Brought to you by: Jonah T. Johnson, MS, CPAg, CCA - Sales Agronomist, PCT | Sunrise
November 12, 2019: Do you know your number? SCN sampling time!
Soybean Cyst Nematodes, SCN for short, have become more and more of an invasive pest among soybean growers in Ohio. Dr. Anne Dorrance at OSU always says….”Soybean cyst nematode (SCN), Heterodera glycines, was first identified in Ohio in 1981 and has now been found on soybean in 72 of the 88 Ohio counties. SCN damages soybeans by feeding on roots, robbing the plants of nutrients, and providing wound sites for root rotting fungi to enter. The severity of symptoms and yield losses are dependent on several factors including: the number of nematodes present in the field at planting, the soybean variety, tillage practices, soil texture, fertility, pH, and environmental conditions during the growing season. Once SCN is established in a field, it rarely is eradicated. SCN is the leading cause of soybean yield loss in North America and now occurs in all major soybean production areas worldwide.”
Symptoms and Signs
Symptoms are highly variable. Symptom development depends on several factors, especially population densities of the nematode, the presence of other pathogens, soil nutrient status, resistant soybean varieties planted previously, and rainfall. Often, yield is reduced when there are no visible symptoms. SCN injury can also easily be confused with other crop production problems such as nutrient deficiencies, injury from herbicides, soil compaction, or other diseases. Moderate symptoms include circular to oval patches of yellowed plants with reduced yield (Figure 1). Affected areas may increase in size each year, usually in the direction of tillage. Severe symptoms include patches of very stunted plants and lower yields.
Figure 1. Severe symptoms of soybean cyst nematode can be seen with chlorotic and necrotic plant symptoms.
These symptoms, however, are non-diagnostic, thus soil sampling needs to be done for proper diagnosis.
Figure 2. Cysts, or mature females, can be seen as small, bright white dots on the outside of the roots.
Step 1: Identify the fields that have cysts and monitor egg populations. SCN populations are highest in the fall after soybeans are harvested. SCN populations can increase as much as 10 to 40 times in a single growing season. The number of SCN cysts or eggs found in the soil sample will determine the best management plan for the field (Table 1). Techniques for sampling soil for SCN by the Soybean Cyst Nematode Coalition are as follows:
- Use a 1-inch diameter soil probe to collect samples (6 to 8 inches in depth).
- Follow a zigzag pattern; collect 10 to 20 soil cores per 10 to 20 acres.
- Collect cores from areas of similar soil types and crop history.
- Dump cores from each 10- to 20-acre area into a bucket or tub and mix thoroughly.
- Place 1 pint (2 cups) of mixed soil in a soil sample bag or plastic zippered bag and label with a permanent marker.
- Store sample in a cool, dark place until shipped to an SCN analysis lab.
|Labs in Ohio that process SCN soil samples collected in Ohio include:
|C. Wayne Ellett Plant and Pest Diagnostic Clinic (OSU)
8995 East Main Street
Reynoldsburg, OH 43068
|Spectrum Analytic Inc. (confirmed 2018)
1087 Jamison Road NW
Washington Court House, OH 43160
|Brookside Laboratories, Inc.
200 White Mountain Drive
New Bremen, OH 45869
|Table 1. Best SCN management strategies for Ohio soybean producers
|Egg Count Per 100 to
200 cc* of Soil
|Continue to monitor field after two crops of soybean.
|May begin to measure some yield loss in susceptible varieties at or above 200 eggs/200 cc.
|Plant SCN resistant variety or rotate to a non-host crop. At or above 2000 eggs, some yield loss may result on SCN resistant lines.
|Rotate to a non-host crop next year and return with SCN resistant soybeans the following year. Losses of 25-50 percent have been recorded in Ohio on susceptible varieties when grown at these populations.
|5000 and over
|Rotate to a non-host crop for two to three years, then sample the soil to determine nematode populations before planting SCN resistant varieties.
|*100 to 200 cc = approximately ½ to 1 cup
Step 2: Rotate crops. Once SCN is identified in a field, the best disease management strategy is to keep the numbers low. The best way to keep numbers low is to rotate, rotate, and rotate. Rotating host crops with non-host crops (corn, small grains, and alfalfa) is the most effective method of controlling SCN. Under average Ohio conditions, SCN populations may decline by 50 percent per year under non-host crops. In fields where SCN populations are high, it may take three years or more of non-host crops between soybean crops to reduce SCN populations significantly. SCN populations will not be eliminated in these fields. If soybeans are repeatedly planted for several years, SCN will again become yield-limiting. SCN populations can increase 10 to 40-fold per year on susceptible soybeans. SCN can also reproduce on many legume crops and weeds, including purple deadnettle and henbit. These are common winter weeds of no-tillage fields. They emerge from September through early November, and they can increase the SCN population before winter. Because of this, winter annual weeds should be controlled as soon after crop harvest as possible.
Step 3: Use resistant soybean varieties wisely. There are currently two major sources of resistance in commercial varieties: PI 88788 and Peking. Ohio now has some fields with sizable SCN populations that can reproduce on the soybeans developed with PI 88788 resistance. Resistant varieties should be used in crop rotation with non-host crops to prevent the buildup of SCN in that field. The resistance in commercial soybean varieties is not complete resistance; it’s a type of resistance known as partial resistance.
Step 4: Use best management practices.
- Fertility—Maintain optimum fertility based on a soil test. Under high SCN populations, even the most fertile fields will be severely affected; fertilization will not eliminate the problem.
- pH—Studies in Wisconsin have shown that soil pH has an effect on the level of yield loss caused by SCN. SCN populations were highest in areas of the field with the highest soil pH (7.1–8.0 vs. 5.8–6.4). Likewise, the yield advantage of SCN resistant varieties was greatest in high pH soils and lowest in low pH soils.
- Optimize planting/harvesting dates for the maturity group for your region.
- Optimize drainage for proper plant growth.
Step 5: Manage other diseases. Sudden death syndrome and brown stem rot interact with SCN. With SCN many of these diseases can have a larger impact than if the plants were infected separately. Choose varieties that are resistant to these and other Ohio soil-borne pathogens.
Step 6: Prevent introduction. This is the first line of defense. Nematodes can move no more than a few inches a year on their own, so they depend on “hitching rides” on anything that can move soil, such as field machinery, migratory birds, floodwater, or wind. Plant seeds that have been thoroughly cleaned to remove soil particles. SCN can also be introduced into a field by animals, flooding, or wind-blown dust.