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SCN management update

November 30, 1999  By Blair Andrews

Providing soybean cyst nematode (SCN) resistance in a broader range of varieties and using more sources of resistance are the main goals of the latest soybean breeding efforts to help farmers manage this destructive and expensive pest. Touted as the most yield-limiting disease of soybeans in North America, SCN continues to expand its reach into Ontario’s growing regions.

Albert Tenuta, field crops pathologist with the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA), says the small, parasitic roundworm that attacks the roots of soybeans is no longer limited to just southwestern Ontario. “In the past, we talked about SCN being in the lower six to eight counties from Woodstock down to Windsor. Now, we’re starting to see it move up into Huron and Bruce counties as well as shifting into the Niagara Region,” he explains.

SCN has also moved into the Greater Toronto Area, and near Trenton and in Northumberland County. Most recently, Tenuta says detections have been made in the Ottawa Valley. “The moral of the story is, down the road, Ontario will be like any soybean area that has SCN: it will eventually find its way through most of the regions.”


As SCN has spread through the province, soybean breeders have been working to develop varieties across a broader range of heat unit zones to keep pace with the pest. “SCN is a key disease target for us in our soybean breeding program, and the trend I’ve seen with our lineup is we’re getting the SCN protection in the germplasm with earlier and earlier maturities,” says Mark Lawton, Monsanto technology lead for Eastern Canada. “The survey work of Albert Tenuta and Tom Welacky of Agriculture and Agri-Food Canada (AAFC)’s research station at Harrow, Ontario, has shown that the distribution has changed over time and we’ve made adjustments to ensure we have SCN-resistant genetics included in much earlier maturities than we would have had five or 10 years ago.”

Growing SCN-resistant varieties has been the chief tool to help farmers manage the economic risks of the pest.

When SCN emerged as a major threat in the early 1990s, Tenuta says it took a heavy toll, slashing yields by about half. He notes that growers, who were accustomed to yields of 50 or 60 bushels per acre, were calling in with reports of only 20 to 25.

Since the introduction of SCN-resistant varieties, Tenuta says that farmers have been able to realize the higher yield levels again. The higher yields are also demonstrated by research conducted by OMAFRA and AAFC in Harrow.

According to data from 2009, SCN-resistant varieties outyielded susceptible varieties by about 53 percent at Highgate and 20 percent at Leamington.

Tenuta says the results underscore the importance and effectiveness of SCN varieties when used even under low SCN pressure. He says that the greatest increase in SCN population densities occurred when a susceptible variety was used. In contrast, a general reduction in SCN populations was observed for both the PI88788 and Peking sources of resistance.

In addition to offering SCN resistance in more maturity zones, another effort is focused on adding more sources of resistance to the germplasm.

According to Monsanto, nearly all of the SCN varieties derive resistance from a single source. This makes the germplasm vulnerable to shifts in SCN populations.

Results of collaboration
Monsanto is collaborating with BASF to research transgenic approaches that would allow the company to deploy additional SCN resistance mechanisms. “The transgenic approaches being investigated are focused on controlling current SCN populations as well as minor and emerging races of the pathogen,” says John Pitkin, Monsanto SCN project discovery lead. “Both greenhouse and field SCN screens are being deployed to evaluate genes identified in several independent gene discovery screens.”

Pitkin adds that Monsanto would determine when and where to deploy the new and improved sources of SCN resistance through an annual SCN population survey throughout the US.

The companies are also collaborating on an effort involving conventional germplasm, but at a reduced rate compared with the transgenic work. More of the conventional, or non-GMO, research is being left up to smaller companies or public breeding programs.

A prime example is the program of Dr. Vaino Poysa, a soybean breeder with AAFC, based at Harrow. “I think we are making significant progress. We’re getting an increasing number of breeding lines and varieties that have yield levels equivalent to non-SCN resistant material moved into commodity beans as well as the beans that are dual purpose, or food-grade beans,” says Poysa.

The challenge for the conventional breeders is to strike the balance of boosting yields while maintaining the quality traits desired by the non-GMO, food-grade markets.

In addition to using traditional Asian sources, Poysa is also working with some materials developed in the US Midwest that have been used for commodity soybeans. “So we are developing lines that have improved yield. But it’s a struggle to get it back into the overall quality package that is required for the Identity Preserved end users.”

Poysa adds that the AAFC program is also trying to incorporate several alternate sources of SCN resistance into its breeding lines. He is optimistic that the work will result in the introduction of commercial varieties in the near future. “We are looking at releasing, this next winter, an alternative source of SCN resistance. And during the next five years, I think we can see a number of alternative sources coming out,” says Poysa.

For now, the current sources of resistance seem to be standing up well to SCN. However, Tenuta is starting to see shifts in the nematode populations in southwestern Ontario. “We’re seeing some of these new races, or HG (Heterodera glycines) types starting to develop in Essex, Kent, Elgin, Middlesex and Lambton that bypass the 88788 PI resistance,” says Tenuta. “That’s where Peking and some of these other resistance sources down the road will come in.”

Besides alternative sources of resistance, the crop protection industry is also working on seed treatments that incorporate nematicides to help protect the soybean roots. Although these treatments have not been registered for use in Canada yet, Tenuta is hopeful that the products will be available within a few years.

As the industry works on new efforts to help farmers control SCN in the future, Tenuta reminds producers of the key steps to follow. Noting that the emergence of new HG types of SCN is in its infancy in Ontario, Tenuta says the development of new populations can be delayed by rotating the use of SCN-resistant varieties. “Because there is no resistant variety that is immune to SCN, they all will have some degree of cyst reproduction on them. When you plant that same variety over time, you increase that percentage of the population that can feed on it and reproduce on it,” he says. “The trick here is not to use the same variety, because the cysts will adapt to it, and by the time the cysts adapt, we’re in trouble.”

In other words, Tenuta says, the idea is to keep the populations of SCN in the field “off balance” by rotating different soybean varieties as well as different SCN resistance sources. “You don’t want to shift those populations to some of these other races or types that we don’t have resistant genes for. The evil you know is better than the one you don’t know,” he adds.


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