Ten years ago, Dr. Laura Sweets wrote "This would be a good year to sample fields for SCN." It's good advice for 2022 as well. Drought conditions favor soybean cyst nematode (SCN) reproduction.
Shortly after harvest is a good time to sample. SCN eggs will persist in the soil overwinter. Sampling in soybean stubble in 2022 can provide information that will remain relevant for planting decisions in 2023.
MU's SCN Diagnostics can provide farmers with 4 free SCN egg count test this fall. The effort is supported by the clinic, Missouri Soybean Merchandising Council and the "What's your Number" campaign of the SCN Coalition.
Candidate fields for sampling include:
- Fields that have not been sampled for SCN within the last 3 to 5 years
- Fields that are known to have SCN populations
- Fields with a history of SDS
- Fields where soybean are grown annually
Why sample?
SCN was first confirmed in Missouri in 1956. For more than 3 decades, the primary source of SCN defense has been to grow soybean varieties that contain PI 88788 resistance. Juvenile nematodes can feed on roots of soybean with the PI 88788 resistance source but maturation to female SCN is impeded. The SCN lifecycle is disrupted, and egg production is stopped.
However, thirty years is a long time to rely on one management tactic. In a 2015-2016 statewide survey Dr. Melissa Mitchum's team confirmed that Missouri SCN populations can reproduce on soybean that contain PI 88788. Some juvenile nematodes are feeding on the soybean and maturing into females, which continues the SCN lifecycle.
The PI 88788 source of resistance is breaking or is broken, depending on whether you hold a 'glass half full' or 'glass half empty' outlook.
Spring 2022 sampling results
Figure 1 Counties in which we collected soil for SCN egg counts during the spring of 2022.
We collected 84 samples from 21 Missouri fields (Figure 1) in spring 2022. Egg counts ranged from 0 to 46,250 per 250 cc of soil. The average number of SCN eggs per sample was 2,217 SCN. Thus, we can assume yield loss is occurring at some of these locations.
How much yield loss? In-field variations in topography and environment make it difficult to provide concrete answers. We have Missouri small plot research that we can extrapolate from to get us 'in the ballpark' with rough estimates. Dr. Kaitlyn Bissonnette's team conducted research in central and northern Missouri in recent years. The trend between spring SCN egg counts and end-of-season soybean yields from those small plot research trials is shown in Figure 2.
Figure 2 Trends between spring soybean cyst nematode egg counts and soybean yield in small-plot research conducted in Missouri in 2019 and 2020. Linear and exponential trends are graphed (gold and black, respectively.) Sample size = 232
If I extrapolate from that data and apply those results to the 2022 in-field egg counts, the rough estimates on yield losses in SCN-infested areas are:
- 2,217 SCN eggs per 250 cc soil would result in ~1 bushel/acre
- 10,000 SCN eggs per 250 cc soil would result in ~4 bushel/acre
- 46,250 SCN eggs per 250 cc soil would result in ~16 bushel/acre
This fall we plan to resample each of these fields to determine how in-season management practices influenced SCN levels.
How to sample?
Sampling can be frustrating because nematodes are not distributed uniformly in the field, and they travel such short distances. A detailed method and explanation on sampling produced by the SCN Coalition can be found here. The summary:
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Divide the field into 10- or 20-acre sections. Create sections that are logical, such as:
- Different topographies within the field
- Areas where SDS or SCN symptoms have been observed
- Traditionally low-yielding areas of the field
If you suspect you have an SCN problem, we suggest going with more divisions and more samples in order to gather the best information for making management decisions.
- Collect 15 to 20 soil cores per section. Cores should be 1" x 8" deep and collected in a zig zag pattern across the section.
- Mix the 15 to 20 cores thoroughly and place in a sampling bag.
- Submit samples for each section separately to SCN Diagnostics. The sample submission form can be found here.
- Contact SCN Diagnostics with any questions (scndiagnostics@missouri.edu)
Understanding whether you have a problem is the first step. We plan to share research on management tactics in subsequent newsletters.
Header image of SCN eggs by Jeff Barizon, SCN Diagnostics