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Soybean cyst nematodes

Soybean cyst nematodes have been a long-time problem in U.S. crops. Mario Tenuta says it’s inevitable that we’ll find them in our fields


Soybean cyst nematodes are a very big problem in the U.S., accounting for over double the loss of soybean crops as compared to any other soybean disease. In 1987, Tom Welacky, field crops biologist for Agriculture and Agri-Food Canada, was one of two scientists who discovered soybean cyst nematodes (SCN) in Canadian farmers’ fields. 


“SCN is a minute microscopic, worm-like tube called ‘vermiform,’” says Welacky. “You have to multiply it by some 40 times to see them under a microscope. They survive in the soil by the female nematodes forming a lemon-shaped cyst full of eggs (with 20 to 200 eggs), and they can survive in the soil anywhere from two to 14 years.


“You can see the lemon-shaped cysts with the naked eye — if you dig the root, put it in a bucket of water, or shake it gently — and they look just like Christmas lights.”


Fighting soybean cyst nematodes


Once soybean cyst nematodes were found in Canada, Welacky says, “We took quick action to try to find resistant varieties, motivated to at least reduce the impact and spread of it as quickly as possible. Once you get it, you have to learn to live with it.”


Welacky has been working with breeders to develop soybean lines that are resistant to the nematode. “The pathology part of it is in relation to root rot diseases, especially the root rot of soybeans. Another disease we just started working on a couple years of ago is what we call sudden death syndrome, which is another root rot organism — one of the few variants common in legumes, as well as being specific to soybeans and closely related to SCN.


“The last few years, we’ve also been looking at the impact of SCN on edible beans, like white, kidney and black beans. When we sample the soil, we look for these lemon-shaped cysts and count the eggs.


“Once they’re in the soil, if there’s a food source available (such as soybeans, edible beans and some wheat roots), if the root’s there, they’ll become activated, hatch, go into the root, form a feeding site inside the root and suck the nutritional fluids out of the root.”


Welacky says often, SCN brings very little damage. “I’d say 80 to 90 per cent of the time, damage to soybean or edible bean fields is minimal (without any major symptoms expressed except maybe a little patchiness, which requires soil testing to find out if there’s a problem).


“The two best crop-control tools we’ve found are rotation and growing resistant varieties. Once you find SCN, there’s nothing you can do that year or that crop, but in following years, you can rotate a non-host crop.


“Here, we sometimes recommend four years of no soybean or edible bean crops, depending on how severe the field infestation is. If there’s a fairly low infestation, the usual recommendation is two-three years of non-host crops.


“I’m very hopeful, from various scientific studies and sources, that we’re on the verge of a breakthrough in the genetics for resistance. We’ll be seeing, especially seed companies, developing real sources of resistance through molecular biology, to the point of suppressing the SCN population.”


The life of soybean 
cyst nematodes


Mario Tenuta is a professor of soil ecology at the University of Manitoba’s Department of Soil Science. 


“With SCN, the female nematode buries herself partially into the root, where she feeds to produce eggs within her body,” says Tenuta. “She eventually dies and her body becomes protection for dozens to hundreds of eggs of new worms. The overall structure of the dead female with protected eggs inside here is called a ‘cyst.’ 


“Male nematodes feed, for a short time, on soybean roots to grow and become an adult. Then they leave the root to find a female attached to roots to mate and deposit sperm, which she uses to produce eggs.”


Damage from soybean cyst nematodes often appears as patches of stunted, yellowing leaves (chlorotic symptoms). Roots will have black/brown lesions (necrotic symptoms). Tenuta says yields can be decreased from zero to 30 per cent, maybe higher in heavily infested fields.


“Soybean acreages have recently expanded in Manitoba and we haven’t examined fields for the presence of the nematode,” Tenuta says.


Last fall, researchers started surveying southern Manitoba soybean fields for soybean cyst nematodes. Tenuta says they’re currently processing the samples. 


“I believe it’s inevitable we’ll have SCN in Manitoba,” Tenuta says. “We need to know when and where it’s present to make farmers aware of control options like rotations and tolerant varieties.”


Tenuta suggests farmers scout their fields; where there are poor- performing patches, there may be SCN. “Also farmers should be aware soybean may produce its own nitrogen but not other nutrients. Definitely don’t forget to pay attention to phosphorus fertility.” 


Another line of defence, Tenuta says, is to “keep good rotations and know your field’s issues.” 


Tenuta’s lab is also developing rapid, accurate molecular tests for identifying and quantifying SCN, in partnership with Actlabs in Ontario and Agriculture and Agri-Food Canada (AAFC) Growing Forward. These tests are expected to be commercially available in the next couple of years, giving farmers more options when it comes to testing fields accurately and tracking whether the nematode is present. 


“We hope to develop rapid tests for races (‘HG types’) of SCN too,” said Tenuta. “This is important, as the selection of tolerant soybean varieties is determined by HG type.”


The SCN survey in Manitoba is sponsored by the Manitoba Pulse Growers Association (MPGA) and the Canadian Agricultural Adaptation Program (CAAP) through the Manitoba Rural Adaptation Council (MRAC). The molecular tests are being supported by Actlabs and AAFC Growing Forward, via the CAAP. † 


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