Catching a Silent Yield Robber

If you have mysterious yield loss this season, a destructive soybean pest could be at work in your fields.

“Soybean Cyst Nematode (SCN) is a silent yield robber,” says Marijke Vanderlaan, Syngenta Agronomic Sales Rep. “Growers could lose up to 30% of yield without even seeing above-ground symptoms.”

As Vanderlaan explains, even though you can’t see it, SCN could be damaging 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 has infected a field, it must be managed.

Sampling is critical to confirm if the pest is present and to get on top of it before the nematode population gets out of control. 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 Vanderlaan.

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.”

Vanderlaan suggests collecting the SCN sample at the same time as probing for soil fertility. “I often tell 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.” 

SCN is spreading

Vanderlaan expects that many Ontario soybean growers will receive a positive SCN result in 2019 because of the significant number of Sudden Death Syndrome (SDS) cases she witnessed this year and last. The correlation is that SCN feeding creates wounds on the roots that allow other pathogens to enter, like SDS.

“Last year, basically every sample we took where there was SDS, there was SCN. What’s surprising is that it was even found in areas not known to have SCN in the past,” says Vanderlaan. “It was not always high levels, but it showed me there is a lot more SCN out there than we think. And it continues to spread.”

U.S. soybean growers know this reality 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 Ontario 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

Tylka’s advice to growers to manage the issue includes rotating resistant varieties. “Farmers should seek out the best varieties, and by best I mean high yielding, but with good nematode control,” he says. “Farmers spend a lot of effort finding the right varieties and then to be told you really shouldn’t ever grow it again is a tough pill to swallow.”

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. In Ontario non-host crops include corn, wheat, barley, oats and alfalfa.

The other practice he recommends to keep SCN at bay 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

Vanderlaan agrees that seed treatments have a role to play in managing SCN and resistance.  “Whether it’s SCN, fleabane or western bean cutworm, 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.”

SCN is a complex issue to be sure, but probing for the pest this fall will help take the mystery out of your fields and allow you to make informed seed choices.