Crop scouting is always the first step when managing blackleg. Assessing the level of incidence and severity of blackleg in the field can help growers to develop an estimate of blackleg risk for the future. There are two main management strategies once growers know what they’re dealing with, one is crop rotation. A minimum two-year break between canola crops allows for crop residue housing the blackleg-causing pathogen to break down. A second way to manage the disease is by growing blackleg-resistant canola hybrids.
Fortunately, as blackleg advances, so do we. One of the biggest steps some of the industry has taken recently is labelling the major blackleg resistance genes found in their canola hybrids, something BrettYoung has been doing for years. This is important information as it can be used to rethink the approach towards effective blackleg management.
Canola hybrids use two sources of resistance — quantitative and qualitative (major gene). Quantitative resistance is a sort of “catch-all”, meaning it has numerous genes working together to slow the infection of blackleg in your canola plants. Because quantitative resistance has so many genes working within it, it’s more difficult to classify and harder to screen for. Qualitative resistance, on the other hand, are major genes that stop blackleg right at the site of infection. BrettYoung Regulatory & Agronomic Services Manager Justine Cornelsen said the industry’s shift to labelling major genes and paired with quantitative resistance is a good thing as it provides more concrete protection for growers.
“With quantitative resistance, you have multiple genes working together to slow the pathogen down as it moves through the plant,” said Cornelsen. “This minimizes the overall severity of the disease but doesn’t eliminate it. Qualitative resistance is when a major gene matches an avirulence gene within the blackleg pathogen to initiate a defense response within the plant, that stops the pathogen at the site of infection.” One of the most common avirulence genes found in fields across Western Canada where blackleg is present is AvrLm7, and with the Rlm7 major resistance gene recently deployed in Canada, a few canola hybrids now have a new tool to defend with.
The blackleg pathogen is known as Leptosphaeria maculans. Blackleg stubble tests determine the pathogen genotype and phenotype, the phenotype being the important information for growers with blackleg concerns. The phenotype results will show growers the blackleg races present in their field, which is useful when making hybrid decisions for the field.
Growing a canola hybrid with blackleg resistance is the main way to combat the pathogen. When purchasing seed for the year, Cornelsen said it’s imperative to choose a hybrid with at least one major gene that will match the avirulence genes present in your field — having just one match will ensure your crop is protected from blackleg.
“If you don’t have that information, you’re pretty well guessing,” she said. “Stubble tests are the only way to know which L. maculans races are in your field, so they’re the only way to know how which major genes you need.”
One of the most predominant blackleg avirulence genes in Western Canada is AvrLm7. With the addition of the Rlm7 resistance gene to canola hybrids, Cornelsen said there’s a good chance most growers with blackleg issues will have much stronger success in minimizing disease pressure if they’re growing a hybrid with that major gene as it matches 90% of L. maculans races detected in the region.
Like most pathogens, L. maculans changes with time to build tolerance towards resistance genes. Cornelsen noted the Rlm3 resistance gene is a good example of this — the gene has been used in Western Canada for so long, it’s rarely seen in L. maculans races anymore.
BrettYoung currently has four canola hybrids with the Rlm7 resistance gene:
- BY 6216TF
- BY 6217TF
- BY 6219TF
- BY 7204LL
BY 6217TF has both the Rlm3 and Rlm7 genes, while BY 6216TF, BY 6219TF, and BY 7204LL have just the Rlm7 gene. The quantitative resistance in each hybrid also helps slow the rate of infection from the few remaining races that don’t include AvrLm7.
| L. maculans phenotype | Major Resistance Genes | BrettYoung Hybrids to Utilize |
| AvrLm4-5-6-7-11 | Rlm4, Rlm5, Rlm6, Rlm7, Rlm11 | BY 7204LL, BY 6219TF, BY 6217TF, BY 6216TF |
| AvrLm2-3-5-9-S | Rlm2, Rlm3, Rlm5, Rlm9, RlmS | BY 6214TF, BY 6211TF, BY 5125CL |
Growers across the Prairies can submit canola plants to the following testing labs for blackleg detection and L. maculans race identification:
- Manitoba’s Pest Surveillance Initiative (PSI) Lab
- Discovery Seed Labs
- 20/20 Seed Labs
- SGS Canada Labs
Consult the Canola Council of Canada’s website (blackleg.ca) for the latest information on blackleg management.
References
Soomro W, Kutcher HR, Yu F, Hwang SF, Strelkov SE, Fernando WGD, McLaren D, Peng G. 2021. Race structure of Leptosphaeria maculans in western Canada between 2012 and 2014 and its influence on blackleg of canola. Can J Plant Path. 43: 480-493.
Liu F, Zou Z, Peng G, Fernando WGD. 2021. Leptosphaeria maculans isolates reveal their allele frequency in western Canada. Plant Disease 105: 1440–1447.
