Plant breeders battling blackleg with disease resistant varieties

Life science companies have much to brag about when it comes to the resistance they’ve deployed in Canadian canola varieties, which has successfully kept blackleg pressure relatively low in this country.

In the early 1990s, blackleg throttled canola production by as much as 50 percent in some fields.

Resistant varieties then became available to producers, who also extended rotations on fields where blackleg was prevalent.

Canola breeders use both quantitative and qualitative resistance to develop blackleg resistant varieties.

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“Quantitative resistance has been doing the heavy lifting to minimize that (blackleg) pressure,” said Justine Cornelsen of the Canola Council of Canada.

“The nice thing about quantitative resistance is it’s going to work with any race within the field; it will provide some protection.”

Quantitative resistance, also known as minor gene or adult plant resistance, has been the resistant backbone of Canadian cultivars because blackleg races became virulent to the major resistant gene deployed.

However, even quantitative resistance is starting to break down on some fields.

“Once you get that disease pressure up or you get a really aggressive strain in the field, you will even start to see it in the quantitative, where it’s just not working, where there is extreme pressure. Something needed to change,” Cornelsen said.

Qualitative resistance, also known as major gene or race-specific resistance, is extremely effective when it matches up with the prevalent blackleg race in a field.

Multiple major genes are being brought into Canadian canola lines.

“They’ve discovered lots around the world; I think they are up around 16 major resistance genes for blackleg now. We just have incorporated them into our breeding programs here in Canada. However, something like that takes time,” Cornelsen said.

When the genes are brought into a canola’s genome, they can also bring in extra unwanted genetic material that need to be cleaned up throughout the breeding program.

However, four or five new major genes are already in play in Canadian canola lines, Cornelsen said.

To help extend the use of these major genes, the canola industry has implemented a major gene-labeling scheme, which enables growers to better manage these genetic assets.

“The major gene labelling is to help preserve the awesome genetics that we have and create a stewardship plan for them. So if you can successfully rotate your varieties every two or three years and help minimize that pressure, or match that pathogen and keep it low, that’s really going to prevent that buildup within the field,” Cornelsen said.

A test is now available at a handful of diagnostic labs across the Prairies, which enable growers to find out the predominant blackleg race found on their canola stubble.

This will help growers, agronomists and seed company match canola varieties to the predominant race within a field.

“With the blackleg major genes, all you need is one major gene to match up to the race in the field to initiate the response within the plant. It’s a pretty complicated system, but once you’ve worked through the steps, it does make sense,” Cornelsen said.

The Pest Surveillance Initiative in Manitoba, in conjunction with the Manitoba Canola Growers Association, is offering a free blackleg test to any MCGA member.

For producers who want to purchase a race test, it works on a two-tier system. For $100, producers can submit a sample and the lab will tell them if it is indeed blackleg.

The second level kicks in if the sample has blackleg. The lab will then find the predominant race for another $100.

Not every seed company in Canada is currently labelling the major gene resistance that their canola varieties use, but Cornelsen said three or four companies are doing so, and a few more will be labelling for next season.

Cornelsen said the main tools for fighting blackleg continue to be longer rotations, identification of the disease when scouting and resistance to the pathogen bred into Canadian cultivars.

While it’s a good idea for growers to find out which blackleg race is prevalent in their fields, it’s the growers who use tight rotations such as a two-year wheat-canola rotation who should be most concerned about testing the pathogen in their fields and then using rotations and matching the blackleg resistance that they use.

“Those would be the fields I would check first,” she said.

“If you have seen any premature ripening or something just not quite right in the field and you plan on growing canola on it, maybe next year or a few years down the road, that would be the place to test,” Cornelsen said.

Growers who have been struggling with blackleg can also use a fungicide to combat the disease.

“With the varietal rotation and with a fungicide, those are kind of the two backup things to use when you haven’t been doing the first main three management tools,” Cornelsen said.

She said she hasn’t seen many farmers use a fungicide for blackleg because they often don’t see a return on investment.

“Fungicide works really good on susceptible varieties; we don’t grow susceptible varieties,” Cornelsen said.

She said fungicide for blackleg has to be applied extremely early, before the disease starts to infect the plant, and growers are often too busy getting the first pass of herbicide down to move into a blackleg fungicide program.

Putting blackleg to the test

Blackleg race testing is complicated because there is not just one blackleg race present in a field, which means the entire blackleg population has to be examined.

Researchers have found that races interact with each other so just knowing the races isn’t enough. PSI also determines the pheno-type — how the races in a field may affect the variety resistance in that field — to help guide the choice of a canola variety that offers the best resistance for that field.

So far, PSI has found these races within the blackleg population in Manitoba fields:

Contact robin.booker@producer.com

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