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Disease Management

Once established in a field, SCN cannot be eradicated. However, there are various practices that can be implemented in an integrated pest management (IPM) program to minimize SCN reproduction and maximize soybean yields in infested fields.

Scout for early detection (look for symptoms and signs in the field). Aboveground symptoms can range from nonexistent to severe, and they are influenced by many factors. (One cannot rely solely upon aboveground symptoms, however, for definitive identification of SCN infestations). If the soybean yield obtained in a particular field has leveled-off or decreased for no apparent reason, or if SCN has been confirmed on nearby land, it is recommended that soil samples be taken from the plant root zone (Figure 18). A professional diagnostic lab should examine the samples for the presence of SCN. Soybean roots should also be examined directly for the presence of SCN females on the root surface (Figures 4 and 5).

Figure 18

Genetic resistance

Grow SCN-resistant soybean cultivars. Resistant soybean cultivars are an effective management tool (Figure 19). Planting resistant soybeans in SCN-infested fields will usually reduce reproduction of the nematode. Most SCN juveniles do not feed and hence are unable to complete their life cycle on the roots of resistant cultivars; a few, however, will survive and reproduce. Some resistant soybean cultivars may yield slightly less than susceptible cultivars in noninfested fields, but they will yield significantly better than susceptible cultivars in fields infested with SCN. If the same resistant cultivar is grown in the same field year after year, SCN has the ability to adapt to the cultivar, or “break resistance.” To reduce the possibility of this happening, some university researchers recommend that growers alternate use of the soybean cultivars with different sources of SCN resistance, and also that a susceptible cultivar be grown once after all types of available resistance have been rotated. The “HG-Type” system that has replaced the race system indicates which genetic sources of soybean resistance any given population of SCN can infect.

Figure 19

Cultural practices

Grow SCN nonhost crops. SCN is an obligate parasite; the nematode is unable to mature and reproduce in the absence of host roots. Consequently, SCN population densities decline during any year that nonhost crops are grown. For example, a 1-2 year rotation with corn (nonhost) has proven effective for many growers (Figure 20). The magnitude of decline of SCN population densities during a year that a nonhost crop is grown is somewhat unpredictable, however, because the change in population density varies from year to year and is greatly influenced by environmental conditions. SCN-resistant soybean cultivars often are incorporated into a multi-year cycle of rotations with nonhosts crops – this combination of practices is an excellent integrated management strategy.

Figure 20

Maintain a healthy crop. Plants that have adequate moisture and nutrients are better able to withstand infection by SCN. In land infested with SCN, maintaining proper soil fertility and pH levels and minimizing other plant diseases, insect, and weed pests that weaken the plants is more critical to maximizing soybean yield than when land is noninfested.

Manage the movement of soil. 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. Soil on equipment should be thoroughly removed with high-pressure water or steam, if available, after working in infested fields (Figure 21). Also, seed grown on infested land should not be planted in noninfested fields unless the seed has been properly cleaned; SCN may be spread in the seed-size soil peds (clumps) mixed in with the seed (Figure 17).

Figure 21

Chemical control

Use nematicides. No nematicide will kill all SCN in the soil. There are a few nematicides that are labeled for use against SCN, including the fumigant 1,3-dichloropropene (Telone) and the nonfumigants aldicarb (Temik or Bolster) and oxamyl (Vydate). Different products may be labeled in different states. 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, however, SCN numbers may be as high or higher than they were at planting. The performance of the nematicide will depend on soil conditions, temperatures, and rainfall. Yield and economic benefits are not guaranteed, and nematicides are expensive. All nematicides are extremely toxic, especially the nonfumigants like Temik and Vydate that are nerve poisons. Only licensed applicators may use these pesticides.

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