed or the plants may die.

The most definitive characteristic of SCN is the presence of the swollen female nematode on the roots. If the plant is dug from the soil (not pulled up) and the soil carefully removed from the roots, the whitish, cream colored to yellowish, lemon shaped nematodes (about 1 mm diameter) on the root surface are visible. They are smaller than nitrogen-fixing nodules. In light soils they are usually easy to observe, while in heavy soils it is more difficult to remove the soil without damaging the nematode. At high populations of nematodes it is easier to find them on the roots than at low populations. The swollen live females are usually observed from about mid June to mid August. Later in the season the female nematodes turn into dark brown cysts and it is harder to see them and they are easily detached from the roots when removing the soil. The roots of SCN infected plants are usually dark, necrotic and stunted and there is poor nodulation.

LOSSES

Losses of 15 to 30 percent have been reported even when there are no obvious above ground symptoms. In southern Minnesota, about 20 percent of the soybean acreage on selected farms in 1989 was estimated to lose 10 bushels/acre or more due to SCN. When the nematode is abundant, losses can exceed 20 bushels/acre. If nematode populations are extremely high and the environment is conducive to disease development, almost no crop will be produced.

SCN LIFE CYCLE AND DISEASE DEVELOPMENT

The soybean cyst nematode overwinters in the soil as eggs within the female carcass. When eggs hatch, a second-stage larva emerges and moves a short distance through the pores in the soil to the root tips of soybean. The roots release chemicals that attract the nematode. The nematode has a hollow stylet (mouthspear) that is used to penetrate the root tip cells and allows the larva to enter the vascular tissue. A specialized feeding site is established in the root and the nematode becomes sedentary then enlarges and molts three times before becoming an adult. The female adult enlarges and her body breaks through the root tissues becoming visible on the root surface, but she remains attached to the feeding site. The first adult females may be visible by June and will be observed throughout the summer, but their numbers decline in the late summer and fall. The male adults become slender roundworms and thus are not visible to the unaided eye.

The female produces hundreds of eggs and some are deposited outside her body. Those eggs can hatch and start the life cycle again resulting in new infections of roots. In southern Minnesota, about 3 generations are expected in a favorable year. Many eggs, however, are retained within the female body which upon death becomes a brown cyst. The cyst is a durable structure that protects the eggs and promotes survival in the soil.

SCN populations have historically been classified into different "races" that were characterized by their ability to reproduce on soybean varieties (known as differentials) with three different sources of genetic resistance. Because the variability of SCN populations were not fully described by the race system, a new system of characterizing SCN populations has recently been devised known as the HG type test (HG is for Heterodera glycines ). With the HG type test, SCN populations are characterized by their ability to reproduce on soybean varieties (known as indicator lines) with seven different sources of genetic resistance. After a SCN population has been characterized using the HG type test, a grower can determine which sources of resistance to grow that would minimize the buildup of SCN in a particular field. SCN populations present in Minnesota and South Dakota have not been characterized using the HG type test at the current time; however, they have been characterized using the old race system. Races 1, 3, 5, 6, and 14 are present in MN, with race 3 being the most common throughout the state. Race 3 is also present in SD.

The nematode feeding in the roots disrupts the normal physiology of the soybean plant, and results in disease. Roots become very susceptible to other root rots, they have less nitrogen fixing nodules, and are less capable of taking up water and nutrients. The severity of SCN damage is dependent on the population of the nematode, the soybean cultivar, weather conditions, soil conditions, and cultural practices. Warm, drier soils, and sandy soils usually favor disease development. Symptoms tend to be more pronounced when plants are under stress from drought, low soil fertility, soil compaction or other types of damage. Since many factors, especially growing conditions, affect disease, the amount of damage each year can vary substantially.

SCN is easily spread from infested to non-infested fields through contaminated field equipment, in wind blown or water carried soil, by animals or by soil "peds" (small clumps of soil) in seed harvested from infested fields.

MANAGEMENT OF SCN

Preventing the infestation of fields should be the first step in managing this pathogen. Stopping or slowing the spread of SCN can prevent losses in the following years.

Early identification of SCN is important to developing an SCN management plan for profitable soybean production. Management of SCN has two goals: Preventing the infestation of fields by SCN and reducing the nematode populations in infested fields. In addition, the use of cultural practices, such as adequate soil fertility, and reducing plant stress from weeds, insects, etc, to promote good growth of soybean will reduce the damaging effects of SCN. Research results have been variable when comparing the effects of conventional versus reduced or no-tillage operations on SCN survival or disease development; therefore, there are no recommendations on tillage. No-till, however, may slow the spread of SCN in a newly infested field.

If some acreage is infested with SCN, always work non-infested fields before the infested fields to reduce the risk of carrying SCN between fields. Equipment used in infested fields should be thoroughly cleaned of soil before entering non-infested fields. Avoid using seed harvested from an infested field because the nematode could be in soil "peds". Also, avoid 'bin-run' seed. Use tillage or other practices that reduce the wind or water transport of soil and debris from infested to non-infested fields.

