Above: When mature, female Soybean Cyst Nematodes protrude from the root forming tiny white/brown cysts Source: Craig Grau, Bugwood.org
The Problem
Soybean Cyst Nematode is the number one pest of soybeans across the US causing more yield loss than the next five diseases combined. Many farmers are unaware that they even have a SCN problem. That is mainly because, in many cases, the only symptom is yield loss.
Farmers can have up to a 30% soybean yield loss and not even see visual symptoms of SCN. Furthermore, SCN can promote the onset of other diseases such as Sudden Death Syndrome and Brown Stem Rot. Yield loss then gets blamed on these diseases – not realizing they are secondary results of the larger problem of SCN.
One Solution
Currently, two types of SCN resistance are commonly bred into soybean varieties. The first and oldest trait is known as Peking. Peking is complicated from a breeding perspective, so it is difficult to bring into new varieties. Furthermore, historically, Peking was linked to some lower-yielding soybean characteristics.
For these reasons, breeders moved away from this trait and towards a newer trait called PI88788, which is easier to breed and did not display yield drag. Since 2006, over 95% of the soybeans grown in the US have used PI88788 as SCN protection. It’s no surprise then that we have selected for SCN populations that are resistant to PI88788. Some estimate that nearly 90% of SCN populations display at least some resistance to PI88788, which means those soybean fields are losing yield.
Where is SCN Found?
We have found SCN to varying degrees in nearly all soil types throughout our area. The impact of SCN on soybeans is a function of the egg number and soil texture where coarse soils (more sand) tend to have higher yield losses. To determine the potential impact of SCN on your farm, start by sending a soil sample to the lab to determine egg counts.
Take a 6-inch soil sample at a 45-70° angle into the previous roots of the soybean row. You don’t have to refrigerate the sample but it is best to ship it as soon as possible. The highest egg counts will be in the spring after soybeans, about a month after the corn is planted. The SCN Coalition has an online calculator that can be used to estimate the yield impact of the egg count in your soil texture.
Cover Crop Impact on SCN
There are a few ways that cover crops may impact pest nematodes.
- They may release some anti-nematode compounds into the soil that inhibit nematode growth or their ability to hatch.
- Perhaps, they support the growth of microbes that attack nematodes.
- Alternatively, they could have properties that promote hatching when there are no hosts in the soil to sustain them.
- Lastly, they may act as a true “trap crop” that encourages hatching, attracts, and is infected by juvenile nematodes, but is not a source of food for them.
Any of these mechanisms would crash SCN populations.
Plant Cover Crops to Improve Soil Quality and Biological Diversity
Several university studies have shown that cover crops of red clover, sunn hemp, crimson clover, cereal rye, oats, daikon radish, or ryegrass reduce the number of eggs in the soil and/or reduce the number of female SCN on soybeans in the following season.
There are likely specific cultivars/varieties of these crops that have a larger impact on SCN but that kind of detailed research is complicated. So actual data is weak concerning specific, optimal varieties. This is one of those areas where attentive farmers will probably know the best varieties before confirmation by researchers.
On the other hand, field peas, green beans, pennycress, deadnettle, and henbit are all hosts of SCN to varying degrees. These crops or weeds need to be carefully considered in nematode management and rotation plans.
In short, there are no silver bullets for SCN control. So, farmers need to adopt a holistic management plan to deal with the problem.
Below are a few ideas that I think should be part of the plan.
Management of Soybean Cyst Nematode
- Follow the numbers: Soil sample for eggs after soybeans. Agvise Labs in Minnesota is reliable and quick. Just keep good track of these data over time to know how your farm management is influencing SCN. Don’t forget about yield data, quality, and profitability – these are likely better indicators of your ability to manage SCN than soil egg counts.
- Stay Green: Keeping diverse, non-host, green plants on the field as much as possible improves soil health and diversity and exposes SCN to a variety of challenges. Fallow is not a good choice.
- Rotate Crops: Use as many non-host crops as possible like corn, alfalfa, small grains (wheat, oats, rye), or sorghum. While SCN eggs are viable over a long period, non-host crops will limit population booms.
- Rotate SCN Resistant Varieties: Try a Peking bean in your field. Peking is becoming easier to find and yield-competitive. Besides, most SCN populations are resistant to PI88788.
- Keep Up with Water and Fertility: Follow a solid nutrient plan based on regular soil samples and complete fertility including potash, phosphorus, sulfur, and micronutrients. Don’t forget timely foliar fertility. This will reduce stress and limit yield loss due to SCN infestations.
- Watch your pH: SCN likes high pH. Use a recent soil sample to drive lime applications. Don’t spread lime when it isn’t needed. Be careful with lime applications on weakly-buffered soils and high-pH manure.
- Seed Treatments: Use seed treatments to increase seedling vigor and limit the impact of secondary diseases like Sudden Death Syndrome.
- Weed Control: Certain winter annuals like Field Pennycress, Chickweed, Purple Deadnettle, and Wild Mustard are known to be SCN hosts.
- Limit the Movement of Soil: Clean machinery regularly, especially if you’re acquiring used machinery. Plant and maintain waterways to limit water erosion. Winter cover crops and minimal tillage may also be used to limit wind erosion in sensitive areas.
- Rethink double-crop soybeans: Many in our area struggle to get a consistent return on investment from double-crop soybeans after wheat. A better idea may be to use a cover crop after wheat to improve soil, increase nutrient cycling, and reduce SCN numbers.
Article posted by Ryan Lee. Read his bio here.
Questions? Don’t hesitate to get in touch with us.