Alyssa Koehler, Extension Field Crops Pathologist; akoehler@udel.edu
Soybean Cyst Nematode consistently ranks as the most yield limiting pathogen of soybeans across the US, with average annual yield losses estimated over $1 billion dollars. SCN and other nematodes are often silent yield robbers, being present without noticeable aboveground symptoms. If symptoms from SCN do occur, they can look similar to other production challenges like 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 unknown until severe infestation develops (Figure 1). Scouting soybean roots for SCN females in season and conducting fall soil samples are two ways to check your field for SCN. Yellow to white females can be found on roots from about six weeks after planting through the end of the season. While females on the roots confirm the presence of SCN, they 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 provide a good snapshot of end of season populations and can be collected when already out for routine fertility sampling. We will discuss the steps to collect soil samples for SCN in an August article. Today I will introduce the steps to scout for SCN females on roots:
Figure 1. Soybeans with healthy looking foliage, but high levels of SCN in the soil
When to Sample
Scouting for SCN females on roots can occur 6 weeks after planting up until 3-4 weeks before harvest. 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.
Where to Sample
When scouting a field that has never been checked for SCN, you can target any areas with yellowing or stunting, but it is also a good idea to include healthy looking plants since SCN can be present without any aboveground symptoms. Areas of the field that tend to be 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, 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 in the soil. Gently shake off the soil and check the root system for white to light-yellow lemon-shaped adult SCN females (Figure 2). SCN females are much smaller than the nitrogen-fixing nodules (Figure 3). A hand lens or magnifying glass can make looking for SCN females easier, especially when scouting in sandy soils where sand particles can resemble SCN females. Gently swirling roots in a bucket of water can help to remove soil particles without dislodging the females.
Figure 2. Soybean root system with SCN females indicated at arrows
Figure 3. 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. For many years, nematode populations were managed through a single source of resistance, PI88788. Over the past few decades, we have seen a break down in this resistance and nematodes are reproducing at far higher rates than they should. If high levels of SCN are present, rotation of crop and variety are the best steps to reduce populations. Corn and wheat are both non-host options. While the PI88788 resistance gene still accounts for over 95% of soybean acreage, there are new resistance genes coming out on the market. Seed treatments are another control option. We are currently screening multiple seed treatment products for efficacy in our region and we will post those results as they become available later this year. From 2019-2021, we also conducted regional surveys across 311 field sites, results will be published soon and we will share an overview in a WCU later this summer. If you are interested in learning more about SCN, check out thescncoalition.com to talk todes.