Catch a Silent Yield Robber

With Soybean Cyst Nematode (SCN) confirmed in a small number of municipalities in southern Manitoba, we need to look at several important management factors in the future as the disease spreads.

“Soybean Cyst Nematode (SCN) is a silent yield robber,” says Bryce Rampton, Soybean Product Development Agronomist. “Growers can often lose yield potential without even seeing above-ground symptoms.”

As Rampton explains, even though you can’t see it, SCN could potentially damage your crop’s root system. The pest feeds on soybean roots and disrupts the plant’s ability to take up water and nutrients. Unfortunately, once SCN populations start to build within a field, it must be managed.

With the initial findings for Manitoba showing extremely low populations in a small subset of acres, dramatic changes in management for 2020 are likely unwarranted.  In the meantime, some testing and planning can help our management decisions in the long run.  Sampling is important to first confirm if the pest is present and to get on top of it before the nematode population starts to build up to threshold levels. Testing for SCN is something growers can do at any time. “As long as you can get a soil probe in the ground, you can sample for them, but the best time is in the fall,” says Rampton.

Sample in the root zone

The advantage of SCN testing in the fall is that you can sample in the root zone. “When you put your probe in at a 45-degree angle to the root zone, you’re more likely to get a positive result because that’s where the cysts build up. When you sample before planting, post-harvest, or in a field that does not have soybeans, you’re just random sampling and you might not get an accurate result.”

Rampton suggests collecting the SCN sample at the same time as probing for soil fertility. “I have told growers when you do your fertility sampling in the fall, just split the sample in two. Get the lab to test one for SCN and the other one for fertility.” This video includes a helpful example of how to collect an effective sample for testing.

Midwest SCN shifts

U.S. soybean growers know the reality of SCN all too well. They’ve been dealing with the pest since it was first discovered south of the border in the 1950s. More recently, they’ve been challenged with the problem of resistance. SCN-resistant soybean varieties, which were first introduced more than 60 years ago to keep the pest in check, are losing their effectiveness. It can be a difficult concept to grasp, but it’s not unlike herbicide-resistant weeds that some prairie growers are dealing with today.

Resistance is breaking down

“Quite literally, the same phenomenon is happening with SCN resistance,” explains Greg Tylka, a professor and nematologist with Iowa State University. “It’s only a little more complicated in that it’s not a single chemical – it’s a set of resistance genes. There was always a tiny portion of the population that could feed on the resistance and by using it over and over and over again for decades, we simply have allowed it to build up.”

He makes it clear that growers aren’t to blame for this situation. “They didn’t have a choice. At one time, there was only one source of resistance available,” he says. According to Tylka, in the fall of 2018 there were 830 SCN-resistant varieties available for Iowa soybean farmers, and 796 were from the PI 88788 breeding line. The other 34 were varieties from Peking breeding line, which isn’t yet commonly available on either side of the border.

Rotation is critical

Rotating to non-host crops is another suggestion. “Any year you don’t grow a host crop – one that the nematode’s able to feed on – then SCN numbers will drop,” says Tylka.  Western Canada has many non-host crop choices including canola, corn, wheat, barley, oats and others.

The other practice he recommends to keep SCN populations low is the use of seed treatments. “When farmers use a seed treatment, it may provide extra protection that will help slow the loss of effectiveness of resistant varieties,” says Tylka.

Seed treatments reduce SCN feeding

Rampton agrees that seed treatments have a role to play in managing SCN and resistance.  “Whether it’s SCN, kochia or any other diverse crop pest, we need to be using multiple modes of effective action against the pests in our fields to delay resistance as long as possible. Seed treatments are more than just insurance. Using the appropriate seed treatment will protect against SCN and leave you with a healthy root system that’s going to benefit you throughout the season, especially when stresses arise.”

For more information on seed treatment activity on SCN visit https://www.syngenta.ca/Productsdetail/clariva-pn