Soybean cyst nematode management guide
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- Where is the soybean cyst nematode (SCN) found?
- How important is the SCN?
- What is the SCN? What is its life cycle?
- How does the SCN damage soybeans? What symptoms does it cause?
- How do SCN populations change over time?
- How does the SCN spread?
- How do I know if I have the SCN in my fields?
- How do I take soil samples for detecting the SCN?
- How often, when, and how should I take soil samples for SCN counts to help make management decisions?
- What are HG Types?
- How can I manage the SCN?
- How should the SCN-resistant varieties be used?
- How can crop rotation be used to manage the SCN?
- Can spreading of the SCN be prevented?
- How important is crop health management for minimizing SCN damage?
- Do chemical and biological controls have potential in SCN management?
- What are scientists projecting for the future of SCN?
Where is the soybean cyst nematode found?
The soybean cyst nematode (SCN), Heterodera glycines, has been found in most soybean-producing areas in the world. The SCN was first found in North America in North Carolina in 1954 and since then has spread to at least 31 soybean-producing states (Figure 1) and Canada. In Minnesota, SCN was first detected in 1978 in Faribault County. By 2010, its presence had been confirmed in 64 counties in the state (Figure 2).
How important is the soybean cyst nematode?
SCN is the most destructive pathogen of soybean in the United States. Annual yield losses in soybean due to SCN have been estimated at more than $1 billion in the U.S. Because the nematode can be present in fields without causing obvious aboveground symptoms, yield losses caused by SCN are often underestimated.
Yield losses caused by SCN can vary from year to year, and are influenced by soybean variety, climatic conditions, and soil biotic and abiotic factors. In heavily infested fields, SCN can cause soybean yield losses of more than 30 percent, and in some sandy soils complete yield loss can occur, especially in a droughty year. In addition, SCN can also infect dry beans and snap beans, and cause significant yield loss to these crops.
What is the SCN? What is its life cycle?
The soybean cyst nematode is a microscopic roundworm that attacks roots of soybean and a number of other host plants. The life cycle of SCN includes the egg, four juvenile stages, and adult stage (Figure 3).
The first-stage juvenile develops within the egg and molts to form a second-stage juvenile (J2). The J2 hatches from the egg, moves through soil pores in the film of water surrounding soil particles, is attracted to actively growing roots, and infects by penetrating the host plant root, usually near the root tip. After penetrating the root, the nematode establishes a feeding site in the vascular tissue, where it becomes sedentary. It then enlarges to become sausage-shaped, and molts three more times before becoming an adult.
The adult female is lemon-shaped. When fully developed, the female's body protrudes outside of the root and is visible without magnification. The adult male undergoes a metamorphosis during the last molt to become a slender, motile worm. The mature male stops feeding and exits the root.
A pheromone released by the female attracts the male for mating. The female exudes a gelatinous matrix from the posterior portion of its body into which it deposits a small portion of the total eggs that it will produce. The gelatinous matrix containing eggs is referred to as an egg mass. Eggs in the egg mass hatch, and the resulting juveniles infect soybean roots the same year they are produced.
Several hundred additional eggs are retained inside the female body. As the female ages, its body changes color from white to yellow. When the female dies, the body (now referred to as the cyst) changes color to a dark brown. The cyst protects the eggs from damage by environmental stresses and serves as the over-wintering and long-term survival structure for the nematode eggs. In addition to the protection afforded by the cyst, the egg itself is durable and resistant. Some eggs within the cyst have been shown, under laboratory conditions, to be able to survive for more than 9 years before hatching.
The length of the SCN life cycle is typically about 4 weeks depending on geographic location, soil temperature, and nutritional conditions. Optimal soil temperatures are 75°F for egg hatch, 82°F for root penetration, and 82-89°F for juvenile and adult development. Little or no development takes place either below 59°F or above 95°F.
In southern Minnesota, SCN can complete three to four generations during a soybean-growing season. In central to northern Minnesota, the nematode probably completes only three generations.
How does the soybean cyst nematode damage soybeans? What symptoms does it cause?
