From 2005 to 2015, Canadian soybean production increased by almost 98 per cent. But as soybean acres spread across Canada, so does the risk of yield-crushing soil-borne diseases. Knowing what could be lurking beneath the soil, and what to do about it, can help define your crop’s success.
“Soybeans have been grown in Canada for 100 years, but the crop has really only taken off in the past 30 years,” says Albert Tenuta, field crop pathologist with the Ontario Ministry of Agriculture, Food and Rural Affairs.
“It is now grown successfully in Quebec and Manitoba and is expanding into the Maritimes and western prairies. Because soybean commodity markets have been strong, growers are showing increased interest in producing the crop, and breeders have been able to develop shorter season varieties that work in those regions.”
According to Statistics Canada, in 2016, soybeans were the fourth most seeded crop in Canada in terms of acreage, and the third largest field crop in terms of cash receipts. More than 2.2 million hectares were seeded last year, producing six million tonnes of soybeans, making Canada one of the top 10 soybean producing countries in the world.
Currently the biggest impediment to the crop’s success is disease, with soybean cyst nematode (SCN) topping the list — its impact is greater than the next five diseases combined. In Canada, SCN is only an issue in Ontario and some areas of Quebec. However, once established it’s impossible to eradicate. Even the most extended rotations can’t eliminate it since cysts can live in the soil for decades.
“The cysts are very hardy and easily overwinter in the ground,” says Tenuta. “And most of the damage from SCN happens below ground. Each cyst can contain 40 to 300 eggs that produce juvenile nematodes that feed off a soybean root. The female will migrate inside the root and set up a feeding site where a lemon shaped cyst will form on the root of the plants.”
Above ground, soybeans can look normal or, in particularly stressed areas, become stunted. The plant will yellow, mimicking a potassium deficiency. “Identification is one of the big challenges with the disease,” says Tenuta. “It looks like so many other things. People wonder if it’s compaction, or pH issues, fertility problems or herbicide stresses.”
Partner in cri me: SDS
Another soil-borne disease that requires particular attention is sudden death syndrome (SDS), which is aided and abetted by SCN. The root damage caused by SCN provides an entry point for SDS, as well as other early-season diseases.
“SCN and SDS go hand-in-hand,” says Holly Derksen, field crop pathologist with Manitoba Agriculture. “While you typically find SDS in SCN-infected fields, it can occur on its own. But they like different conditions. SDS prefers cool and wet soil and shows symptoms later in the season, SCN likes a wide variety of weather conditions and shows up earlier in the season. Both can dramatically reduce yield.”
As with SCN, most SDS damage happens underground, so correct identification after emergence is important. But that can be difficult. SDS symptoms are very pretty: veins on soybean leaves stay green while the tissue between them turns a » brilliant yellow then brown before falling off entirely, leaving the green veins behind.
Sudden death syndrome rates have been steadily increasing in Ontario, particularly in the Toronto-Windsor corridor, and has not yet been confirmed in eastern Ontario, or any other growing region in Canada. But as populations build, it’s only a matter of time.
Watching and waiting
“Manitoba soybean growers are experiencing a prolonged honeymoon period in terms of disease concerns,” says Derksen. “While we have had to deal with some phytophthora root rot, in a region that struggles with high levels of disease in cereals and oilseeds, we haven’t seen any SCN or SDS so far.”
Manitoba Agriculture conducts an annual disease survey to try and get a picture of what growers can expect in the coming season. “We really need to keep a sharp eye on our neighbours to the south,” says Derksen. “We know that there is SCN right up to the Canadian border of North Dakota and Minnesota. So, it’s likely in Manitoba, but we just haven’t found it yet.”
SCN spreads through the soil and so any opportunities to minimize soil movement will help. “Growers need to be on top of their biosecurity,” says Derksen. “They should be careful about transporting soil from the U.S. and they need to be cleaning their equipment carefully. But they can’t control flooding or erosion or movement from rivers, so it’s impossible to completely prevent its movement north.”
Keeping disease under control
Once these diseases are in your field, it becomes a matter of managing their effects going forward. A soybean crop can see a 30 per cent reduction in yield with no visible symptoms above ground, so step one is proper identification.
“If you think you have SCN, which is the first disease you’ll likely find, use a shovel and dig up some of the roots,” says Tenuta. “Don’t pull the plant as that will leave many of the tiny cysts in the ground. They are the size of a period at the end of this sentence so they are easy to shake off or miss on a visual inspection.”
Growers can send in a soil sample to confirm disease presence and levels in the soil. After that, it is all about managing for the disease, starting with choosing a resistant soybean variety. There are a number of them available for all lengths of growing seasons in Canada.
And even though it won’t eliminate SCN, crop rotation is an important tool to reduce soil inoculum. Corn and wheat are good rotational crops, while beans, peas and legumes are less successful since they’re also identified hosts and provide a reproductive source.
Seed treatments also help to suppress disease levels, but they must be used within an integrated pest management system as they can only reduce, not eliminate, the risk of disease development. There are nematicides available for SCN, and a new seed treatment this season has shown to suppress both diseases. (see sidebar)
“Any time a plant has to defend itself it has to divert resources,” says Tenuta. “Little stressors can create bigger stressors as the pressure on that seedling or plant accumulates, so making sure your seed is fully covered against potential diseases will give it the best chance to tackle whatever is hiding in the soil.” FF