Soybean cyst nematode (SCN) is the single most damaging pest of soybeans in the United States. Over the last 30 years, SCN has moved increasingly northward from the southern US, and is now damaging tens of millions of soybean acres. All major soybean-producing states are affected, and the economic impact is likely over a billion dollars per year.
SCN is neither a disease nor an insect, but a tiny worm-like parasite with a three-stage life cycle - egg, juvenile and adult. The juveniles burrow into the roots of host plants and feed on young root cells. This feeding damages soybean plants in several ways:
The extent of above-ground damage resulting from root feeding is variable. Under conditions of moisture stress, low soil fertility, compaction, disease pressure or other environmental stresses, above-ground damage is usually dramatic. But if soil moisture is plentiful and soil fertility is high, there may be no visible above-ground injury, even though yields are reduced.
It takes a period of years for SCN to build up in fields to damaging levels. Preventing this buildup requires early detection and management. However, SCN may decrease yields substantially without inducing obvious symptoms, so many fields may be infested without the knowledge of the grower. Sampling fields with no symptoms is the only way to detect SCN before it becomes an economic problem.
In addition to soybean, SCN can survive on a variety of crops and weeds (Table 1.)
|Beans (adzuki, bush, dry, green, lima, mung, red, snap)||Henbit|
|Chickweed (common, mouse-ear)||Lespedeza|
|Clover (alsike, crimson, scarlet, sweet)||Lupine (white, yellow)|
|Ground cherry||Vetch (common, crown, hairy, winter)|
|Hemp sesbania||Winged pigweed|
(Iowa State Univ. Soybean Extension and Research Website) (http://extension.agron.iastate.edu/soybean/diseases_SCNbiology.html)
Corn, alfalfa and small grains are the most common crop choices for reducing SCN numbers. However, because SCN persists in the soil for many years, it cannot be totally controlled by crop rotation or any other practice. Rather, growers must try to keep SCN numbers as low as possible while maintaining soybean yield. This Crop Insights will explain SCN management using resistant soybean varieties.
Researchers have identified a number of soybean lines that have the ability to resist nematode reproduction on their roots. Currently, there are three main sources for genetic resistance to SCN ? PI 88788, PI 548402 (Peking), and PI 437654 (Hartwig and CystX ®). The PI 88788 source is used in over 90% of existing SCN-resistant varieties marketed in the US. Only a small number of varieties currently use the PI 548402 source, and even fewer use the PI 437654 source.
SCN populations are genetically diverse and have historically been placed into population classes or races by their ability to reproduce on soybean tester lines. The PI 88788 source is resistant to races 3 and 14, while the Peking source is resistant to races 1, 3, and 5. The Hartwig source is resistant to almost all races, but this resistance is not complete immunity. The three sources differ in their ability to control SCN reproduction. PI 88788 reduces nematode reproduction by around 80 to 90%, while Peking and Hartwig sources reduce nematode reproduction by nearly 100% for the races they control.
In addition to the source of resistance, the genetic background also plays a role in a variety's ability to control SCN. For this reason, rotating resistant varieties is often the best strategy for controlling an SCN population.
Variation in SCN Populations
Race 3 is the most common SCN type in the US. Therefore, varieties with the PI 88788 source of resistance will provide good protection in the majority of SCN environments. If an alternative race exists or the SCN population is quite high, however, this source may not provide adequate protection and the use of a Peking or Hartwig source may be warranted. Appropriate resistant varieties can increase soybean yields by more than 50% in heavily infested fields. A combination of resistant varieties, crop rotations, and soil sampling can help manage SCN race variability and population levels.
Doerge, T. 2005. New Soybean Varieties Moderately Resistant to SCN. Crop Insights Vol. 15, #8. Pioneer Hi-Bred Int'l., Inc., Johnston, IA.
CystX® is a registered trademark of ACCESS Plant Technology, Inc.