Soybean cyst nematode infection causes damage to plants not only physically by penetrating and moving through the roots, but also physiologically by altering the metabolism of the root cells surrounding the nematode. These modified root cells, called syncytia, produce the nutrients needed for the nematode's growth and development. SCN infection also can induce secondary infection by one or more microbial pathogens resulting in a disease complex. As a result, function of soybean roots is reduced, and the soybean plant may show nutrient deficiency symptoms.
Figure 4. Chlorotic and stunted soybean plants in plots with 17,050 eggs (left four rows) and healthy soybean in plot with 175 eggs/100cc of soil (right three rows).
"Yellow dwarf" is an appropriate description for symptoms that are commonly caused by SCN. When soybean plants are severely infected, the plants become stunted, canopy development is impaired, and leaves may become chlorotic depending on soil and weather conditions (Figure 4).
Figure 5. The interaction of SCN with high soil pH resulting in iron-deficiency chlorosis. Pot on left of each image was infested with 10,000 eggs/100cc of soil, and pot on right had no SCN.
In Minnesota, iron-deficiency chlorosis (IDC) is a common problem that may be induced or made more severe by SCN infection in high pH soil (Figure 5). Similarly, SCN may induce potassium-deficiency symptoms in soils with low potassium levels. Unfortunately, these symptoms are caused not only by SCN. Other stresses such as actual nutrient deficiencies, injury from agricultural chemicals, feeding of the soybean aphid, and infection by other plant pathogens can cause similar symptoms.
SCN populations are not evenly distributed throughout fields. Areas of severely affected and symptomatic soybean plants are often round or elliptical in shape. Those heavily infested areas are often elongated in the direction of tillage due to localized mechanical spread of cysts by tillage equipment. These uneven distributions are often observed in a field where the nematode was recently introduced and a field with various soil types.
SCN infection may limit nodulation by nitrogen-fixing bacteria. Because SCN damages roots and limits nutrient uptake by the soybean plants, iron, potassium, and nitrogen deficiencies may increase in severity. Severely infected plants may die before flowering, especially during dry years in soils with poor water holding capacity.
Good soil fertility and adequate moisture increase tolerance of soybean plants to SCN and reduce the severity of aboveground symptoms. Producers may not realize that SCN is present in highly productive fields. Environmental stresses can accentuate the effects of large SCN populations that have developed during previous growing seasons.
Belowground symptoms include dark-colored roots, poorly developed root systems, and reduced nodule formation. SCN infection may increase susceptibility of plants to microbial pathogens by altering plant metabolism or by creating wounds for other pathogens to enter the plant. Several important diseases including brown stem rot, sudden death syndrome, and other fungal root rots of soybean are associated with or increased in severity by the presence of SCN.
How do populations of SCN change over time?
Figure 6. Seasonal change of SCN egg population densities in plots planted with susceptible soybean and in fallow in a Minnesota field in 2008.
SCN population density is affected by a number of environmental factors as well as host status. The most important environmental factor is probably the temperature. Consequently, seasonal changes in SCN population densities vary in different geographic locations.
Figure 7. Relationship of SCN egg population density at harvest with number of years of corn following SCN-susceptible soybean during 1996-2004 in a field in southern Minnesota.
In Minnesota, after the soil has thawed and temperature increased in April, second-stage juveniles (J2) start to hatch from eggs. After planting soybean, J2 hatch increases due to chemical stimulants from soybean roots. Egg population density in soil declines gradually due to the hatch of J2 until late June to early July when the females of the first generation become mature and produce eggs, and egg population density starts to increase in SCN-susceptible soybean. From late July or early August to the end of the season, SCN egg population density can increase rapidly (Figure 6).
Egg population densities in susceptible soybean at harvest can be as low as a few thousand to as high as tens of thousands per 100cc of soil (Figure 7). Average annual reduction of egg population density in nonhost corn plots is about 50 percent. It takes about 5 years to lower the egg population density from 10,000 to approximately 300 eggs/100 cc of soil, a level at which there is limited or no damage to soybean.
How does the SCN spread?
