Soybeans: Soybean Cyst Nematode

Excerpt from Agronomy Guide for Field Crops (Chapter 4)

Order OMAFRA Publication 811: Agronomy Guide for Field Crops

Table of Contents

1. Incidence
2. Appearance
3. Disease Cycle
4. Management Strategies
   • Hosts for Soybean Cyst Nematode - Table 4-21
   • Potential Risk of Yield Loss for Various SCN Population Levels - Table 4-22
5. Updates on Soybeans: Soybean Cyst Nematode
6. Related links...

Incidence

Since it was first identified in 1988, soybean cyst nematode (SCN) has been identified in nine counties in Ontario. Infected counties include Essex, Kent, Lambton, Elgin, Perth, Haldimand-Norfolk, Middlesex, Huron and Oxford. Unfortunately, soybean cyst nematode will continue to move across the province into previously non-infested counties. The disease can be managed effectively, but the first step is identification and awareness. All soybean producers should be scouting and testing for SCN. Losses to SCN in Ontario have ranged from 5% to 100%. Unfortunately, by the time SCN symptoms become visible on the plants, the producer has lost 25% to 30% of the yield potential. Once a field has SCN, eradication is impossible.

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Appearance

These microscopic, worm-like nematodes damage the root system and prevent the uptake of water and nutrients. Typical SCN symptoms include yellowing of the leaves and stunting of plants, particularly on lighter soils under dry conditions. Damage occurs in a circular pattern ranging from a few metres to over 50 m in diameter. SCN damage is often confused or misdiagnosed as nutrient deficiency, flooding, herbicide injury, compaction or root rot damage. Yellowing of the leaf margins can resemble potassium deficiency symptoms, however, the addition of potassium will not reduce the damage from SCN or eliminate symptoms. Poor stand establishment can be the result of SCN infection and is often confused with seedling diseases. Never try to pull up a plant to see SCN. Too much root will be lost and the nematodes will be stripped off as the roots are pulled through the soil. Instead, use a shovel and dig up the plant along with the soil surrounding the roots.

Plate 72. Plants infected with soybean cyst nematode can be stunted, with yellowish leaves. Frequently in circular patterns in the field.

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Roots of infected plants may be dark due to secondary root rot infection and have fewer nodules. White to yellow-brown cysts less than 1 mm in diameter (pin-head) are also visible on the roots.

Plate 73. Soybean root with SCN cysts. Yellow-brown, lemon-shaped cysts (pin-head size) are produced on the roots of plants infected with soybean cyst nematode.

Nematode injury symptoms (including plant death) are most obvious under stressful growing conditions, especially under hot, dry circumstances. Under good growing conditions with little stress, the visual damage from soybean cyst nematode may go unnoticed. This is in contrast to what happens in high-stress years. Under high-stress conditions, lower SCN numbers cause considerably more visual damage and hence the corresponding yield loss. A soybean plant's ability to compensate for the SCN feeding injury is less likely under high-stress conditions than when good growing conditions exist (see Figure 4-6, Soybean Yield Response to High and Low SCN Populations Under High and Low Stress Conditions).

Figure 4-6. Soybean Yield Response to High and Low SCN Populations Under High and Low Stress Conditions

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SCN infection symptoms may not be obvious and yield reductions of 25% to 30% on susceptible fields can occur without visual (above-ground) symptoms. Areas of the field where above-ground SCN symptoms will most often occur include entrance points for equipment into the field, equipment and vehicle storage areas, tops of knolls, compacted headlands, and along the fencerow where wind-blown soil tends to accumulate.

Disease Cycle

The life cycle of SCN has three major stages: egg, juvenile and adult. The cycle begins when eggs hatch to release worm-shaped juveniles in the soil. This is the only stage when SCN can infect soybean roots. Soybean plants release chemicals that attract the larvae to the roots. Once they have penetrated the roots, the young nematodes migrate to the water and nutrient-conducting tissue (vascular system) and establish a feeding site (syncytia). Syncytia form when female juvenile nematodes inject secretions into the root cells, modifying and transforming them into specialized feeding cells. At this stage, the female nematode begins to swell and eventually breaks through the root surface. Adult females that remain attached to the root to feed, produce eggs in a mass or egg sac outside of the body. As the life cycle nears completion, eggs also develop within the female's body cavity. These egg-filled bodies of the adult females are called "cysts." Initially white, the cysts turn yellow and brown as females mature. Each cyst can contain 100 to 300 eggs. The number of cysts per plant varies from a few to many hundreds. In infected soil, cysts are distributed throughout the root zone.

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Management Strategies

The following practices will decrease the likelihood of this pest causing significant economic losses:

1. Plant certified or good-quality, clean seed that is free of soil peds.
2. Wash soil off farm equipment when moving it between infested fields or farms.
3. Use proper soil conservation practices to reduce soil movement between fields.
4. Practice prudent weed control. Many weeds may serve as hosts to SCN.

If SCN has been diagnosed in a field, establish a rotation that uses SCN-resistant soybean varieties and non-host crops such as corn, wheat, alfalfa, clover or vegetable crops like tomatoes. This will lower SCN populations and improve yields on SCN-infested fields. It is not advisable to substitute edible (white, coloured) beans or peas into the rotation instead of soybeans since these other crops are also hosts for soybean cyst nematode. Refer to Table 4-21, Hosts for Soybean Cyst Nematode, Table 4-22, Potential Risk of Yield Loss for Various SCN Population Levels, and Figure 4-7, Crop Rotations for SCN Management for Fields with Race 3 or a Combination of Races 1 and 3.

Table 4-21. Hosts for Soybean Cyst Nematode

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Source: Welacky, Anderson and Tenuta. Agriculture and Agri-Food Canada and OMAFRA, 2000.

Monitoring of the soybean cyst nematode population in the soil is necessary. These samples should be taken every 2-3 years. Producers should ask for both an egg count and a total cyst count. Send samples for nematode analysis to any of the laboratories listed in Appendix G, Soybean Cyst Nematode-Testing Laboratories

Continuous use of resistant varieties will pressure the nematode population to adapt and shift in the field thereby making the variety ineffective in combating this pest. Consult the Ontario Soybean Variety Trials report for SCN-resistant varieties and their performance on infested soil.

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Figure 4-7. Crop Rotations for SCN Management for Fields with Race 3 or a Combination of Races 1 and 3

Caution: SCN will significantly reduce yields, therefore when selecting soybean varieties for fields with a history of SCN, make sure that the variety has SCN resistance. This is especially true for new technologies or traits such as herbicide resistance. For example, when using a herbicide-resistant variety, minimize SCN losses by selecting a variety that also has SCN resistance.

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Updates on Soybeans: Soybean Cyst Nematode

• Field Crop Protection Guide - Publication 812
  • Soybeans: Soybean Cyst Nematode

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Related links...

• Disease
• Soybeans

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