Soybean Cyst Nematode - Disease Symptoms, Development and Management Consideratons

Soybean cyst nematode (SCN) is neither an insect nor a disease, but rather, a tiny worm-like parasite that attacks soybean roots. It is usually classified with diseases, however, because of its manner of infection and spread. SCN is the single most damaging pest of soybeans in the United States. Over the last 25 years, SCN has moved increasingly northward from the southern U.S., and is now damaging tens of millions of soybean acres. All major soybean-producing states in the midwestern U.S. are affected by SCN, and the spread of this pest is continuing. The most northern U.S. states and Canada are also affected.

Above Ground
SCN infestations can result in circular or oval-shaped patterns of stunted, yellow, less vigorous plants. In the affected areas, rows are slow to close and may remain so throughout the season. These symptoms, however, are not unique to SCN, and could also be caused by other diseases or stresses.

Below Ground
Soybeans may have dwarfed or stunted roots and fewer nodules, which can also result from a number of other causes. Therefore, the only truly unique symptoms of SCN infection are the presence of cysts on the soybean roots. Cysts appear as tiny, lemon-shaped structures that are white initially, then yellow and finally brown as they mature. These cysts are about the size of a pin-head, and can be seen with the naked eye, but more clearly using a magnifying glass. The cysts are much smaller than nitrogen nodules attached to the roots. Roots should be dug and removed carefully from the soil for examination – otherwise, cysts may be dislodged.

SCN has a three-stage life cycle – egg, juvenile and adult. Juveniles, the only stage that can infect the plant, are capable of moving only a short distance through the soil as they search for a host plant. Upon finding one, they burrow into the plant root and begin to feed on young root cells. As the female nematode feeds, its body begins to swell – eventually becoming so large that it breaks through to the outside of the root. This greatly enlarged, adult female is referred to as the cyst stage. The eggs inside the cyst are protected by the tough cyst wall and can remain viable in the soil for many years. These cysts move readily with the soil, making the spread of SCN very difficult to control.

Under optimum conditions, only 24-30 days are required to complete the entire life cycle, so several generations may occur during the growing season. The number of generations depends on the presence of a suitable host, planting date, soil temperature and moisture and the length of the growing season. The ability of SCN to multiply prolifically is one reason for its rapid spread

In an SCN-infested field, management goals include improving soybean health and yield, reducing SCN infestation levels, and preserving the yield potential of SCN-resistant varieties. A combination of management strategies is usually required to meet these goals. In addition to prevention, rotation and resistant varieties, any management practice which promotes good soybean growth will also help against SCN. Avoiding low fertility, compaction, drought stress and other disease or insect pressure will enable soybean plants to better withstand the effects of SCN.

Resistant soybean varieties are available across a wide range of maturities to combat this pest. Resistance is race-specific, with many varieties resistant to more than one race. A combination of resistant and susceptible varieties along with crop rotation with non-host crops can keep SCN population levels in check.