This article covers the current status of blackleg disease in Western Canadian canola crops, the new blackleg major gene identification system, practical scouting and severity assessment methods and effective management strategies as discussed by Eric Gregory of BrettYoung Seeds and Justine Cornelsen of the Canola Council of Canada.
Blackleg Disease in Canola
The Current Status in Western Canada
- Blackleg levels have risen: Over the past 10 years, blackleg levels have risen.
- Pathogen found in most fields: Provincial surveys show about 70 to 75% of fields display the pathogen.
- Field incidence is still low: Incidence within fields is low (10 to 15%), only 10% of surveyed plants actually show blackleg.
- Incidence still above target: This low incidence is positive but trade partners require incidence below 5%, so the current incidence is still higher than desired.
- Severity is typically low: Averages a 1 rating out of 5, meaning plants are infected but not severely.
- Yield loss threshold: Yield loss occurs at a severity of 2 but current levels mean only minimal yield loss is being observed.
- Goal is lower incidence: The goal remains to reduce blackleg incidence back to lower levels seen in the past.
The Blackleg Major Gene Identification System
Background and Development
- Labeling system introduced in 2017: Around 2017, a major gene labeling system for blackleg was introduced by the Canola Council with input from the Canadian Blackleg Steering Group and the broader industry.
- Offers more information than old Canadian system: This new system provides more information than the previous Canadian system, which only labeled varieties as resistant, moderately resistant or susceptible based on a percentage of Westar (a susceptible variety).
- Old labels limited producer choices: The previous labels did not help producers choose between resistant varieties when faced with blackleg problems.
How the System Works
- Participation is optional: Use of the major gene labelling system is voluntary for seed companies.
- Expanded resistance labels: Traditional ratings such as R (resistant) and MR (moderately resistant) are now supported by major gene group identifiers (e.g., A, C, G, E1).
- Gene-specific identification: Each letter represents a distinct major resistance gene present in a canola hybrid.
- Proven global model: A comparable system is already in use in Australia, where it helps guide crop rotation decisions and anticipate disease pressure.
Advantages Over Previous Systems
- Informs better hybrid decisions: The system enables producers to make better informed hybrid decisions.
- Adds more detail: It adds a layer of detail, helping match hybrid genetics to specific blackleg pathogen races found in the field.
- Helps prolong resistance gene value: The system increases longevity of resistance genes by allowing for strategic rotation.
Selecting the Right Varieties Using the System
- Hybrid selection is complex: Hybrid selection is complex, requiring knowledge of both the variety’s genetics and the predominant blackleg races in the field.
- Producers should monitor levels: Monitor blackleg levels over time. If issues increase, current hybrid genetics may not match prevalent blackleg races.
- Take and submit field samples: Take field samples and submit them to diagnostic labs to identify the predominant pathogen race.
- Choose hybrids that match major genes to races: Choose hybrids containing major genes corresponding to the races found.
- Major genes only work when matched: Major resistance genes function like a light switch, they are only effective if they match the race present, stopping the pathogen at the infection site.
Integrating the System into Farm Practices
- Requires some adaptation: Adopting this process requires adaptation by producers and close work with agronomists or independently.
- Especially useful after major outbreaks: The process is especially valuable after significant blackleg outbreaks, areas with lodging or yield loss likely have a mismatch in genetics and pathogen race.
- Allows for longer gene usefulness: Using this system can prolong the usefulness of resistance genes, in Australia, effective rotation allows reintroduction of genetics after 8 to 10 years.
Detecting the “Silent Yield Robber”: Scouting for Blackleg
Challenges
- Harder to detect than other diseases: Blackleg is harder to detect than some other diseases like sclerotinia or clubroot, as it is not always obvious in the field.
- Proper detection requires pulling plants: Detection requires pulling plants and inspecting root tissue.
Effective Scouting and Rating Steps
- Pull representative plant samples: Pull representative samples, typically 50 to 100 plants per field.
- Inspect stems for lesions: Inspect lower plant portions, look for stem lesions.
- Cut through root for internal damage: Cut through the root (about a quarter to half an inch below the base) to check for internal blackening.
