It’s the costliest soybean disease in the United States, and has been in Brown County for around 20 years now. These facts alone give us reason to stop and think more about this disease and how it may impact our operations. This blog post will cover the impacts of soybean cyst nematode and some of the best management practices you can practice in your fields. Do you have SCN? Have you been noticing a yield drag in spots in your field and are not sure what is causing it? Keep reading to find out more about SCN.
History
Soybean Cyst Nematode (SCN) likely entered the US from Japan. It was first found in North Carolina in 1954. Reports of SCN in Doniphan and Atchison counties surfaced between 1985-1989, and in Richardson county in 1986. SCN was discovered in Brown and Nemaha counties around 15 years later between 1998 and 2005. SCN is the most significant disease in soybeans in the U.S. While the exact damage in dollars is very hard to determine, largely because many farmers are not aware they have SCN in their fields, it has been estimated that it costs producers $1.5 billion in the United States. There are 16 different races or strains of SCN and they can cause as much as a 30% reduction in yield.
Description
Soybean cyst nematodes can go through 3-4 life cycles during a growing season (approximately every 24-30 days). The first cycle starts with a juvenile in the egg, then a second juvenile stage emerges from an egg. This is the stage that infects the plant roots. Here in the root, the juveniles feed and mature to the adult stage. If the juvenile becomes a male, it will leave the plant and live outside the root. The damage that it will have on the plant is limited to its juvenile stages. If the juvenile becomes a female, it will stay inside the plant and molt several more times as it reaches adulthood. Inside the plant, they will create feeding sites by modifying plant cells to provide them as much nutrients as they need to grow and mature. Inside the plant, the female will swell up and start to take the form of what is called a lemon body. Eventually, this form will burst through the wall of the root and live on the outside of the root. This is what we call a cyst. The appearance of cysts can be confused with nitrogen fixing nodules. They contain 200-400 eggs for the next generation. These eggs can live protected in the cyst for several years. This creates a challenge for producers for managing and suppressing populations. While hatched nematodes require a host root system to live on, hatching doesn’t occur until the eggs or larva receive a chemical signal from the root system that there is a host plant there to live on. Until that point, they can stay in egg form for years. This is one of the major factors in what makes managing SCN so challenging.
Impacts
Another challenging aspect of SCN is the lack of visible symptoms. Many producers are unaware they have an issue. Their soybeans look the same as 10-20 years ago, but you may have noticed one thing different; they don’t yield as much as they used to. This yield drag is the main symptom of SCN. Yield can be reduced by as much as 30%. Since visual symptoms are not always present, this yield drag can be attributed to many different issues such as nutritional deficiencies, compaction, or drought stress. Yield drag will always be more dramatic in your higher yielding areas where SCN is present.
In situations where visible symptoms are present, they will manifest themselves often as yellow or stunted growth of soybeans. This will generally be in a circular patch, or an oval elongated in the direction of tillage and planting practices. Plants in these spots will have poorly developed root systems and often experience early senescence.
Additionally, areas with SCN are more prone to infections of Sudden Death Syndrome. This combination of diseases can impact yield dramatically. Essentially, the SCN creates wound sites that the SDS is able to enter through. When SCN is reduced, infection by SDS is also reduced.
Management Practices
Soil Sampling
The first step in dealing with SCN is finding out what kind of levels of SCN are present in the field. Take several soil samples across the field. Each sample should not represent more than 10-20 acres. Alternatively, if you know of an area where you have decreased yields or suspect SCN, you can target samples in that area and also an area where you think the yield has been “normal”. Having an idea of the distribution of SCN in the field as well as levels of eggs will be very useful moving forward. This also provides a baseline level of SCN populations that can be used as a benchmark in future years to determine if populations have increased or have decreased. Samples can be sent to commercial soil labs or to Universities with a nematology lab.
Resistant Varieties
The most useful treatment of SCN is the use of resistant soybean varieties. Remember, resistance doesn’t mean that the plant is immune to SCN but rather that it has some ability to reduce the impact of SCN. There are 3 main sources of resistance, PI88788, PI548402 (Peking), and PI437654 (Hartwig) currently in use. PI88788 is the most common type of resistance and is the trait in 98% of resistant soybean varieties. Rotating sources of resistance is recommended in order to not create resistant strains of SCN in your fields. Decreased efficacy of the PI88788 is a real concern for producers. In fact a recent study from Nebraska showed that almost half the fields tested showed that SCN was able to reproduce on PI88788. There are 5 genes that make up PI88788 and they are not all expressed equally in different varieties carrying this resistance. If PI88788 is the only form of resistance available to you, make sure you are rotating varieties with this same trait as they will all react a little differently to SCN in the field due to the balance of the 5 different genes. When possible, rotate to varieties with a completely different form of resistance like Peking or Hartwig. This should help prevent resistance from building up in SCN populations.
Finally, while it is tempting to think of planting a susceptible variety of soybeans so the SCN in the soil will not grow resistant to the strains of resistant varieties being planted, this is an unwise decision. One year of a susceptible variety will negate 5 years of intense management.
Non-Host Rotations
SCN can use a variety of plants as host crops. This includes weeds. Consequently, rotating to a non-host crop as well as managing weeds in a timely manner are both important practices for reducing the impact of SCN. Several non-host crops are common in our rotations: corn, alfalfa, forage grasses, sorghum, and small grains like barley oats, rye, and wheat. Host crops include some that we include in our cover crop mixes: birdsfoot trefoil, sweet and crimson clover, cowpea, and hairy, crown, and common vetch. The following weeds are also considered host crops and need to be managed closely: field pennycress, henbit, pokeweed, purslane, and wild mustard to name a few.
The suggested rotation for fields with SCN is first a non-host crop, then 1 type of resistant soybeans, non-host crop, and a different type of resistant soybeans. SCN levels will likely decrease in a non-host crop year, but will not be eliminated. Some studies have looked at switching to a non-host crop like corn long term. While this would seemingly make sense, SCN will not be completely removed due to the length of time eggs can survive in cysts in the soil.
Seed treatment
Several seed treatments are marketed as effective against SCN. ILeVO is the most common and likely the most effective seed treatment. A study from Pioneer showed that in fields with high levels of SCN, ILeVO increased yields by 4.9 bushels. The new LumiGen offering from Pioneer includes ILeVO in its product lineup. If you have fields with known SCN, this would be a good option for you.
Sanitation
Want to prevent the spread of SCN from field to field? Cleaning equipment is key. SCN moves through the soil only an inch a year on its own. The way SCN moves farther through the field and from field to field is through soil movement. Soil on tires, equipment, shoes, even moving on animals and migratory birds can all be the means of transport. While some of these sources we are not able to control, we are able to clean tires and equipment off before heading to the next field to reduce the spread of SCN.
Resources
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