Soybean cyst nematode is such a serious problem, it’s worth taking a closer look at what the soil test for SCN means. Sam Markell, North Dakota Extension pathologist, answered our questions about it.
Q: What would be a low level of SCN where you can still grow soybeans, but need to rotate crops to keep eggs numbers from growing?
A. The key here is what a low enough number that it might not be “real?” We know that other nematodes are in the soil, and when soil sampling for SCN is done someone literally counts eggs in a microscope. So, it is possible that nematode eggs might not all be SCN, even if the soil sample is from a soybean field. In our SCN egg maps, we have categories of 0, 50-200 and so on. If you have 50-200 eggs, there is reason to be suspicious and you probably want to test again. A low number could be a false positive. Conversely, if you have a 0, it means there were no eggs picked up in the sample. It doesn’t necessarily mean you don’t have SCN.
If you have SCN, to me it doesn’t really matter whether you have 500 eggs or 30,000 eggs, you want to start managing it. Five hundred eggs can explode into extremely high numbers in one growing season (30,000+ is possible, and we have seen that in our trials). You might not have much yield loss that first year, but you will when you put soybeans back in the ground. Also, once SCN numbers are that high, they are really hard to beat back down. So, I would recommend a resistant variety at any level, realty.
Q: What is considered an extremely high level of SCN where you would want to avoid planting soybeans?
A: Again, no hard and fast rule. When you get to 10,000, it’s time to rotate. But, if you had super high levels and already rotated out a couple years, egg levels could be really high. At that point, going back in with a stout resistant variety is probably the course of action a grower could take.
Q: What gene confers SCN resistance to soybeans?
A: Short answer — PI 88788 or Peking. PI 88788 is in almost everything, like 95% or more of the resistant soybean varieties in the U.S. Perking is probably in most of the remaining varieties.
The better answer is a bit more complicated, but extremely important for growers to understand. With SCN resistance, it’s not a single gene that gives the variety resistance. What we have is a "source" of resistance. Basically, this means that a plant (the source) has resistance to SCN that may be conferred by many genes. In the case of SCN, the most common source of resistance is PI 88788. This literally means a "P"lant "I"ntroduction number 88788. So, PI 88788 is the wild soybean plant with resistance, and there are lots of little genes in it that contribute to this resistance.
There are lots of implications here. If you are a plant breeder and are trying to breed in resistance to a new variety, you need to try and get resistance genes from the source into the new line. Sometimes, you are not able to get all or many of the genes, but sometimes you can. This is a lot harder than just selecting for one gene such as for phytophthora, for example.
The negative result of this is that if 10 varieties have the same source of resistance (PI 88788, for example), they are likely not all equal. Some might be better than others simply because they have more of the genes that confer the resistance. Some will be worse.
If you are a farmer and are rotating PI 88788 varieties, you might actually be rotating the resistance genes. This has helped the resistance in PI 88788 last a long time. It is being overcome in much of the U.S. but seems to be holding in our area, for now.
Q: Are SCN seed treatments worth the investment?
A: They may have a place in management in the Dakotas, but they are not a substitute for resistance rotation, if you can help it. Our data is a bit scattered, but if you can control SCN right away with a seed treatment, that would be very helpful. I have seen data where this is the case. In our region, where really high egg levels exist and some management tools are hard to come by, seed treatments may be an option. There is not a ton of local data yet, but when growers have hots spots in fields, seed treatments present an option not present even five years ago. Also, I imagine they will only get better with time. Seed treatments are not perfect, every grower knows that you can still get root rot with a fungicide seed treatment. If you can get some help, though, that’s beneficial.