Watch for the Soybean Cyst Nematode
Craig Grau
Plant Pathology
This is a good time of year to inspect soybean plants for the soybean cyst nematode (SCN). The SCN is the number one pest of soybean in the USA and causes yield loss each year. Often SCN is missed because symptoms not obvious in many cases. In fact, normal looking fields may experience a 10 to 20% yield loss without obvious symptoms. Another problem is that symptoms caused by SCN are not unique and are frequently blamed on other potential problems. I have observed flushes of weeds, especially foxtail, in pockets in fields known to be infested with SCN. SCN prevents complete canopy closure resulting in less competition for weeds and thus the appearance of weeds. SCN will likely be found in all counties with soybean production.
Diagnosis
The white stage of the SCN can be observed on soybean roots at or shortly after flowering. The number of SCN on roots will increase until late pod stage and then decline. SCN will change color progressing from white to a yellow color. Plants must be dug, not pulled, to properly inspect roots for SCN. It is important to first inspect roots on the outside of the ball of soil on the spade before removing all the soil from the roots. SCN will be most common on the finer feeder roots, and less common on the taproot and larger lateral roots. The presence of SCN on soybean roots is a positive identification, but a soil sample is required to determine the level of infestation.
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SCN appear as white or yellow “pearls” on roots. Each SCN is the size of a pin head.
Risk assessment of soybean cyst nematode by soil population and soil typeA
|
Risk |
Egg count range per 100 cm3 of soil |
Potential yield loss for SCN susceptible variety |
Potential yield loss for SCN resistant variety |
|
None |
0 eggs |
None |
None |
|
Low |
1 to 500 |
0-10% silt or clay soil
5-20% sandy soils |
None |
|
Moderate |
500 to 2000 |
10-20% silt or clay soils
10-40% sandy soils |
0 to 5% all soils |
|
High |
2000 to 5,000 |
20-50% all soils |
5 to 10% |
|
Very High |
Greater than 5,000 |
30-70% all soils |
10 to 15% |
aYield loss associated with resistant varieties will be modified by the virulence of SCN population in a specific field.
Analysis of Soil
Plant symptoms and inspection are important, but a laboratory analysis of soil compliments a field diagnoses. A laboratory analysis defines the population density of soybean cyst nematode. Soil should be collected in a systematic pattern within a 15 to 20 acre area. Fields can be sampled anytime of the year, but spring samples may be more predictive of the level of risk due to soybean cyst nematode. A small trowel may be used to collect soil, but a 1-inch soil probe is preferred to collect soil at 15 to 20 sites within a 15 to 20 acre block of land. The soil should be collected at a depth of 6 to 8 inches within the rows if possible, and placed in a container. The soil should then be mixed thoroughly, reduced in volume and placed in bags that retain moisture to prevent the drying of the soil. Soil samples do not need to be refrigerated, but should be kept cool by not placing the samples in direct sunlight or near other sources of heat. The samples should be submitted quickly to a laboratory that analyzes soil for soybean cyst nematode. Send samples to the: Plant Disease Diagnostic Lab, Department of Plant Pathology, 1630 Linden Drive, University of Wisconsin – Madison, Madison, WI 53706-1598
Additional labs that test for SCN can be found on page 85 and 87 of “Pest Management in Wisconsin Field Crops” UWEX no. A3646.
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The areas shaded white are intended to provide examples of where SCN can be concentrated in highest numbers within a field. These are areas that should be sampled to increase the likely-hood of detection in soil samples. SCN populations are higher if soil pH approaches 7.0 and decline as soil pH approaches 6.0 and below. The following are suggestions to increase your chances for finding SCN in a suspect field. Field entryway: SCN introduced with soil on equipment; Low spots and flooded areas: Soil is washed into low spots and SCN becomes concentrated; Fence lines: SCN can be carried with windblown soil; Presence of weeds: SCN reduces the soybean canopy allowing weeds to develop: Areas with stunted plants: High populations of SCN are a possible cause of poor soybean growth and development.
Additional Sources of Information on SCN
• 2005 Wisconsin Soybean Variety Test Results; UWEX A 3654
• Pest Management in Wisconsin Field Crops 2005; UWEX A3646
• Soybean Plant Health Website
– www.plantpath.wisc.edu/soyhealth
• Soybean Plant Health Initiative; North Central Soybean Research Program http://www.ncrsp.com/planthealth