Determine SCN population ( more information- )

Determining the population of the nematode in the soil is the first step in managing SCN. This is accomplished by soil sampling and sending the soil samples to a qualified laboratory that can determine the number of nematode eggs in the sample. You cannot estimate the population of SCN in a soybean field by looking at the presence of the nematode on the roots. Soil sampling in the fall following a soybean crop is a good time to take samples. Use a soil probe or small trowel and collect 20 soil samples 6-8 inches deep (about 1 cup per sample) in a zig-zag pattern every 10 to 20 acres. Collect samples from areas of similar soil type or crop history or possibly only sample areas where you suspect SCN problems. Combine the samples from every 10 to 20 acre set in a bucket and mix very thoroughly. Place about 1 pint in a plastic bag, label the samples and store in a cool, dark place until shipped to a lab. Include your name, address, phone number, sampling date, acres sampled, crop history, location of each field and indicate the sample numbers and labels. A list of laboratories that determine SCN populations is included in this bulletin. At low nematode populations (below 200 eggs/100 cm3 of soil) it is difficult to detect eggs in the soil. A soil sample that is negative for SCN, therefore, does not guarantee there is no SCN. If SCN is reported in your immediate area, keep monitoring your fields for SCN. Maintain records of egg populations to determine the best rotations for your farming operations.

Reducing SCN populations (more information- )

Crop rotation to non-hosts and the use of host resistance are the principal methods of reducing nematode populations. Examples of non-host crops are small grains, corn, sunflower, sugar beet, flax, potato, alfalfa, forage grasses, sorghum and canola. Field pea is considered a poor host, thus may be used in a rotation. Dry beans are hosts of SCN, and should not be in a rotation (see list of hosts). Effective weed control is also necessary because there are some weed hosts. Research from adjacent states suggests that a three to four year rotation to non-host crops will reduce moderate levels of nematode populations below the economic threshold. Heavily infested fields might require 4 or more years in rotation to non-hosts. Before a decision is made to plant susceptible soybeans again, sample the soil and have it analyzed for the egg population.

SCN resistant soybean cultivars can also be used in the rotation. Resistant soybeans allow growers to produce a soybean crop and either reduce or not allow the SCN population to increase. In Minnesota, resistant soybeans have performed well in soils with egg counts up to 2,000 eggs/100 cm3 of soil. At higher populations, resistant soybeans might be damaged by SCN. Resistant cultivars, however, may not be the best yielding cultivars, therefore, only use them when SCN populations are too high for growing susceptible cultivars. With low infestations of SCN your high yielding susceptible cultivar may still yield more than a resistant cultivar. However, at the end of the season the egg counts may be to high to grow another susceptible cultivar the following year. This is where soil sampling to determine egg counts is needed to make cultivar decisions.

An example of a rotation for an infested field might be: year 0 - identification of SCN; yr 1 - non-host crop; yr 2 - non-host crop; yr 3 - resistant soybean; yr 4 - non-host crop; yr 5 - susceptible soybean (if egg population is below threshold - see following paragraph). Rotations using non-hosts and resistant soybeans can be tailored to the infestation levels of each field. There are few SCN resistant soybeans adapted to North Dakota, but such cultivars are under development by both public and private breeders. Some cultivars reported to be resistant to SCN which could be used in ND are listed below. To prevent shifts in SCN races within a field, alternate sources of resistance when continually using resistant soybeans in a rotation.

This list may not be complete. Mention of a tradename or product does not constitute recommendation by North Dakota State University or the NDSU Extension Service.

Thresholds (more information- )

Research from adjacent states suggests that 100 to 150 eggs per 100 cm3 of soil can cause some yield loss in susceptible soybeans. Yield loss increases as egg counts increase. Start a rotation to non-hosts or grow resistant soybean when the egg counts are near this level. Remember that growing a susceptible soybean in a field with low eggs counts may not cause high yield losses, but the high reproduction of the nematode on that cultivar can result in greater than a 5 to 10 fold increase in the egg population by the end of the season. That results in a greater SCN population that must be managed for the following years. Fields with high egg counts (> 2,000 eggs/100 cm3 of soil) should be rotated first to non-hosts to reduce SCN levels before planting a resistant soybean. Although resistant soybeans can be grown in fields with high egg counts (2,000-5,000 eggs/100 cm3 soil), SCN may cause some yield loss. Growers need to balance the need for high yields against the need to manage the SCN population.

Nematicides

Nematicides are not commonly recommended to control SCN in the northern soybean growing areas. In general, they are not as effective or as economical as other control methods. Nematicides can provide early season protection, but not season-long control. Many factors such as soil type, and weather affect the results obtained from nematicides. Always determine if chemicals are registered for use in your state before attempting to use on a crop.

HOST PLANTS FOR SCN (only a partial listing of hosts)

LABORATORIES THAT ANALYZE SOIL FOR SCN

These laboratories are close to North Dakota; there are others in the Midwest. Call and request information on cost, time for sample analysis, and any information or payment that may be required before processing the samples. Ask specifically if egg counts are performed. Some laboratories may count cysts or eggs. Request egg counts. Generally laboratories will express the results as number of eggs per 100 cc (cubic centimeters) of soil. Some labs will express the results as eggs and juvenile larva per volume of soil.

Acknowledgment: The authors thanks Drs. Senyu Chen, Len Francl and A. Lamey for helpful comments.

This bulletin was originally published in 1999 and revised in May 2003.

Photo credits: Field symptoms and resistant and susceptible soybeans courtesy of J. D. Smolik; nematodes on roots courtesy of G. Tylka; white females and cysts together courtesy of T. Niblack; other photos by the B. Nelson.