Brent Jaenisch

Controlling Soybean Cyst Nematode: Do you have a resistance problem?

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This article originally appeared in the AGVISE Laboratories Winter 2022 Newsletter

This is the third year of our soybean cyst nematode (SCN) resistance project. Each year, we have flagged spots in soybean fields and collected paired SCN soil samples in June and September. If the SCN egg count increases through summer and into fall, we can quickly learn if the soybean SCN-resistance source, either PI88788 or Peking, is working or failing. University SCN surveys have found that the PI88788 resistance source has begun to lose its effectiveness at controlling SCN populations in much of Minnesota. This is a particular problem because 95% of SCN-resistant soybean varieties still use the PI88788 resistance source.

SCN egg count and soybean yield data from the 2021 AGVISE SCN resistance project. Bars of the graph represent SCN egg count, lines of the graph represent soybean yield. Click on the graph for a higher resolution version.

In 2021, paired soybean variety comparisons with SCN soil samples and soybean yield data really helped us see the difference in these SCN resistance sources. Among the sites, the Peking resistance source always had a lower SCN egg count than the PI88788 comparison, indicating that the Peking soybean varieties had better control of the SCN population at 4 of 5 sites. The Alberta site had similar SCN population control with both PI88788 and Peking resistance sources, so the soybean yield was similar at the site. However, the other sites demonstrated SCN resistance to PI88788, and the resulting soybean yield with the Peking resistance source was better with 7-bu/acre soybean yield increase on average.

For 4 of 5 sites, it is apparent that a Peking-traited soybean variety is the better choice. To learn if you have SCN resistance problems in your field, the simple early-late SCN soil sampling exercise, like we did in this project, is a quick way to learn if your current soybean variety is still controlling SCN and delivering the best soybean yield.

 

 

Richard Jenny

Sampling Fields for SCN

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Soybean cyst nematode (SCN) is a microscopic, parasitic worm that attacks the roots of susceptible soybean and dry edible bean, causing unseen or unexplained yield losses. Soybean and dry edible bean are naturally susceptible to SCN, but through plant breeding, most soybeans have some level of resistance, varying in level from good to poor. The most common source of resistance to SCN in soybean is PI88788, which is about 30 years old, and many soybean growing areas have SCN populations that are becoming resistant to this source. The Peking source is a very effective SCN resistance source but is only available in less than 5% of all soybean varieties.

Soybean cyst nematode cysts each harbor hundreds of eggs. Cysts and eggs of SCN can survive in the soil and remain viable for many years even without a soybean or dry bean host. Any activity that moves soil around will move SCN, meaning that areas with a history of soybean production likely have or will have this pest. Soybean cyst nematodes were first reported in Minnesota in 1978, South Dakota in 1995, North Dakota in 2003, and Manitoba in 2019.

During the growing season, the developing SCN cysts containing the eggs can be seen on susceptible plant roots, as seen in the picture below. To get an accurate assessment of the infestation level of the field, you need to collect soil samples and submit them to a laboratory to get a measure of the SCN egg count.

Photo of soybean roots with SCN cysts. Photo courtesy of NDSU.

Sampling strategies

If you have never tested for SCN before, you will want to sample fields intended for soybean or dry bean for the presence of SCN and gather a baseline SCN egg count. The best time to collect this sample is at the end of the growing season, right before harvest or just after (before any tillage). Sampling in the fall coincides with the highest egg levels in the soil and typically falls in the months of September and October. Collect 10-20 soil cores (6 to 8 inch soil depth) right in the soybean row from areas of the field that are likely to have SCN. Since SCN is a soil-borne pathogen, it moves wherever contaminated soil can enter the field. Therefore, the areas you will want to collect samples from are field entry points where soil can be transferred on equipment and tires, places where blown soil accumulates (e.g., fence lines), ditches and flooded areas, and locations in fields with consistently low soybean yields. Mix the soil cores together and take a subsample to fill a soil sample bag.

If you know you have SCN, you will want to sample soybean fields twice during the year: once in June to get an initial SCN egg count and then again in the fall to get a final SCN egg count. The early and late SCN samples allow you to measure if SCN populations are being effectively controlled (i.e., no increase in SCN egg count) or if the soybean variety SCN resistance source is failing (i.e., SCN egg count increases). Choose a single point in the soybean field and collect 8-10 soil cores (6 to 8 inch soil depth) taken within the soybean row at that spot. Mix the cores together and fill a regular paper soil sample bag. Mark that point with a flag and collect its GPS coordinates. Come back to that exact spot in the fall and collect a second sample. This will help you assess how your SCN management strategies, including the soybean variety SCN resistance source and soybean seed treatment, are working in the field.

Preparing and sending SCN samples to AGVISE Laboratories

You can submit SCN samples via paper form or online through AGVISOR. AGVISE provides special paper forms for SCN sampling and special stickers for online AGVISOR submission at no charge. The bright yellow forms and stickers help us sort samples and ensure samples submitted for SCN analysis are not dried and ground. All SCN samples analyzed by AGVISE Laboratories are analyzed at the Benson, MN laboratory. You can either send the SCN samples directly to the Benson Laboratory (see addresses below) or to the Northwood Laboratory, where they will be routed to Benson for analysis. AGVISE Laboratories reports SCN results in “eggs/100 cc” of soil and provides interpretation on our reports informed by university research.