SCN can move only a few centimeters in the soil by itself. However, soil containing cysts and eggs can be moved long distances within a field or between fields by any means that moves soil. Soil movement in runoff water, by wildlife, and in windborne dusts are all natural mechanisms that spread SCN. Since the nematode cysts can survive passage through a bird's digestive system, birds may spread SCN over long distances. Human activities that move soil between fields on equipment, tools, and vehicles are probably the primary means by which SCN spreads. Seed contaminated with soil peds infested with SCN is another way SCN can move long distances.
In Minnesota, SCN has been reported throughout soybean-producing areas in the southern and central regions of the state. In recent years, the nematode has been found in several counties in the northern soybean-growing areas in Minnesota. Thus, the colder climatic conditions will not limit the spread of the nematode in the Red River Valley where soybean production is increasing. Indeed, frequent flooding in the Red River Valley may favor rapid spread of the SCN in that area.
How do I know if I have SCN in my fields?
In Minnesota, SCN has been found in most (64) soybean-growing counties. However, 50 percent of soil samples near-randomly collected from soybean fields throughout the soybean-growing area in Minnesota in 2007-08 were not infested with SCN or had undetectable low SCN population densities. While most fields in southern Minnesota are infested by SCN, a large proportion of fields in northern Minnesota may have no or low SCN infestation.
Early detection is important for managing SCN and minimizing yield loss to the pest. To determine whether your fields have an SCN problem and how severe it is, you may need to look for any plant symptoms in the field, scout females (cysts) on roots, monitor soybean yield, and/or have soil samples tested for the presence of SCN eggs.
Stunting and chlorosis are typical symptoms of soybean induced by SCN (See Section 4). However, SCN can cause yield loss in the absence of visible symptoms. Declining yields from a field or portion of a field are sometimes the first clue that SCN could be causing a problem. Subtle signs such as areas of uneven soybean heights, slow row closure or expanding, or out of place nutrient deficiency symptoms may also be clues to SCN infestation.
The most accurate diagnosis of an SCN problem is to find the nematode on plants or in soil. The unique, diagnostic sign of SCN infection is the living mature female nematodes or cysts attached to roots. These tiny, lemon-shaped, white to yellow females usually can be seen on the roots beginning 4 to 5 weeks after planting. The cysts on roots are usually abundant in July and August and then decline in numbers as roots senesce. Visible females on the roots increase and decrease as generations of SCN are produced.
Plant roots to be examined for the presence of females need to be gently dug rather than pulled from the soil to prevent loss of the cysts. Gentle rinsing of soil from the roots in a bucket of water will help reveal their presence. Adult females and cysts are about 1/40 inch long and 1/60 inch wide and are large enough to be seen with the unaided eye (Figure 8). They are easily distinguished from the much larger bacterial nodules on the roots.
Females may be difficult to find on roots when the SCN population density is low, when sampling is done too early or too late in the growing season, or when the SCN population density is extremely high. In the latter case, soybean roots that are severely damaged due to the actions of the nematode and associated microorganisms will no longer be capable of supporting SCN. Under such circumstances, analysis of soil samples from suspect fields by a professional laboratory may be necessary to detect the presence of SCN.
How do I take soil samples for detecting SCN?
Soil sampling is an efficient way to determine if SCN is present in a field when SCN is suspected but cannot be observed on roots. This type of sampling can be done at any time of the year when the physical conditions of the soil will permit use of a soil sampling tube or, less desirably, a shovel.
A sample consisting of 10 to 20 one-inch diameter by 6-8 inch long cores should be collected from each of several localized "most likely" sites in a field. One "most likely" site would be a short distance from entry point where tillage equipment enters the field because contaminated equipment is the number one method by which SCN is introduced into a property. Infested soil scoured from equipment at such locations allows SCN buildup to begin there. Another "most likely" site would be on the leeside of a small hill or knoll, or along a fence or tree line. Wind-blown soil containing cysts tends to settle out at such locations much like wind-driven snow accumulates behind a snow fence or similar obstacle. A third "most likely" sampling site would be low areas that flood when streams overflow. Egg-filled cysts can be carried and spread by moving water.
In addition, soil cores should be collected from in-row locations rather than from between crop rows that are 15 inches or more apart because nematode populations are much more likely to be larger in soybean rows than they are between the rows where plant roots are scarce.
