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Staying ahead of Asian soybean rust

Asian soybean rust (ASR) has not turned out to be quite the spectre of death that was anticipated in Canada. Still, these are early days for its spread and adaptation to northern North American conditions. It was first sighted in Louisiana in 2004, and only detected in Ontario in 2007, so it is still a relative newcomer. “We’re still in the early stages of the disease developing in North America,“ says Albert Tenuta, plant pathologist with the Ontario Ministry of Agriculture, Food, and Rural Affairs. “It usually will take a good 10 years or so for us to fully understand what’s going to happen with many of these new diseases and pest problems.”


November 30, 1999
By Heather Hager


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Asian soybean rust (ASR) has not turned out to be quite the spectre of death that was anticipated in Canada. Still, these are early days for its spread and adaptation to northern North American conditions. It was first sighted in Louisiana in 2004, and only detected in Ontario in 2007, so it is still a relative newcomer. “We’re still in the early stages of the disease developing in North America,“ says Albert Tenuta, plant pathologist with the Ontario Ministry of Agriculture, Food, and Rural Affairs. “It usually will take a good 10 years or so for us to fully understand what’s going to happen with many of these new diseases and pest problems.”

Tenuta says that there was a lot of worry about ASR as it moved northward because of how quickly it became problematic in other areas during its spread from Asia through Africa to South America. Although it has not yet affected soybean plants in Canada, spores do blow up from the United States each growing season. An extensive spore-trapping network, funded by the Grain Farmers of Ontario (GFO) and the Ontario and federal governments, has detected ASR spores in Ontario each year since 2007, as well as in the Prairies, Quebec, and the Maritimes in some years, says Tenuta.

“The amount of spores reaching Canada will dictate whether or not we have any issues,” explains Tenuta. “To date, the number of spores has been low, which has been ideal. That decreases the likelihood of the spores being alive and finding a suitable environment for infection when they do arrive. The higher the number of spores, the greater the probability that we’ll see infection.”

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Like many Canadian snowbirds, ASR overwinters near the Gulf of Mexico coast, from Florida to Texas and northern Mexico. Tenuta cites several factors that influence the numbers of spores that reach Canada each growing season, including winter and spring environmental conditions and the presence of plant hosts for the fungus.  Conditions in the southern US have been fairly unfavourable for the fungus during the last few years, with relatively dry growing seasons and unusual cold events in winter. For example, in January 2010, freezing temperatures that threatened the orange crop reached the tip of Florida and killed off some of the kudzu vine, which is an abundant and widespread host for ASR in the southeastern US. Tenuta also notes that some of the US kudzu populations seem to be fairly unsusceptible to ASR infection.

These conditions have so far kept ASR populations low. However, that could change. In time, the fungus could become more adapted to North American conditions, perhaps overwintering further north. It could also become more virulent on the unsusceptible kudzu. If conditions become more favourable for the fungus, that could increase the potential for it to become troublesome in Canada. 

Resistance in reserve
Ontario-based public researchers from the University of Guelph’s Guelph and Ridgetown campuses and from Agriculture and Agri-Food Canada (AAFC) at Harrow and Ottawa continue to breed soybeans for resistance to ASR. Hundreds of lines have been sent to the United States Department of Agriculture (USDA)’s screening trials at the University of Florida Research Station in Quincy, Florida, where they are exposed to ASR naturally during the growing season and evaluated for resistance. Some of the lines have been rated as resistant, and a few appeared to be tolerant of ASR, showing infection symptoms but less yield loss compared to ASR-intolerant lines. The work has been ongoing since 2007, funded in part by the Grain Farmers of Ontario through Growing Forward, a federal-provincial-territorial initiative administered through the Agricultural Adaptation Council. The idea behind the breeding program is to be ahead of the curve with some ASR-resistant soybean material adapted for Canada if or when ASR becomes more of a threat to Canadian growers.

Dr. Elroy Cober, soybean breeder at AAFC Ottawa, is breeding specialty soybean material for testing at Quincy. He is crossing soybean lines for the 00 maturity area with about 17 different potential resistance sources from around the world that have been identified by the USDA and other research groups. He says the difficulty is that many of the promising resistance sources are from tropical regions and thus from very late maturity groups such as VII, VIII, and IX. “I make a cross and get less than five percent of the plants that will mature here in Ottawa. So I’ve had to make big crosses and then just select out of those crosses the lines that would mature here in Ottawa,” he explains. “There’s always the chance that I’m losing the resistance because I’m making such a hard selection for maturity.” 

Dr. Vaino Poysa, a soybean breeder at AAFC Harrow, who works with non-genetically modified soybean material, says that he has been making crosses “since ASR was identified as a problem for Canada and sources of resistance were identified.”

Poysa explains that once the initial crosses are screened at Quincy, those that show resistance are crossed again with lines adapted for Canada. Those are again selected for maturity and screened at Quincy. “Some of those had good resistance and reasonable yield, were reasonably adapted,” he says. “So the top lines of those have been distributed to the other public breeders in Ontario.” 

Poysa notes that further breeding is needed to bring those to a level of yield that would satisfy growers. He also cites three activities that will help the Canadian soybean industry to stay prepared for ASR down the road. One is to maintain breeding efforts to introduce resistance. “We’ll make the crosses and keep on sending them down to Quincy to make sure we have resistance in the best material that we can.”

The second is to “piggyback” on work that US researchers have been doing to characterize the genetics of the original resistant lines and develop molecular markers for the resistant genes. “If they identify a major gene that was in one of the sources that I’ve used, and they have a molecular marker, then we can quickly see whether or not that molecular marker is present in the line that we’ve developed and use that for backcrossing and keep the resistance in material that is near cultivar release,” he explains.

The third initiative is to maintain the network of soybean sentinel plots throughout the US and Canadian soybean-growing areas to follow the development and distribution of ASR each growing season. Information from the sentinel plots is updated regularly on the USDA’s PIPE website. (www.sbrusa.net)