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Soybean Cyst Nematode Symptoms and Management Guide


​​​​​​​​​​​​​​It’s no surprise many scientists consider soybean cyst nematode (SCN) the most damaging soybean disease in the U.S. The microscopic plant parasitic roundworm can reduce yields by 50% when not managed. SCN seriously impacts soybean productivity because populations establish themselves years before the crop is visually affected. SCN can survive on a limited range of host plants, including several bean varieties, lespedeza, hairy vetch and some winter weeds, such as henbit, purple deadnettle and field pennycress.

   Soybean Cyst Nematode.​​​​​​​

Identification and Life Cycle
The SCN life cycle encompasses 3 main stages, including egg, juvenile and adult, and the nematodes can have up to 6 generations per growing season. Overwintering occurs in the egg stage. Juveniles are microscopic roundworms that hatch from spring eggs when soil temperature and moisture are right. SCN enters a host root and moves to vascular tissue to form a feeding site. Male juveniles feed for several days, then leave the plant through the soil, never to return. Female juveniles feed for several days and develop into an adult. They swell and burst through the root tissue. Their bodies become lemon-shaped and are just visible on the root surface. N-fixing nodules are larger and have an irregular shape. SCN cysts are initially white and gradually darken with age. They eventually die, turn dark brown and become an egg-filled cyst. The cysts overwinter, allowing the eggs to survive for many years without a suitable host. Typically, 50% of the eggs hatch each year.

      Cysts pictured on left, followed by cysts on soybean roots at right.
​​​​​​​Symptoms and Impact
Below-ground symptoms include:
  • Stunted roots
  • Fewer N-fixing nodules
  • SCN cysts on roots, which are much smaller than N-fixing nodules
Above-ground SCN symptoms are difficult to observe during the first few years of infestation. Lowering yields are typically the first sign significant SCN are present. Initial visual symptoms include stunting and yellowing, and are easily confused with other conditions, such as compaction, nutrient deficiencies, drought stress, herbicide injury and other plant diseases. SCN is distinguishable from other injury by areas typically elongated along tillage direction, since SCN spread by tillage.

SCN symptoms are enhanced when associated with:
  • Sandy, well-drained soils
  • Low fertility
  • Drought
  • High soil pH
SCN infection can also increase brown stem rot, sudden death syndrome and Fusarium root rot disease potential.

Soil Sampling
If you can’t find SCN cysts on your roots, it doesn’t mean your field isn’t infested. Soil testing is the only way to determine SCN pressure. Sample every year to catch infestations early. To track the growth of your SCN population, sample at the same time every year and after the same previous crop. The best time to sample nematodes is in the fall when egg levels are highest. Sample before or after harvest, but not after tillage. SCN aren’t evenly distributed through fields, so collect from multiple sites. Consider taking separate samples from stressed and unstressed, SCN injury and non-infested areas.

   Cream-colored cysts on soybean roots.​​​​​​​

Managing SCN
The keys to successful SCN management are early detection and keeping egg numbers low. It takes years to drop high SCN egg counts to non-damaging economic levels. The earlier you can detect populations, the fewer years it will take to reduce the count to a manageable level.

1. Growing varieties with SCN resistance: PI88788 is by far the most common source of SCN resistance provided in commercial soybean varieties. Peking is another source of variety resistance, but not near as available as PI88788. Both of these sources provide partial resistance because they allow a few nematodes to develop and reproduce. PI88788 varieties differ in their resistance to SCN. Even if sources of resistance other than PI88788 are not found, rotating to different PI88788 varieties may help slow SCN development.

2. Rotating to a non-host crop: Rotating to a weed-free/non-host crop is the best way to decrease SCN populations. This practice can drop SCN egg counts by 55% or more in 1 season. SCN will starve if there aren’t host roots close by because they only disperse a short distance. The more non-host crops that are worked into the rotation, the more SCN populations will drop.

Once SCN establishes in a field, consider the following rotation:
  • Non-SCN host crop (corn, small grains, canola or alfalfa)
  • Resistant soybean variety
  • Non-SCN host crop
  • Resistant soybean variety
     A cyst under the microscope showing the hundreds of eggs, and some
    juveniles contained inside.

3. Reduce crop stress: SCN will impact soybean health more in soybeans that are stressed by adverse weed, moisture or fertility conditions. Any practices that reduce these stresses will decrease SCN’s yield impact. 

4. Minimize soil movement: SCN can’t move far on its own. The nematodes need help from machinery or weather to infest other fields. Since SCN is bound to the soil, anything that moves it encourages SCN dispersal. The biggest movers of soil are erosion and farm equipment. Anything that reduces erosion, such as less tillage or terracing, decreases SCN movement. Clean tillage or planting equipment before leaving a field to decrease the spread of SCN. As a result, tilling and planting the worst infested fields last is a good strategy.

Contact your Golden Harvest Seed Advisor with questions or for additional agronomic insights.

ReferencesPhotos are either the property of Syngenta or used under agreement.

Syngenta hereby disclaims liability for third-party websites.

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