Soil samples enclosed in individual sealed plastic bags should be submitted to a professional laboratory for processing. Although the dark brown cysts can be seen with the unaided eye, they are very inconspicuous when mixed with soil. The laboratory will use a procedure to "float" any cysts out of a soil sample. Eggs, if any, will be released from the cysts, and counted. The lab's report will report number of eggs per 100 cm3 (approximately one-half cup) of soil.
How often, when, and how should I take soil samples for soybean cyst nematode counts to help make management decisions?
Fields infested with SCN should be managed to minimize yield loss. In order to manage SCN populations effectively, it is important to monitor SCN populations over time. Distributions of SCN are generally uneven in most fields, and nematode egg numbers can vary with sampling technique. So, to minimize the variability for a representative SCN egg count, it is very important to use recommended sampling procedures:
- Divide a field into 5- to 10-acre areas.
- Use a 1-inch-diameter soil probe to collect soil cores to a depth of 6 to 8 inches.
- Collect soil cores from about 20 different locations in a zigzag pattern for each area to be sampled (Figure 9).
- Place the soil cores in a plastic bag.
- Label the sample with field identification using a water-resistant marker.
- Store the samples at a cool temperature if they cannot be sent within a few days to a professional laboratory for analysis.
- If a soil sample is used for both SCN and soil fertility analyses, mix the soil sample thoroughly before sending subsamples to different laboratories.
There are a number of factors that contribute to the variability of egg numbers from soil samples. SCN eggs are deposited in a cluster, and the spatial distribution of SCN in many fields is an aggregated pattern. In some fields, soil cores may contain high egg numbers from hot spots and low, even zero, egg numbers from non-infested or recently infested areas.
Increasing the number of soil cores collected in each 10-acre area or reducing the size of the area for each sample can increase the precision of the sample. If the hot spots in the field cannot be managed separately from the rest of the field, the best option is to manage the entire field according to the higher population density.
The efficiency of extracting SCN from the soil is dependent on soil characteristics such as texture and moisture content at the time of sampling. Some variability may be associated with the actual laboratory processing of the sample, leading to a rough estimate of the average SCN population rather than an exact measure. However, an effective management program can be implemented using the rough estimate of the average SCN population in a field.
There are other reasons why SCN population densities may vary in two soil samples taken from the same field. SCN egg counts will be highest if samples are collected in the soybean row at the end of the growing season. Due to the variability, it is difficult to compare SCN samples taken from a field at different areas and times of the year.
Long-term SCN management based on soil samples is best done with a sampling plan that is consistent in area(s) sampled, crop sampled, and time of year samples are taken. Since SCN egg population densities are reduced during a year when a nonhost crop is grown, SCN egg counts from samples taken after corn harvest, but before soybean planting, are the most useful in estimating potential soybean yield loss. Sampling in the fall rather than spring allows more time for the soybean producer to develop an appropriate SCN management plan.
What are HG types?
SCN field populations vary in their ability to develop and reproduce on soybean lines that differ in their resistance to SCN. The variability of SCN virulence is described by HG Type schemes, in which the virulence phenotypes of SCN populations are determined by the number of females that develop on seven indicator lines as compared with susceptible Lee 74 or other suitable susceptible soybean varieties.
Table 1. Indicator lines for HG Type classification of soybean cyst nematode.
|Number||Indicator Line||Example: HG Type 2.5.7|
The soybean lines and varieties are inoculated with nematode eggs and maintained in the greenhouse under favorable conditions for about one month. The females formed on the soybean roots are collected and counted. Based on the number of females, Female Index (FI) is calculated:
FI = (number of females on indicator line) × 100 / (number of females on Lee 74).If the FI is less than 10, the response of the soybean line is "–", and if = 10, the response is "+". The description of HG Type indicates the positive response of a population on the individual lines (Table 1). If no FI is more than 10 on any of the indicator lines, the population is described as HG Type 0.
Which HG types are found in Minnesota?
