Soybean Cyst Nematode
(PP1732 April 2022)Soybean Cyst Nematode is the biggest disease threat to soybeans in the US, and is spreading through North Dakota. This publication introduces the nematode and management tools to our growers.
Contact your county NDSU Extension office to request a printed copy.
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Signs and Symptoms
The most common above-ground symptom of SCN is no symptoms (Figure 1). Up to a 30% yield reduction can occur without any obvious above-ground symptoms. When above-ground symptoms do appear, they are not unique to SCN (Figure 2). Symptoms often manifest as general yellowing or stunting and are mimicked by other diseases, herbicidal injury, drought stress and/or nutrient deficiency. They are often first observed in circular to oval patches of yellowed or stunted plants late in the growing season. Patches may be as small as several feet or as large as many acres. Commonly, affected areas increase in size if left unmanaged.
Below-ground SCN cysts (dead adult females) may be visible in mid-to late part of the growing season. The lemon-shaped, cream to white-colored females (cysts) are very small (1/40th inch long by 1/60th inch wide) and may
be visible to the naked eye, but a hand lens and light (sunlight or flashlight) will make identification easier (Figure 3). SCN cysts are much smaller than a nodule (Figure 4) and their characteristic lemon shape can help differentiate them from soil particles (Figure 5). SCN cysts will eventually darken from yellowish white to brown as they mature (Figure 6). To look for SCN, carefully dig the plants roots and remove soil gently to avoid shaking off SCN females (Figure 7). However, the best way to detect SCN is to soil sample (see page 4).
SCN Sampling
Where to sample
Distribution of SCN is generally uneven and in an aggregated pattern in most infested fields. Therefore, depending on your situation, different sampling strategies may be appropriate.
- If you are SCN-sampling for the first time, target high-risk areas of the field, where SCN populations might first be introduced, such as field entryways, previously flooded areas, along field borders, shelter belts, fence lines, and near buildings, equipment and storage areas. You can also target low-yielding spots, low spots and areas with high pH where SCN has a greater chance of increasing quickly (Figure 8).
- If you already know SCN is present in your field, you can best understand the distribution and egg levels in your field by dividing your field into smaller subsections. Fields can be divided in a grid, or along naturally occurring geographic zones. Then collect approximately 20 soil cores following a zig-zag pattern (Figure 8).
- If you would like to evaluate the effectiveness of your management tactics, you can divide the field into different management zones and monitor the population changes in these zones. If egg levels are staying approximately the same or less from the time of planting to harvest, the management tools may be working. If a dramatic increase in the population occurs over the course of the growing season, current management tools may be insufficient.
Figure 8. Three approaches to collecting SCN soil samples 1) collect soil cores using a zig-zag pattern, 2) collect soil cores from logical areas of management zones in the field and 3) collect soil cores from high-risk areas in the field where SCN might first be discovered. (www.thescncoalition.com)
Where is SCN in North Dakota?
Between 2013 and 2021, the North Dakota Soybean Council has supported a grower-based SCN-sampling program. In that time, approximately 4,700 samples have been submitted by growers and their advisors throughout the state. Results are presented as the number of eggs plus J2 found in 100cc of soil (a little less than half a cup). To date, approximately two-thirds of the samples have been negative. However, the remainder have been ‘positive’ (>50 eggs/100cc) and levels as high as 100,000 eggs/100cc or more have been observed (Figures 9 and 10). As SCN can increase rapidly in a field, active management of SCN at any positive level is strongly encouraged. Notably, very low levels of SCN (50 eggs/100c to 200 eggs/100cc) could be false positives, and future testing is encouraged.
Management
Crop Rotation
Crop rotation is the most effective tool that can be used to reduce SCN population levels. The greatest reduction in SCN population levels occur in the first year a non-host crop is planted. Further reduction (although less dramatic) in population numbers will occur with additional years of non-host crop planting. All cash crops grown in our region are excellent rotation choices except dry edible beans. Dry edible beans are good hosts for SCN, although differences exist among varieties and market classes. A soybean every-other-year rotation may be used successfully, but is most successful when sources of resistance are rotated in a six-year crop cycle (Figure 11). When SCN levels are high, there is benefit by switching out of soybeans for more than one year. Soybean on soybean rotations are risky and over time are likely to faciltate an increase in yield loss to SCN.
For more information
For more information on this and other topics, see www.ndsu.edu/extension
For more information visit www.thescncoalition.com.
