Soybean cyst nematode frustration

Nematodes continue to rob farmers and frustrate agronomists, but a new biological seed treatment may soon help

It almost plays like a riddle. First are these clues: first confirmed here in 1988; called a disease by some and a pest by others; often ignored until it’s too late; and, impossible to eradicate.

Of course, there’s another clue which should be a dead giveaway. It steals more bushels than any other soybean threat in Canada.

For more than 25 years, soybean cyst nematode (SCN) has been confounding growers, plant breeders, crop pathologists and public and private researchers across North America.

Farmers know its name, and most now know what it looks like and what it can do, and they also understand the need for planting SCN-resistant varieties and for rotating different varieties containing different levels of resistance.

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Yet the losses continue.

For much of its first 15 years in Ontario, the disease (as it’s identified by the Ontario Ministry of Agriculture, Food and Rural Affairs), was found primarily in Essex, Kent, and Lambton in the extreme southern portion of the province before infestations started appearing in large numbers in Middlesex, Oxford and points farther east.

By 2007, the unstoppable pest was even confirmed to have spread east of Toronto, which had been considered only a matter of time. Provincial field crops pathologist Albert Tenuta even tried to put a positive spin on the situation, at the time calling it “good news” that it had actually been confirmed, and saying that with its identification, growers could begin using the available means to reduce SCN’s impact.

That hasn’t really happened, however, largely for reasons that are all too familiar: tighter planting windows, tighter rotations, and tighter operating margins.

All have helped SCN develop hot spots. Worse still, there are signs that SCN is continuing to spread and intensify, and parts of Huron and Bruce counties have been suffering higher concentrations.


photo: Albert Tenuta

Once found, it’s permanent

According to Chris Denys, seed care technical lead for Syngenta in Eastern Canada, SCN is permanent. Once it gains a toehold in your soil, it’s there for good. There is no way to eradicate it from your farm.

Now that it has been confirmed in at least 12 counties in Ontario and into western Quebec, that means a lot of growers have a challenge on their hands.

“We expect it’s going to continue to spread, and eventually it’s going to be in all of the soybean-growing areas across Ontario, especially when you see how the populations have shifted to those non-traditional areas that are much farther north, into Huron, Grey and Bruce coun-ties,” says Denys. “We’re finding some pretty extreme populations up there, sometimes more extreme than what we see down in Kent and Essex.”

That’s a bit of a surprise given the history of soybean production in Grey and Bruce counties and the relatively late spread of soybean production there.

“Part of it may be related to rent and land values, that maybe there’s some reluctance that way, that guys might not want to know when they’re in a situation where they’re renting or thinking of selling the land,” says Denys. He adds that he doesn’t come across that issue much in his travels. “Sometimes it’s just a matter of not being aware of the problem, or they can’t diagnose the problem in the field. You can have 30 per cent yield loss in soybeans due to SCN and not really recognize any above-ground symptom.”

Depending on the field, Denys says SCN losses could be blamed on compaction or a planter problem or water deficiency. Even when you do observe the root feeding, it’s easy to dismiss it as something else, and not SCN.

It’s part of a trend that’s becoming a fact of life for farmers as they’re being pulled in different directions. Some of those directions — like precision ag or traceability for consumers — weren’t on the agenda 10 or 20 years ago.

Still, SCN is a fact of life for growers now, and the consensus is getting stronger than ever; more needs to be done to mitigate the damage it can inflict.

Part of the process is to acknowledge key facts about SCN. They may already be known by a large percentage of growers, but they certainly bear repeating:

  • SCN is the leading yield robber in soybeans, more than the next five diseases combined. Sudden death syndrome (SDS) which can be a “companion disease” to SCN is a distant second on that list.
  • Cyst eggs can survive in the soil for more than 10 years, so even opting out of soybeans for a decade wouldn’t eliminate the nematode threat from a field.
  • When reproducing, a single cyst can release 200 to 500 eggs into the root zone, depending on the environment.
  • SCN can provide an entry point to other diseases, particularly SDS and brown stem rot.
  • Various weed species can be a host to SCN, although most of the susceptible weed species are not found in any significant concentrations in Ontario or Canada (see Table 1 below).
  • Conversely, corn, wheat, alfalfa, barley, oats, canola along with forage grasses, alfalfa, sugar beets, miscanthus and switchgrass are poor or non-hosts for SCN.
Source: Plant Health Initiative (U.S.) and the North Central Soybean Research Program (NCSRP) (U.S.)