The frequency distribution of HG Types (percentage of fields with an HG Type) varies in different regions in the United States. In two previous surveys conducted in 1998 and 2002, SCN populations in most Minnesota fields were HG Type 0 or 7 (Table 2), which have a low level of virulence on the current commercial resistant varieties. The frequency of virulent populations in the state may change over time in response to planting SCN-resistant soybean varieties.
Table 2. Percentage of SCN populations from Minnesota with Female Index more than 10 on the indicator soybean lines.
Figure 10. Relationship between reproduction potential (Female Index) of SCN on 'PI 88788', 'Freeborn', and 'Peking' after the use of the SCN-resistant soybean Freeborn for various years during 1996-2007 in a Minnesota field infested with an original population of HG Type 0 (race 3).
In a field plot experiment, SCN reproduction potential on the resistant soybean variety Freeborn and its resistance source PI 88788 increased with increasing years of growing the variety (Figure 10). After 5 years, the population changed from the original HG Type 0 (race 3) to a population that was able to overcome the resistance of PI 88788 (FI > 10; HG Type 2.5.7). After 10 years, Freeborn that had been moderately resistant (FI ≈ 15) to the original population became susceptible (FI > 60) to the resulting SCN population.
Such a change of virulence phenotypes may occur in other fields where resistant varieties have been planted for a number of years. Across Minnesota, the percentage of virulent populations on the resistance source lines PI 88788 and Peking increased dramatically from 2002 to 2008 (Table 2). Approximately 20 percent of fields in southern and central Minnesota have SCN populations with FI on PI 88788 more than 30, to which PI 88788 varieties are no longer effective. In a few fields (about 2%), the SCN FI are high (>30) on both PI 88788 and Peking. The results of these studies convey a warning that more soybean varieties with alternative sources of resistance are needed for effective long-term management of the nematode in the state.
MN HG type test
Based on field observations and recent surveys, SCN populations in many Minnesota fields have become virulent to soybean varieties carrying resistant genes from PI 88788 and/or Peking. If a resistant variety yields poorly or a field has been planted with the same resistant variety or varieties with the same resistance source (PI 88788) for a number of years (e.g., more than 5 years), it is recommended to have the HG Type in the field evaluated.
Scouting females (cysts) on the soybean roots in field and testing egg population density after harvesting the resistant variety are also useful methods of determining the reproduction potential of the nematode population on the resistant variety planted in the field.
A complete HG Type analysis including seven indicator lines is time-consuming and costly. To reduce the cost, we recommend only including Peking and PI 88788 because most current SCN-resistant varieties are developed from PI 88788 and a few from Peking. Although a few varieties with PI 437654 source of resistance are available in Minnesota, we can exclude PI 437654 from the MN HG Type test because none of the SCN populations in Minnesota could reproduce well on it (FI are 0 to 8.8 with the average only 0.4) based on the soil samples collected in 2007-08. Varieties with PI 437654 source of resistance should be effective in lowering SCN population densities in fields.
Besides the designation of HG Type, the Female Indexes on individual lines will be reported (Table 3). Soil samples can be submitted to a professional lab (e.g., the Nematology Lab, Southern Research and Outreach Center in Waseca) for a MN HG Type test.
Table 3. Example of MN HG Type.
|Indicator line||Females/Plant||FI||HG Type|
|PI 88788 (2)||19||4.0|
How can I manage the soybean cyst nematode?
Photo: Andrew J. Baumgartner
Figure 11. SCN-susceptible (left) and resistant (right) varieties in plots infested with high SCN egg population densities (17,400 eggs/100cc of soil left, 10,500 right).
Figure 12. Guideline for SCN management based on SCN egg population density, and a soybean-nonhost annual rotation system, which is a common practice in Minnesota. For most fields, 5 years of SCN-resistant soybean and nonhost are needed to reduce the SCN egg population density to low enough (< 200 eggs/100cc of soil) for an SCN-susceptible soybean. In some fields, 3 years of SCN-resistant soybean and nonhost (brown arrow) may be sufficient.
