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.
Fig. 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 (Fig. 4).
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 (Fig. 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.
Fig. 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.
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.