Helpful links:

Soybean Cyst Nematode, ISU

Plant Disease Management: Soybean Cyst Nematode, NDSU

Soybean Cyst Nematode (SCN), UMN

Soybean Cyst  Nematode in South Dakota: History, Biology, and Management, SDSU

The SCN Coalition

 

Richard Jenny

Is PI 88788 Working in Your Soybean Fields?

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Soybean Cyst Nematode (SCN) is the number one yield-reducing pest in soybeans. Potential yield loss to SCN is expected to rise as more and more populations of SCN overcome the PI 88788 source of resistance. The Peking source of SCN resistance is not near as common as the PI 88788 source but is used in several soybean varieties.

If you want to see how the SCN resistance source in your soybeans is holding up this growing season, you can do an early and late SCN soil test. If the egg count increases substantially between the early and late SCN sample, your SCN resistance source is likely failing.

Here are the 4 steps to this simple test:

Early SCN sample (June): 

  1. Choose a spot in a current soybean field
  2. Collect 8 to 10 0-6″ soil cores taken within the soybean row at that spot
  3. Mark that spot with a flag or GPS so you can get back to that spot to sample later in the season

Late SCN sample (mid to late August): 

4. Go back to the same spot you collected a soil sample from in June and repeat step #2

Once you’ve conducted this simple test, you will get an idea of whether or not the SCN resistance source in your soybean variety is holding up or if it is time to change the resistance source in next year’s varieties. AGVISE completed a field project using a similar procedure in 2019 and 2020. The data showed that the PI 88788 trait was not preventing SCN populations from increasing in some field sites tested in Minnesota. You can read more about our project here.


Data from the AGVISE SCN field project, 2019-2020

A silver bullet for managing SCN does not exist and will likely never exist. Do your due diligence and figure out if your SCN resistance source is working in your own fields.

You can order SCN submission forms from our online supply store here.

Additional resources:

SCN in Iowa: A Serious Problem that Warrants Renewed Attention

Iowa State University – SCN Resources

 

 

 

Richard Jenny

Soybean cyst nematode: Failing resistance traits, increasing SCN populations

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Originally featured in the Winter 2020-2021 AGVISE Laboratories Newsletter

In 2019, AGVISE Laboratories investigated if popular soybean varieties with PI88788 or Peking SCN-resistance traits were effectively providing protection from soybean cyst nematode (SCN) and found that a number of the varieties failed to do so. We expanded the project in 2020 with cooperation from agronomists in west-central Minnesota.

For over 20 years, PI88788 has been the primary SCN-resistance trait in over 95% of soybean varieties. In the past few years, university research is showing that PI88788 is losing its effectiveness at controlling SCN. Detecting SCN-resistant trait failure with the naked eye is impossible, unlike the detection of failed pesticide control, where you can still see a herbicide-resistant weed that is growing vigorously. Therefore, we wanted to demonstrate how you can measure SCN resistance with soil sampling, even though you cannot see it with your naked eye.

In the project, we had 41 soybean fields with SCN-resistant varieties, 35 with the PI88788 trait, and 6 with the Peking trait. In each field, a location was flagged and soil sampled for SCN egg count in early (June) and late (September) parts of the growing season. From June to September, the SCN egg count increased by 4.9 times on average across all 41 soybean fields (individual field reproduction factor ranged from 1.2 to 12.9). In some fields, the high SCN reproduction rate shows that SCN were successfully reproducing on soybean plants and the SCN resistance trait is failling. We also learned that soybean varieties with the Peking trait had much better control of SCN than those with the PI88788 trait. One cooperator from Benson, MN grew both PI88788 and Peking soybean varieties on his farm. He noted a 2.5 bu/acre soybean yield advantage with the Peking soybean variety (56.5 bu/acre) over the PI88788 soybean variety (54.0 bu/acre).

The project showed that SCN soil sampling in the early vs. late growing season was a simple way to detect a failing SCN resistance trait. The simple protocol only takes a big flag to mark the spot, then a set of soil samples in June and September to compare the SCN egg count results.

 

John Lee

Copper for Small Grains

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Among crops grown in the northern Great Plains, small grains (cereals) are the most susceptible to copper deficiency. Copper (Cu) is an essential micronutrient required in small concentrations for plant growth and reproduction. Copper deficiency symptoms in cereals include pale yellowing, wilted and twisted leaf tips, and malformed seed heads. Severe copper deficiency will stop plant growth and kill plants during tiller formation. During pollination, copper deficiency will cause florets to remain partially open. This creates a vulnerable period for diseases, such as Fusarium head blight (head scab) and ergot, to infect the seed head and reduce grain yield.

Small grains sensitive to copper deficiency include barley, oat, rye, triticale, and wheat (including durum, spring, and winter types). Copper deficiency is most common on soils with less than 0.5 ppm Cu. Soils with low soil test Cu frequently include sandy soils with low organic matter (<3.0%) and organic soils (peat) with very high organic matter (>10%). Between soil and plant analysis, diagnosing copper deficiency with soil analysis is the most predictive. Plant analysis is less helpful because the plant Cu concentrations in sufficient and deficient plants are very close.