There is no way to eliminate SCN once it is present in a field. Instead, the goals of SCN management are to:
- minimize yield losses
- reduce SCN population density
- maintain yield potential of resistant varieties with an integrated approach
Currently, the most effective SCN management practices are:
- use of resistant varieties (Figure 11)
- rotation to nonhost crops (Figure 12)
You can take these steps, which provide the information necessary for making SCN management decisions:
- monitor yield yearly
- scout for symptoms
- take soil samples to determine SCN egg densities
How should the SCN-resistant varieties be used?
Many SCN-resistant varieties in Maturity Groups II and I and a few in Maturity Group 0 have been developed and are available for Minnesota soybean producers. Performance of a resistant variety in an SCN-infested field depends on the genetics of both the soybean and the nematode.
Figure 13. Average yield of top ten SCN-resistant varieties (R) and top ten susceptible varieties (S) in SCN-infested and non-infested fields in 1999-2002.
In the past, resistant varieties produced 5-10 percent less yield than susceptible varieties when both were grown in the absence of the nematode (Figure 13). Although current elite, high-yielding susceptible varieties may still outperform current resistant varieties in fields where there are no soybean cyst nematodes or fewer than 200 eggs/100cc of soil, the yield potential of resistant varieties has been improved, and some elite resistant varieties have fairly high yield.
Most (˜95%) SCN-resistant varieties are developed from the single source of resistance PI 88788, and a few from Peking and PI 437654. Repeated use of the same resistant variety or continuous use of varieties with the same resistance source may eventually lead to SCN populations that can overcome resistance from the common source. Consequently, soybean varieties with resistance genes from different sources should be alternated to slow changes in HG Type composition and increase effectiveness of resistant varieties.
Figure 14. Levels of resistance to HG Type 0 of commercial soybean varieties labeled as SCN-resistant in 2010.
If resistant varieties have been used in a field for a number of (>5) years, the HG Type should be determined to make sure the varieties are still resistant to the population. Different commercial SCN-resistant varieties have different levels of resistance (Figure 14). Check data of the varieties tested in the greenhouse and local fields, and make sure the variety you will use has a sufficient level of resistance to the SCN population in your field.
Not all the varieties labeled as SCN-resistant are resistant (Figure 14). Data on SCN resistance and yield potential is available as part of the University of Minnesota Soybean Breeding Project's contribution in the annual Soybean Field Crop Trials and at the University of Minnesota Southern Research and Outreach Center website at sroc.cfans.umn.edu.
Table 4 offers guidance for selecting varieties to manage SCN based on resistant level of a variety and HG Type of SCN from the field. Yield potential is certainly the most important criterion in variety selection.
Table 4. Determine whether an SCN-resistant variety can be used in a field based on the source and level of resistance of the variety and HG Type of SCN population in the field.
FI of HG Type 0 on a variety (Variety resistance)
|FI of SCN from the field on the source of resistance of the variety (HG Type)|
|< 10||10-30||> 30|
|< 10||Low risk||Moderate risk||High risk|
|10-30||Moderate risk||High risk||Very high risk|
|> 30||High risk||Very high risk||Very high risk|
How can crop rotation be used to manage the soybean cyst nematode?
Crop rotation is used not only for SCN management, but also to benefit general crop management. Although SCN has a wide range of host plant species, only a few crops are its hosts (Table 5). Many crops, including alfalfa, barley, corn, oat, potato, sorghum, sugarbeet, sunflower and wheat are not hosts for SCN and could be included in a crop rotation to reduce SCN population densities (Table 6).
Table 5. Hosts of soybean cyst nematode.
|Crop plants||Weed plants|
|Common and hairy vetch||Common chickweed|
|Soybean||Milk and wood vetch|
|Sweet clover||Mouse-ear chickweed|
|White and yellow lupine||Wild mustard|
Table 6. Poor hosts and nonhosts for SCN management rotation.
|Barrel medic||Flax||Sugar beet|
|Brassica cabbages||Marigolds||Sunn hemp|
|Corn||Red clover||White clover|
The number of years of nonhost crops needed to effectively lower SCN population density depends on many factors, including initial egg density, and soil biotic and abiotic factors that affect nematode mortality. In Minnesota, SCN survives well during winter, and with high populations after a susceptible soybean it may take as long as 5 years, depending on initial egg population density and soil environments, of nonhost or poor-host crops to reduce the SCN population prior to planting below a density (e.g., ~200 eggs/100cc of soil) that will not damage a susceptible variety (Figure 12).
