Alyssa K. Betts, Extension Field Crops Pathologist; akoehler@udel.edu
Soybean Cyst Nematode (SCN) is widely prevalent across the region, especially in sandy soil types. Symptoms of SCN can resemble nutrient deficiency, soil compaction, drought stress, or other diseases. SCN can inhibit Rhizobium nodule formation, causing chlorosis or yellowing of soybeans in affected areas of the field. Due to the lack of consistent or obvious aboveground symptoms, it is very common for SCN to go unnoticed until severe infestation develops. To check your field for SCN you can scout soybean roots for SCN females or collect soil samples. Yellow to white females can first be observed on roots about six weeks after planting. Females on the roots confirm the presence of SCN, but do not provide information on the level of infestation. Soil samples are the best method to assess overall populations across the field. Soil sampling can be conducted at any time, but fall samples are recommended and can be collected when already out for routine fertility sampling. We will follow up with the steps to collect soil samples for SCN in August and today I will introduce the steps to scout for SCN females on roots:
When to sample
You can scout for SCN females on roots 6 weeks after planting up until 3-4 weeks before harvest. I find that digging plants earlier in the season is generally more effective because new roots surrounding the base of the plant are easier to dig and not as deep into the soil profile. The females are also easier to see on these younger root systems.
Where to sample
When scouting a field that has never been checked for SCN, you can target any area with yellow or stunted plants, but it is also a good idea to include healthy looking plants since SCN can be present without aboveground symptoms. Areas of the field at higher risk for SCN include near a field entrance, areas that have been flooded, areas with pH greater than 7, areas where yield has historically been lower, or areas where weed control is not as good.
How to sample
Using a shovel, dig 6 to 8 inches from the base of the plant to try to remove as much of the root system as possible. Avoid tugging or pulling on the plant since you will leave much of the root system behind. Gently shake off the soil and check the root system for white to light-yellow lemon-shaped adult SCN females (Figure 1). Gently swirling roots in a bucket of water will remove soil particles without dislodging the females. SCN females are much smaller than nitrogen-fixing nodules (Figure 2). A hand lens or magnifying glass can be very helpful, especially when scouting in sandy soils where sand particles can resemble SCN females.
Figure 1. Soybean root system with SCN females indicated at arrows.
Figure 2. Soybean root system with nodulation (left arrow) and SCN females (right arrow)
What to do next
If you find SCN females or suspect nematodes are present in the field, a soil test is the next step to estimate population density in the field. If high levels of SCN are present, crop rotation can help to reduce populations. Corn and wheat are both non-host options. For many years, nematode populations were managed through a single source of resistance, PI88788. When first launched, this resistance kept nematode reproduction below 10%. Over the past few decades, we have seen a break down in this resistance and SCN are reproducing at far higher rates, with some fields up to 60-70% reproduction. While the PI88788 resistance gene accounts for over 95% of soybean acreage, there are new resistance genes slowly entering the market. Seed treatments are another control option. Compared to plain seed, stand improvement has been the most notable effect of seed treatment in our trials, particularly in April or early May plantings. In trials from 2019-2022, yield response from seed treatments were variable based on season and ranged from 0 to +7 bu/a. A new profit checker tool has been developed by the SCN Coalition to help estimate the yield loss of individual fields due to SCN. For this tool, I have provided a default female index of 45, which is a conservative estimate for the region. If you have interest in knowing more about SCN reproduction in your field, you can email akoehler@udel.edu.