The most effective strategy to build soil test Cu on mineral soils is to broadcast-incorporate copper sulfate (25% Cu). building soil test Cu for many years. Do not mix copper sulfate with seed-placed dry fertilizer blends for air drills; copper sulfate is a hygroscopic (water holding) material that makes blending difficult and bridging is a concern. For seed-placed copper, use a liquid copper source injected in furrow. Liquid copper sources include dissolved copper sulfate and various chelated Cu products.

On organic soils, soil test Cu is difficult to build as copper readily forms complexes with soil organic matter. To reduce copper complexation, apply seed-placed liquid copper at planting and follow with foliar copper in the first herbicide application. Liquid copper sources include dissolved copper sulfate and various chelated Cu products.

John Breker

Prevented Planting Acres in 2020: Maximizing Cover Crop Effectiveness

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In 2020, there are again widespread acres of Prevented Planting (PP) in North Dakota and northwest Minnesota. Farmers are now making plans to plant cover crops on unplanted cropland in the next few weeks. It is important to establish cover crops on PP fields because growing plants help reduce the chance these fields will be PP fields again next year.

Let’s look at the major reasons why cover crops are valuable tools on Prevented Planting acres.

Soil Water Use

A field without any growing plants is a fallow field. Before no-till, summer fallow was a widespread soil water conservation strategy in dryland agriculture. Actively growing plants transpire (use) a lot more water than evaporation from the soil surface alone does. Cover crops help fill the water-use void by transpiring a lot of water, helping to dry the soil surface and lower the water table before the following year. This also opens space in the soil profile for summer and fall rains to leach soluble salts from the soil surface and reduce salinity in the root zone.

Soil Erosion Control

Tillage is a popular weed control tool, but it also destroys crop residue and leaves soil exposed and vulnerable to water and wind erosion. Planting cover crops protects the soil surface from rain and wind, keeping soil firmly in place. Just because you cannot grow a cash crop on the field this year, you should not let your soil blow into the next field, letting your neighbor farm it next year.

Weed Control

An established cover crop can compete with weeds, helping suppress weed growth and weed seed production. For fields with problematic broadleaf weed histories, a cover crop mix containing only grass species is preferred. In grass cover crops, you can still use selective broadleaf weed herbicides to control the problematic broadleaf weeds of conventional or no-till systems such as Canada thistle, common ragweed, kochia, volunteer canola, and waterhemp while not killing the grass cover crop. For fields with low weed pressure, a cover crop mix containing grasses, brassicas, and legumes will provide more soil health benefits.

Soil Biological Activity

Have you heard about “fallow syndrome” before? Fallow syndrome is an induced nutrient deficiency, often seen in corn following fallow, when the population of mycorrhiza fungi is insufficient to colonize plant roots and help them acquire water and nutrients. Mycorrhizae are especially important in plant uptake of phosphorus, so plants with fallow syndrome often show phosphorus deficiency symptoms. Fallow syndrome is a major concern in corn following summer fallow or Prevented Planting without cover crop.

During the Prevented Planting year, it is important to include grass species in the cover crop mix to support and maintain the mycorrhiza population through next year. Brassica species, like radish and turnip, are often included in cover crop mixes for their deep taproot architecture and high forage value for grazing livestock, but brassicas do not support mycorrhizae. You do not want a cover crop mix consisting of brassica species alone because fallow syndrome might occur next year.

 

In June 2020, excessive rainfall slammed some parts of the upper Midwest and northern Great Plains, drenching soils with 3 to 15 inches of rain over a couple days. On summer-flooded fields, cereal rye is an attractive soil management tool. You can plant or fly on cereal rye well into August or mid-September, and it will continue to use soil water through late summer and fall. Next spring, the overwintered rye will grow again, using more soil water and maintaining soil structure, providing you with a much better chance to plant the field. If soybean is the next crop, you can plant glyphosate-tolerant soybean into green cereal rye then terminate the cereal rye with glyphosate later. This practice has become more and more popular on difficult fields.

Do not forget about soil fertility and plant nutrition for cover crops. A modest application of nitrogen will help cover crop establishment, plant water use, and competition with weeds, as cover crops with adequate nitrogen will grow faster and larger than those without nitrogen. Around 46 lb/acre nitrogen (100 lb/acre urea, 46-0-0) should be enough to establish nice cover crop growth. Prevented Planting fields, being wetter than those successfully planted in spring, lost some, if not most, soil nitrogen via nitrate leaching or denitrification. Although additional nitrogen may have mineralized from soil organic matter during May and June, excess precipitation in June may have caused additional soil nitrogen loss. The best way to know is collecting 0-12 or 0-24 inch soil samples for nitrate-nitrogen analysis.

As you choose the appropriate cover crop mix on Prevented Planting fields, you must consider the pros and cons of each cover crop species and how each will help accomplish your goals. These are some helpful resources that will provide additional information on what cover crop options will work best on your fields.