Some leguminous crops such as pea, sun hemp, and Illinois bundleflower are poor hosts that produce SCN hatch stimulants and are more effective in lowering SCN population density than monocots including corn and wheat. Some crops such as marigolds and sunn hemp may produce compounds that have nematicidal effects.
In most cases in Minnesota where soybean is frequently grown, the short period of rotation with nonhost crops is not long enough to lower the egg population densities below levels that cause yield loss, and resistant varieties must be used to reduce yield loss.
Photo: Andrew J. Baumgartner
Figure 15. The effect of two soybean cyst nematode population densities on the resistant soybean variety 'Freeborn.' Plant growth was significantly suppressed in the two left rows where SCN egg population densities prior to planting were 35,500 eggs/100cc of soil. The two rows on the right were planted into soil with 3,500 eggs/100cc of soil.
Resistant varieties should be used at egg counts between 200-10,000 eggs/100cc of soil. SCN at a high density (> 10,000 eggs/100cc of soil) can cause a significant yield loss (> 2 bu/a) even to a resistant variety (Figure 15). Consequently, fields with SCN population densities at or above 10,000 eggs/100cc of soil should be planted to a nonhost crop for one or more years until the population densities drop below that level. If the egg number is reduced sufficiently by the rotation of nonhosts and resistant varieties, a susceptible soybean can be used.
In some fields, because the soil is suppressive to SCN, 3 years of SCN-resistant soybean and nonhost (Fig. 12, brown arrow) may be sufficient to reduce the SCN population to a low level, and susceptible soybean can be considered. However, SCN population density should be determined before planting an SCN-susceptible soybean.
If nonhost crops are planted in rotation with soybean for sufficiently long periods such as in many organic-farming fields, SCN populations can be lowered to less than damaging levels (< 200 eggs/100cc of soil). At low SCN population densities, susceptible varieties can be considered to help avoid or slow down the development of SCN populations that may overcome resistance.
Can spreading of the SCN be prevented?
It is important to take measures to prevent or slow down spread of SCN to areas where the nematode has not been found. This is especially important in the Red River Valley where SCN was recently introduced, and there are a limited number of infested fields.
If SCN is found in only a few fields in an area or county, the best option is to plant these fields to nonhost crops to slow down spread of the SCN to other fields in the area. Seed contaminated with soil peds from infested fields should be avoided. On farms where both infested and uninfested fields have been identified, farm equipment (Figure 16) should not be used in uninfested fields until contaminated soil has been thoroughly removed by steam cleaning. While sanitation delays the spread of the SCN, it probably cannot prevent the spread.
How important is crop health management for minimizing soybean cyst nematode damage?
Figure 17. Yield difference between SCN-susceptible (S) and resistant (R) soybean varieties was not significant in manure-applied soil, but there was a big difference in the soil without a manure application.
It is important to maintain proper soil fertility, provide good drainage, and control other diseases, insects, and weeds. Insurance pesticide applications, however, are not an effective part of SCN management. Appropriate cultural practices such as maintenance of good soil fertility may enhance plant growth, increase tolerance of plants, and minimize yield loss to SCN (Figure 17).
In Minnesota, no-till or reduced tillage does not reduce or has a limited effect on SCN egg population density. In fact, conventional tillage may improve early season root development, and reduce damage to soybean caused by SCN. These practices, however, do not reduce SCN population density in a field. To limit the growth of SCN populations, they must be integrated in a management program with a rotation of nonhost crops and resistant varieties.
In some fields, SCN management is complicated by the presence of microbial pathogens and nutritional deficiencies. For example, if chlorotic symptoms are observed in a field planted with an SCN-resistant variety, root rot disease and/or nutrient deficiency (such as iron deficiency) may be involved. In this case, action should be taken to identify and manage all of the crop stresses.
Do chemical and biological controls have potential in soybean cyst nematode management?
Photo: Andrew J. Baumgartner
Figure 18. An SCN second-stage juvenile parasitized by the fungus Hirsutella minnesotensis.
