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Soybean cyst nematode: colonizing new frontiers

It could be lurking in a nearby field, producing no obvious visible symptoms but decreasing soybean yield by 30 percent or more. Soybean cyst nematode (SCN) management is practically cost-free and can yield bushels in rewards. However, growers continue to be caught off guard as this soil-dwelling plant parasite spreads to new fields, both within and outside southern Ontario. “It’s just a matter of time until it’s detected both in Quebec and Manitoba,” says Albert Tenuta, plant pathologist with the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA).

In fact, the Canadian Food Inspection Agency (CFIA) found a viable cyst in soil sampled from the central agricultural region of Manitoba in fall 2009, confirms Lois McLean, plant protection specialist with the CFIA. However, SCN was not detected in spring 2010 follow-up samples from that field.

By the time SCN gets to the point that it produces visible damage, its population numbers can be so high that it takes years of careful management to bring them back down to reasonable levels, says Tenuta. By then, the grower has already experienced “many years of yield losses due to it,” he says. “We want to avoid, in these new areas, some of the problems that we encountered in southwestern Ontario,” he explains. “The important thing is for growers to manage it now and detect it while the levels are very low, because at that stage, the nematode populations are much easier to manage than if we had to go through those very high levels that we had seen in southwestern Ontario, which took many, many years to manage.”

That is why Tenuta is participating in a large-scale, international outreach program to improve SCN awareness and management. Begun in 2008, the project involves research and extension personnel and soybean growers from Ontario and 12 north-central US states and is supported by OMAFRA, Agriculture and Agri-Food Canada (AAFC), Grain Farmers of Ontario and the Agricultural Adaptation Council through Canada’s Growing Forward initiative, the US North Central Soybean Research Program, and 12 US land-grant universities. Extension personnel are working with growers at two or three locations known to have SCN in Ontario and each of the 12 states each year. They are using strip trials to compare yield from susceptible and resistant commercial soybean  varieties, count SCN population numbers, and look at how well the SCN populations can develop on the resistance sources.

Substantial yield losses
In preliminary results from the Ontario strip trials, the resistant varieties out-yielded the susceptible varieties by 53, 49, 45 and 20 percent at two locations near Highgate (Municipality of Chatham-Kent) and two near Leamington (Essex County) in 2008 and 2009. Soil tests indicated that the locations used in 2008 had more than 3000 SCN eggs per 100 cubic centimetres of soil, which was considered a high SCN density. In absolute numbers, yield loss from using a susceptible variety in these four fields ranged from about 9.8 to 33.9 bushels per acre.

Even at locations with lower SCN egg densities, susceptible soybean varieties are at risk of significant yield loss, particularly in years when the plants are stressed, for example, by drought, says Dr. Loren Giesler, plant pathologist at the University of Nebraska-Lincoln and one of the project’s co-leaders. “Also, in the years when they don’t have any moisture stress, and this applies across the whole region and Ontario as well, you get really wet years and great bean growing conditions, and people would say: ‘I put a resistant bean out but there’s no difference in yield’. But if they look at the SCN population, it’s just growing exponentially during those really good production years,” he says.

That increase in SCN numbers means that a field could go from having a low risk of yield loss to having a high risk of substantial yield loss. Tenuta says, “Where we had the susceptible variety in one rotation, we’ve increased SCN populations three-, four-, five-fold, or more; 20-fold in some cases. You can go from 1000 eggs up to 20,000 eggs pretty easily.”

“So it’s a long-term commitment and a never-ending battle against SCN,” he emphasizes.

SCN can overcome resistance
Like any pest-control technology, SCN-resistant soybean varieties can lose their effectiveness after repeated exposure of the pathogen to the same source of resistance. Because it is difficult to get SCN resistance into soybeans without losing agronomic performance, current soybean varieties generally have resistance from only one of three sources, or plant introductions (PIs): Peking (PI 548402), PI 88788, or Hartwig (PI 437654). Currently, only varieties with Peking and PI 88788 sources of resistance are adapted for Ontario growing regions. So, a major component of the outreach program, as well as Tom Welacky’s SCN research program at AAFC in Harrow, Ontario, is to see how much SCN populations can change over a single growing season in terms of their ability to reproduce on resistant varieties.