Severity Rating Scale
| Severity Rating (0-5) | Findings |
| 0 | Healthy plant, no internal blackening |
| 1 | Up to 25% internal infection |
| 2 | 25 to 50% infection |
| 3 | 50 to 75% infection |
| 4 | Near complete blackening |
| 5 | Dead plant with cankering and complete infection |
Disease Identification
- Canker appears as hard root lesion: Blackleg canker appears as a hard, woody lesion within the root, causing constriction and blocking water and nutrient uptake, which leads to plant death.
- Find pepper specks (pycnidia) on lesions: Look for pepper-like black specks (pycnidia) on lesions to differentiate blackleg from other diseases like grey stem or verticillium.
Importance of Severity Ratings
- Low infection doesn’t always mean yield loss: Low infection does not always result in yield loss but even subtle infections can silently reduce yield.
- Scouting and rating is critical: Knowing how to properly scout and rate blackleg is essential for managing it.
Additional Blackleg Management Practices
Essential Steps
- Scout and recognize disease regularly: Scouting and disease recognition are crucial, understand blackleg’s appearance throughout the season.
- Crop rotation is vital: Crop rotation is extremely important because blackleg survives on canola residue. A two year break (grow canola every three years) is recommended to allow residue to break down.
- Rotate resistance genes: Resistant hybrids are standard but strategic rotation of major resistance genes is a newer, valuable tool.
Fungicides and Seed Treatments
- Fungicides rarely used: Fungicides are seldom used mainly due to the need for early application before pathogen infection.
- Current seed treatments are limited: Current seed treatments only protect against seed-borne blackleg, not in-field spread.
- New seed treatment products explored: New seed treatment products are being explored to protect plants from cotyledon to four-leaf stage, especially when genetic resistance is no longer effective.
Economic Considerations
- Fungicide may not be cost-effective: Fungicide treatments are not always cost-effective if genetic resistance is matching prevalent races.
- Weigh cost and confirm blackleg issue first: Assess economic impact and confirm blackleg is a significant issue before investing in additional products.
Summary and Key Takeaways
- Blackleg disease still widespread, mostly mild: Blackleg remains widespread but at generally low severity in Western Canada, with some areas still above trade partner incidence thresholds.
- New major gene system helps hybrid selection: The major gene identification system provides growers with more precise information for selecting hybrids matched to field-specific blackleg races.
- Best practice: combine scouting, hybrids, rotation: Effective management includes regular scouting, selecting resistant hybrids with properly matched major genes, following crop rotation guidelines and considering fungicides and new seed treatments only when appropriate.
- Even mild infections can hurt yield: Regular, proper scouting for blackleg is critical, as even low-level infections can reduce yields without obvious symptoms.
- Strategic rotation preserves gene effectiveness: Preserving genetic tools through strategic variety rotation increases their long-term effectiveness.
FAQ
What is the current status of blackleg disease in Western Canadian canola crops?
Blackleg levels have risen over the past 10 years, with provincial surveys showing about 70 to 75% of fields display the pathogen. However, incidence within fields is low (10 to 15%) which is positive but still above the desired level of less than 5% for trade partners. Severity is typically low, averaging a 1 rating out of 5, meaning plants are infected but not severely. The goal remains to reduce blackleg incidence back to lower levels seen in the past.
How does the blackleg major gene identification system work?
The major gene labeling system is optional for seed companies. Labels such as ‘R’ for resistant or ‘MR’ for moderately resistant are supplemented with major gene group labels like A, C, G or E1, each corresponding to a specific major resistance gene in the canola hybrid. This provides more detailed information to help producers match hybrid genetics to specific blackleg pathogen races found in their fields.
What are effective steps for scouting and rating blackleg in canola fields?
Pull representative samples of 50 to 100 plants per field and inspect the lower portions for stem lesions. Cut through the root a quarter to half an inch below the base to check for internal blackening and use a 0 to 5 severity rating scale. Look for pepper-like black specks (pycnidia) on lesions to differentiate blackleg from other diseases.
What management practices are recommended for controlling blackleg in canola?
Effective management includes regular scouting and disease recognition, selecting resistant hybrids with major genes matched to field-specific blackleg races, following a two year break crop rotation to allow residue to break down and considering fungicides or new seed treatments only when genetic resistance is no longer effective or blackleg is a significant issue.