Some nematicides are registered for use in soybean. A few nematicides are effective in lowering SCN population density, but their performance depends on many soil and environmental factors including soil type, rainfall, soil moisture, temperature, and soil microbial activities. Use of nematicides adds significantly to production costs and does not guarantee increased yields. Economics, as well as environmental and personal health concerns, should be considered before using nematicides. For these reasons, nematicides are not commonly recommended for SCN management.
Although there is no widely accepted commercial biological control agent for SCN management, biological control should be considered as part of an integrated management program. Soybean cyst nematode is subjected to attack by a wide range of natural enemies including fungi, bacteria, predacious nematodes, insects, mites and other microscopic soil animals. The species and activities of natural antagonists vary in different fields. In some soybean fields in Minnesota, high percentages (more than 60%) of the SCN second-stage juveniles are parasitized by the fungi Hirsutella minnesotensis and/or H. rhossiliensis (Figure 18).
SCN population densities are relatively low in some soils due to biological factors, and these soils are known as nematode-suppressive soils. Some cultural practices may enhance the activities of nematophagous fungi and suppress nematode population densities. For example, monoculture of susceptible soybean for a number of years may increase parasitism of the nematodes by microbial pathogens, and the soil in the field becomes suppressive to the SCN population.
With SCN population densities reduced by natural antagonists, the required time for planting nonhost crops and resistant varieties can be reduced, yield of resistant and susceptible varieties increased, development of virulent HG Type slowed and/or effectiveness of resistant varieties maintained.
What are scientists projecting for the future of soybean cyst nematode?
Photo: David L. Hansen
Figure 19. Field plots of the soybean cyst nematode and soybean breeding research projects at the University of Minnesota Southern Research and Outreach Center at Waseca.
Soybean production has continued to increase in the past few decades, and it will remain a major crop in Minnesota. Successfully managing SCN will continue to be a key factor for profitable soybean production. SCN management, however, will face serious challenges in the future due to limited sources of resistance, extensive soybean production, and the shift of HG Types.
SCN will continue to spread in Minnesota due to unpreventable natural means and human activities. At the present time, more than 40 percent of soybean fields in Minnesota are not infested by SCN or have an undetectable level of SCN. All of these fields have a high risk for SCN infestation. Early detection of SCN in fields is important to minimize its damage to soybean, especially in the Red River Valley, where SCN was recently detected.
Within the next five years, PI 88788 and Peking will still be the major sources of SCN resistance in commercial soybean varieties. With the extensive use of the SCN-resistant varieties from PI 88788, the frequency of HG Type 2-, and the percentage of the fields with an SCN FI > 30, to which PI 88788 resistant varieties are ineffective, will continue to increase. Within the next few years, the choice for these fields will be to use Peking varieties. Although PI 437654 (CystX) varieties are highly resistant to SCN populations in Minnesota, yield potential of current PI 437654 varieties is still lower than from other sources of SCN resistance.
Peking and PI 88788 carry two distinct types of resistance, and they are good in rotation, at least within a foreseeable period of time. However, it is unclear what the trend of HG Types will be following the rotation of these two sources of resistance. Similarly, it is unclear whether planting Peking varieties in fields having HG Type 2- will change the SCN populations to other Types so that the PI 88788 can be used again or the resulting SCN populations can overcome both PI 88788 and Peking.
Researchers at the University of Minnesota, and other institutions and companies continue to breed for high-yielding soybean varieties with current and new sources of SCN resistance. The development of new, powerful DNA markers and advances in molecular biology will speed up breeding new SCN-resistant varieties. Yield potential of PI 437654 varieties will continue to be improved, and varieties with new sources of resistance will probably be available in 5-10 years.
Long-term effective management of SCN will rely on an integrated program that includes resistant soybean varieties, crop rotation, and possibly alternative strategies such as soil fertility management and biological control. Although it is unclear whether or not there will be any cost-effective commercial biological control agents on the market in the near future, better understanding of the roles of natural parasites in regulating SCN populations in fields may help to develop strategies to lower SCN populations through practical cultural methods.
Published February 2012