Source: Plant Health Initiative (U.S.) and the North Central Soybean Research Program (NCSRP) (U.S.)

Sources of resistance

Another key SCN fact is that there are two primary sources of resistance: PI 88788 and PI548402 (Peking).

PI88788 is the primary source of resistance in up to 95 per cent of the soybean varieties now available across North America, says Denys. The Peking source accounts for most of the remaining five per cent, with a third source — PI437654 (Hartwick or CystX as it’s also known) unavailable in any commercial varieties currently in use.

As Denys points out, one of the reasons why the concentrations in Bruce and Huron counties are so much higher is because those growing regions may not have a wide variety of seed options with resistance available in them.

The issue of resistance is garnering more attention. There are new terminologies too, including HG Type 2, a term that refers to a shift in populations in a field toward nematodes that can thrive on crops that have genes that in past were able to stop them. In other words, you can plant a resistant variety, and a significant portion of the nematodes in the field can still feed on the crop and reproduce.

Don McClure, soybean breeder for Syngenta, also points to the changing definitions surrounding resistance, and how different diseases have different pathways and levels of resistance.

“One variety might have 90 per cent of the plant resistant to the pest, and another variety that’s classified as resistant might have 30 per cent,” says McClure, who’s based at the company’s facility at Arva, just north of London. “And that’s something that is not well understood. Even though they’re considered resistant, it’s a quantitative trait, so there’s more than one gene involved, and each variety may not have the full complement of resistance genes.”

Part of the challenge for breeders and researchers is to find the right combination of disease resistance and yield, says Don McClure of Syngenta.

Part of the challenge for breeders and researchers is to find the right combination of disease resistance and yield, says Don McClure of Syngenta.
photo: Supplied

With such a heavy reliance on the PI88788 source, McClure says, it only makes sense that the industry is starting to see more signs that our defences may be breaking down.

“I don’t think it’s a huge issue yet, but I’m sure there are populations that probably can overcome most of the resistance in PI88788 right now,” says McClure “They’re not widely distributed yet: we’re aware of it and we’re doing everything we can to alleviate that.”

Part of the challenge there is to find the right combination of factors that will meet all needs of the farmer, with superior disease protection plus consistent yield and agronomics all in one package.

Meanwhile, a new seed treatment, Clariva pn will be available for the 2016 growing season. Clariva is a biological, and as such is part of a new wave in the development of traits and bioagents that the pests will have a much harder time getting around.

Billed as a targeted solution to SCN, Clariva releases spores into the root zone that infect and kill any nematodes that come into contact with soybean plant roots. As those initial nematodes die, those ingested spores are released again and help to bolster that protective root zone.

It’s also different in terms of the source of resistance: it’s capable of working on the HG Type 2 populations and the other 10 per cent that the resistance is covering.

Farmers will have access to Clariva pn and can use it in concert with Cruiser Maxx Vibrance Beans or just Vibrance Maxx seed treatments.

This article first appeared in the February 2016 issue of Soybean Guide

Research reveals startling SCN numbers

Late in 2015, Syngenta Canada published a four-pronged strategy for detecting soybean cyst nematode (SCN). The strategy was circulated following the release of data that indicated excessively high concentrations of SCN in parts of Huron County and southern Bruce. In some fields, agronomists found concentrations of more than 4,000 SCN eggs per 100 grams of soil, sufficient to cause yield losses of greater than 50 per cent.

Here’s the text from Syngenta, titled “The Four ways to detect Soybean Cyst Nematode.”

  1. Know the above-ground symptoms: The most noticeable above-ground symptom may start off as circular patches within the field where soybeans are stunted and yellowed. Soybean cyst nematode (SCN) is spread through soil movement, so symptoms may first appear at field entrances or headlands.
  2. Pay attention to your yield monitor at harvest: Since many times visual symptoms may not occur, the best time to diagnose SCN is at harvest. Note areas in the field where harvest yields don’t make sense with the field’s history.
  3. Visually inspect roots: In August and September, carefully dig up root masses. Gently wash the soil off roots and use a hand magnifying glass to identify the cysts, but beware of only relying on a simple visual analysis since cysts can be difficult to see and may sometimes be confused with bacterial nodules.
  4. Have your soil tested: Since visual symptoms are not always evident, either in the field or on the roots, the most accurate detection is to have your soil tested. After harvest is an ideal time to test for SCN since egg and cyst counts should be at their highest for the season.

Above courtesy of Syngenta Canada

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Ralph Pearce

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