Dilantha Fernando has seen the canola debate play out once before.

Six years ago, China set new restrictions on Canadian canola citing concerns about blackleg, a fungal disease that destroys canola crops. Shortly after, the University of Manitoba professor of plant sciences was called on to study the likelihood of the fungal disease spreading. Over the following years, Dr. Fernando – a leading expert on blackleg – would work alongside Canadian officials and Chinese regulators to help assuage concerns and eventually see those rules loosened.

Because of this, Dr. Fernando and many other Canadian scientists are flummoxed at China's recent decision to once again cite blackleg as the reason for imposing new rules on canola. According to Dr. Fernando and others, the science hasn't changed. So what's really at play behind the Chinese decision: plant pathology, or politics?

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"The disease has been growing for a long time. The pathogen has not established there," Dr. Fernando said of China's experience with blackleg. "That's a very good [indication] that it will not."

Others say they're experiencing déjà vu. "We've been down this road before," said Dave Hume, a crop consultant and retired crop physiology professor at the University of Guelph. "It turned out to have occurred in a time when China didn't need to import so much [canola]."

Blackleg, a disease caused by the fungus Leptosphaeria maculans, has been in Canada since at least the 1970s, and is known to cause damage in canola-growing areas. The disease, so-named because infection darkens the plant's stem, is also widespread in other canola-growing countries, including the United States, Australia and France, though not China.

In late 2009, China imposed new rules requiring Canadian exports to carry certificates that proved the product was free of the disease. Part of the rationale was a 2008 study by British and Chinese researchers concluding there was a "serious, if small," risk of the disease establishing in China, and estimating the economic effect of such a spread at $7.9-billion (U.S.).

After the 2009 restrictions, Canadian exports to China dropped to 0.9 million tonnes for 2010-11 from 2.2 million tonnes in 2009-10, a loss estimated at about $1.3-billion (Canadian). With its population of more than 1.4 billion people, China is second only to the United States in purchasing Canadian canola – buying 87 per cent of its supply, or about $3.8-billion worth, from Canada last year alone.

In the weeks after the Chinese decision in 2009, the price of the seed in Canada tumbled by about $15 a tonne, though the price later recovered.

Partly as a result, Dr. Fernando and other scientists were called upon by the Canola Council of Canada to work alongside officials to better understand the disease and how it spreads.

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They were able to demonstrate that seeds from even the most blackleg-infected areas carried only a 0.2-per-cent chance of carrying the infection. Making it even more of a long shot, Dr. Fernando said, is that canola seed is shipped directly to crushing plants in China and not to farmer's fields. "There's lots of epidemiological steps that have to be there for the pathogen to establish itself," he said.

Chinese authorities began relaxing these restrictions in 2012.

This time around, Chinese officials are focusing on what's known in the industry as "dockage" – other plant materials that are mixed in with grains, which currently make up about 2.5 per cent of each shipment. Beginning Sept. 1, Chinese officials will require dockage to be limited to 1 per cent. They say they are trying to further reduce the potential spread of blackleg.

But Dr. Fernando said the change, which Canadian canola producers argue would be costly, would not reduce the likelihood of the disease spreading. The dockage has a slightly higher probability of carrying the infection, he said, but not enough to make a difference. "It doesn't matter whether it is 1 per cent or 2.5 per cent," he said. "It would have a similar impact."

Dr. Hume, meanwhile, said there's also a small chance that the dockage could introduce a new strain of the disease that Chinese canola crops do not yet have resistance to. But the likelihood of that happening, he said, "is not very high."

Most peculiar, scientists said, is that little has changed in the science between the 2009 incident and now. In fact, Canadian and Chinese scientists have been working together on joint research on blackleg between then and now in efforts to further manage the disease.

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Mary Ruth McDonald, a professor in the University of Guelph's department of plant agriculture, said she was baffled at the Chinese decision. "There's nothing out of the ordinary that's happened in the past few years to make this more of a serious problem than it has been in the past."

She echoed Dr. Fernando's argument about blackleg not establishing itself in China in the past. "You'd think that the system was working and there would be no reason to have to tighten the criteria."

In recent weeks, traders and canola producers in Canada have speculated that the real reason behind the new policy may be that China is dealing with a bumper crop this year. Dr. Hume said many in the industry are referring to China's move this time as "a non-tariff-barrier." China, meanwhile, has denied the decision is a political one.

Regardless, most Canadian experts seem to agree that China's recent position is all bluster.

Patti Miller, the president of the Canola Council of Canada, said she's not aware of any other country in the world already producing canola at the 1-per-cent dockage level that China is asking for. According to the council, Australia (which has had a history with blackleg) is the only other country required by China to meet the new dockage standards.

"I think it's a bit of a bluff by China to say, 'Look, we can just go somewhere else.' If they could do that, they would probably already be doing that," said Stuart Smyth, a University of Saskatchewan assistant professor in the department of agricultural and resource economics.

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He pointed to the fact that no other canola-producing countries export as much of the product as Canada – meaning China would have a hard time fulfilling its demand otherwise. Other countries, such as the United States and France, each export less than 1 per cent of China's imported canola, according to the Food and Agriculture Organization of the United Nations.

"It's simply a political trade issue that the Chinese have chosen to push forward," he said. "It's not based on science."

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By the numbers:

249,000: The number of jobs in Canada related to the canola industry

$19.3-billion: The amount canola is estimated to contribute annually to the Canadian economy

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25: Portion of all farm cash receipts in Canada generated by canola

90: Portion of Canada's canola that is exported around the world, mainly to the United States, China, Japan and Mexico

3-4 million tonnes: The amount of Canadian canola seed exported to China annually, worth about $2-billion

17 million tonnes: The amount of Canadian canola expected to be produced this year

Source: Canola Council of Canada, Statistics Canada

Editor's note: The original version of this article stated that the estimated economic effect of the spread of blackleg was $7.9-million (U.S.). In fact, the estimated impact is $7.9-billion (U.S.). This digital version has been updated.