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Soybean cyst nematodes are microscopic, parasitic round worms that attack soybean roots. Potential yield loss is the primary concern with SCN infestation. Visible symptoms associated with SCN are rare, although yield may still be impacted. When symptoms are present, they include stunting, leaf yellowing, and wilting, which can be easily confused with other diseases and management problems (Figure 1). Symptoms will be more severe during times of stress because SCN infestation reduces the plant’s ability to tolerate stresses such as drought and nutrient deficiencies. Nematode feeding can also increase the incidence of diseases.
Soybean cyst nematode eggs overwinter in the soil inside a cyst, which is the former body of an adult female. The eggs hatch in the spring and early summer and juveniles migrate through the soil. When soybean seeds begin to germinate, juveniles penetrate the young roots and begin to feed. Once established in the roots, nematodes stimulate development of nurse cells, which become physiological sinks that take energy away from the roots, leaves, and grain production and compete for water and nutrients. At high levels of infestation, nematodes can severely compromise the tap root and secondary root development may be stunted. Penetration by the juveniles makes soybean roots more susceptible to soil-borne pathogens such as Phytophthora root rot, Pythium root rot, Rhizoctonia root rots, sudden death syndrome, Cylindrocladium black root rot (red crown rot), and southern stem blight. SCN can also reduce nodule formation, reducing the plant’s ability to fix nitrogen.
As female juveniles grow large inside the roots they become lemon-shaped. The body of the female eventually breaks through the root system where she is fertilized by the male (Figure 1). Eggs will hatch and release second-stage juveniles, which will re-infect the soybean plant. SCN may produce 2 to 5 generations per year, depending on the length of the growing season, temperature, and moisture levels. Eventually, the fertilized female dies, detaches from the roots and turns to a brown cyst. The dead female body or cyst protects the eggs, which will be ready to hatch the following spring and can remain viable for several years.
Soybean cyst nematode is not likely to be eradicated once present in the field. The goal is to manage population densities to help minimize yield loss, while using good agricultural management practices to promote plant health and reduce stress.
Crop Rotation-Crop rotation is one of the most effective methods of managing SCN. Planting a non-host crop, such as corn or sorghum, for one year can significantly reduce SCN populations. However, two years of a non-host crop may be necessary to reduce SCN populations to acceptable levels, particularly in sandy soils. Rotation is recommended even if resistant soybeans are planted. Resistant soybeans should suppress the population density of SCN so a susceptible soybean can be grown every third or fourth year in the same field.
Resistant Soybeans-Resistant soybean products can be an economical means of managing SCN. Soybean plants with SCN resistance will not provide complete control. Resistant soybeans limit reproduction of SCN but are still attacked by these nematodes. A resistant soybean product planted following a susceptible product may be damaged by SCN, but will still out-perform a susceptible product in infested fields (Figure 2).
SCN resistance is not the same as SCN tolerance. SCN reproduction is limited on resistant soybeans whereas SCN reproduction is unrestricted on most tolerant soybeans. The yield potential of tolerant soybeans will be higher than that of susceptible soybeans under SCN pressure.
Races of SCN and the HG Type Test-In the past, SCN populations were given a race designation in order to provide soybean growers with recommendations for soybean products conferring resistance to the specific race of SCN in their field. The practice of giving SCN a race designation has been replaced by the HG (Heterodera glycines) Type Test. The goal of the HG test is the same, to indicate which sources of soybean resistance would be beneficial for a specific field and which soybean resistance would not work.
Growers should be aware that SCN populations have the ability to adapt to previously resistant soybean products. Continued use of one resistant soybean product may result in a change in the nematode population and leading to that soybean product no longer being useful for that field. This can be avoided by not planting a resistant product two or more consecutive years in the same field.
Cultural Practices-Optimum soil fertility, proper seedbed preparation, avoidance of drought stress, and excellent weed control can reduce the effect of SCN feeding.
Proper weed control is especially important when managing SCN. Some weeds can serve as alternate hosts for SCN, which can further increase population densities in the field. In greenhouse trials, some weeds (purple deadnettle and henbit) have been shown to be as an efficient host as a susceptible soybean products.4
Plant pathologists have also noted that SCN damage caused to soybean plants tends to be greater in early planted products than in late planted products, as the SCN population density declines from spring to early summer. Since later planted soybean products tend to yield less than earlier planted products, you may consider planting the land most subject to SCN damage last.
Laboratory analysis is the only way to determine SCN population densities and HG type in a field. The first time a field is checked for SCN, samples should be taken from areas where SCN is likely to become established. Areas such as near a field entrance, along fence lines, where soybean yield was previously low, with poor weed control, where flooding occurred, and in high pH soil. If sampling is done in an area with suspected high levels of SCN damage, it is recommended to sample near the edge of that area. The center of the heavily damaged area may have low population density, since the SCN may have moved on to healthier plants.
Once SCN have been identified in a field, that field should again be tested. Testing is important before SCN-susceptible products are planted or if resistant products are grown in a rotation test for SCN once every three years. The ideal time to collect samples for SCN is as close as possible to soybean harvest when SCN population densities are the highest.