One intention is to determine how fast shifts can occur in SCN populations. “Everybody is financially driven,” says Giesler, “So if you get a variety that looks really great and has yielded well, you tend to not think about the SCN population, and that, long term, leads to problems.”

For example, SCN populations are already able to reproduce well on the PI 88788 resistance source in many fields across the north-central US region. 

Rotation is key to keeping SCN numbers down and prolonging the efficacy of resistance. In addition to rotating soybeans with non-host crops for SCN, growers should be sure to rotate the resistant soybean varieties that they plant to minimize repeated use of the same resistance. Even rotating among varieties that have the PI 88788 source of resistance, for example, can be helpful because multiple genes contribute to resistance, and each variety could have a different complement of those genes, says Welacky, who has been mapping SCN populations in southwestern Ontario for several years.

Another reason for looking at SCN population changes is for future industry planning. All three experts say that it will give an idea of how durable the current resistance is and will provide data to encourage the soybean industry to develop varieties that have new sources of resistance. A number of PIs with SCN resistance have been recognized, although not all have been incorporated into commercial varieties.

How often to test soil
To determine whether management is needed, all soybean growers in regions that have the potential for SCN should be having their soil tested for SCN. Giesler stresses that it is not necessary to test every year; he encourages the growers he works with in Nebraska to test once every five or six years to see if SCN has arrived and get an estimate of the population size.

Tenuta suggests testing once every three to six years, particularly to follow changes in known SCN populations. “If you’re on a two-crop rotation, just corn and soybeans, then you’d be looking at maybe every four years. If you’re on a corn, soybeans and wheat rotation, then you probably would do that every six years. One of the keys is, the more frequently you sample, the more likely you are to catch the changes as they’re developing.”

However, the fact that there are still growers who have no idea whether they have SCN or not continues to astound extension personnel. “Most of the growers in southwestern Ontario have dealt with SCN,” says Tenuta. “But there’s still surprise that every year, we get some growers that aren’t quite aware of the full extent of the injury that can occur with SCN. When we’re on a farm and show them, they are surprised that they did have cysts because they thought they shouldn’t. But the nematode knows no boundaries and doesn’t respect their opinion.”

Rather than digging and examining soybean roots for cysts, Giesler recommends having the soil tested because he thinks this method is more reliable. Even in fields with known SCN, Giesler says it can be difficult to find SCN on roots. For growers who have never sampled for SCN before, he says, “Target some sampling around the entryways, low spots and areas where it floods, or places where soil moves into the field. And then go from there. If they get a negative, they don’t worry about it; come back five or six years later and check it again.”

Tenuta says that looking at soybean roots for cysts can be informative as long as growers examine multiple plants from a number of areas in the field, particularly if aboveground symptoms are evident. He says it can be a helpful diagnostic tool, but growers should not rely on that method alone for determining the field’s SCN status. Welacky and Tenuta recommend having the soil tested every three years or so in regions of Ontario with known SCN problems. For help with soil sampling and information about where to send samples, growers can consult their local extension personnel (e.g., OMAFRA), company agronomy representative or the comprehensive Soybean Cyst Nematode Management Guide, Fifth Edition, available from the Plant Health Initiative of the North Central Soybean Research Program (see below for further resources).

“I really think there’s such a need for awareness,” says Giesler.  “Just that growers are paying attention and are checking for SCN. If we could somehow get that message out and producers would do that, I just think it would make them so much money and save them so much over the long run that it would be so worthwhile.”

Further resources:

• SCN Management fact sheet: www.planthealth.info/pdf _docs/SCN_mgmt_NCSRP.pdf
• SCN Management Guide, Fifth Edition: www.planthealth.info/pdf_docs/SCNGuide_5thEd.pdf
• Plant Health Initiative website: www.planthealth.info
• OMAFRA Publication 811: Agronomy Guide for Field Crops, Chapter 14 – Diseases of field crops, soybean diseases: www.omafra.gov.on.ca/english/crops/pub811/14soybean.htm