Revised September 2006
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Figure 1
Your field could look like this, yet you could lose 30 percent of your yield to soybean cyst nematode.
Figure 2
Stunted, yellow soybean plants (foreground) are a
sign of infection by soybean cyst nematode.
Soybean cyst nematode (SCN) is the most devastating and yield limiting pest of soybean in Missouri. A three-year study done in the United States estimated that soybean cyst nematode (Heterodera glycines) caused annual losses of $720 million (120 million bushels).
It is often difficult to identify fields with SCN infestations because lower yield at harvest may be the only aboveground symptom (Figure 1). Yields may decrease slowly for a number of years before the reduction becomes obvious. Suspect fields usually have plants of different heights, but environmental conditions may make stunting less obvious.
When SCN is present and plants are under stress, symptoms such as chlorosis (Figure 2), plant stunting, and (in extreme cases) plant death can occur. However, these indicators are similar to those observed with other crop production problems such as nutrient deficiencies, herbicide damage and drought stress.
White (or yellow) females on roots are the only visible sign of SCN infection (Figure 3). From late June through July, plants exhibiting chlorosis or stunting can be dug in the field and the roots examined for the presence of white females or young cysts on the roots.
Figure 3
White cyst of SCN attached to soybean root adjacent to soybean nodule.
Note
Do not rely on visual inspection for diagnosis, because once the cysts
have matured, they turn brown and fall off the roots.
Many fields in Missouri and other midwestern states are now farmed under reduced tillage. Numerous studies have been conducted in Missouri and other states on the effects of minimum tillage on soybean cyst nematode. Lowered numbers of SCN eggs have been observed in some studies, but the decrease in eggs was not dramatic enough to adopt minimum tillage to manage SCN. Soil erosion management is a much better reason to adopt minimum tillage.
The most accurate way of determining whether SCN is present in a field is to have the soil tested in a nematology lab for the presence of SCN eggs. Sampling efficiency and laboratory cyst extraction are not 100 percent effective, however, and a significant number of SCN eggs need to be present in the soil for lab detection.
Figure 4
Second-stage juvenile (a) hatched from eggs (b). The juvenile infects the roots.
Magnified 500 times.
Figure 5
These mature brown SCN cysts full of eggs were collected from soil.
Nematodes are one of the largest, most diverse groups of multicellular organisms and are second only to insects in abundance. Nematodes are animals that have a wormlike appearance. Most nematodes are beneficial organisms that help decompose organic matter, releasing nutrients for plant uptake.
SCN is a plant-parasitic nematode that changes shape as it goes through its life cycle. The worm hatches from an egg (Figure 4) and penetrates soybean roots. In the root, juveniles become males or females. Males return to the original wormlike shape and leave the roots in search of females. Those that become females lose the ability to move, enlarge into a lemon-shaped "white female," which breaks through the root surface, dies, and turns into a brown cyst or egg case (Figure 5). A single nematode cyst may contain several hundred eggs. The majority of eggs remain within the cyst, where they are protected from drying out and from soil predators. There can be more than one generation of SCN during a single growing season, leading to multiple root infestations.
The number of juveniles entering the plant root soon after plant emergence can have a dramatic effect on plant growth and development. Plant damage occurs from juvenile feeding, which removes cell materials and disrupts the vascular tissue. In short, SCN infection inhibits the growth and functioning of the soybean root system, interfering with nutrient and water uptake. SCN infection can also reduce nodule formation by nitrogen-fixing bacteria and can increase plant damage when other plant pathogens are present in the soil.
The race of a population of SCN is defined by comparing its reproduction on a set of four soybean germplasm lines with that on a standard SCN-susceptible soybean cultivar. The most commonly used race scheme identifies 16 races of SCN. The race designation allows nematologists and soybean breeders to share information about the ability of certain SCN populations to reproduce on soybean varieties that contain certain genes for resistance to SCN.
In 2003 an improved SCN race test was developed. The HG Type Test is an expansion of the old race test. This new test includes seven sources of resistance (germplasm lines) and the results are shown as a percentage, indicating how much the nematode population from a soil sample increased on each of the seven lines. This test indicates which sources of resistance would be good for the field being tested and which would be poor. Since the genetic sources of resistance are limited in commercially available soybean varieties, it is important to rotate these "sources of resistance" to delay the build up of a virulent SCN population.
Not all varieties with the same source of resistance have comparable yields, nor do they respond identically to SCN. Consult Soybean Variety Trial data for SCN-resistant soybeans that are adapted to your region.
A soil analysis is the only way to determine whether SCN is present at detectable levels in a field. It is estimated that even when 2 million eggs are present per acre of soil, there is only a 63 percent chance of detecting one egg if one pint of soil is examined. Distribution of SCN in a field is neither random nor uniform. Numbers of SCN and extent of SCN-related plant damage will depend on soil type, soil temperature, soil moisture, overwinter survival of SCN, soil nutrient level, crop host status (host or nonhost), and the presence of other plant pathogens or natural enemies of SCN. A good soil sampling pattern will ensure that a reliable average of the SCN populations is obtained. One cannot reduce yield loss from SCN if soil samples are not representative of the field.
Once a field is known to be infested with SCN, soil samples need not be collected each year. Soil samples from these fields should be collected before SCN-susceptible varieties are grown again or once every three years if resistant varieties are grown in a rotation.
Although soil samples for SCN may be collected at any time, the ideal time to sample is as close to soybean harvest as possible. SCN numbers tend to be highest when the plants are almost mature to shortly after harvest. Sampling near harvest allows sufficient time for the nematode laboratory to process the sample and provide you with information, and enough time for variety selection or choosing alternative crops for the next year.
Large fields may be subdivided into sections of about 10 acres each and a single sample from each of the different sections submitted for analysis. Collect 10 to 20 soil cores six to eight inches deep in a zigzag pattern across the area to be sampled. Bulk the cores in a bucket and mix thoroughly. Place about one pint of mixed soil in a plastic bag and label the outside of the bag with a marker to identify the field number and owner. Store the sample away from sunlight in a cool area until it is shipped for testing in the lab.
The Plant Nematology Laboratory will determine the number of nematodes or nematode eggs present in soil samples.
Uninfested fields
Infested fields
G4450, revised September 2006