Blackleg

Blackleg (primarily caused by the fungus or pathogen Leptosphoeria maculans) is the cause of substantial yield loss in Western Canada. Because the disease can survive over winter in infected crop residue and continue to create sources of infection, blackleg can persist in an infected field for years.

2016 Provincial disease survey results showed that blackleg prevalence in Western Canada is increasing. Blackleg basal stem cankers were found in:

  • 90% of the fields surveyed in Alberta1
  • 61% of the fields surveyed in Saskatchewan2
  • 82% of the fields surveyed in Manitoba3

These stem cankers can completely girdle the base of the stem during the critical pod filling period, resulting in smaller seeds, reduced standability and lower yields. With the increasing prevalence of blackleg all over Canada, you need to know how to defend your field against Blackleg and how to manage this destructive disease.


Key Factors for Managing Blackleg


Scouting

When it comes to blackleg, the first step in management is being able to identify the disease. Right before swathing is an excellent time to assess blackleg prevalence in field. Look for areas that are lodged or have prematurely ripening plants. Look at the bottom to middle areas of the stems for lesions, which often have pycnidia (small black dots) within them (see photo on the left below). If blackleg infection is severe, canola plants will have woody cankers at the base of the stem. It is this type of canker that will cause yield loss. If you see plants exhibiting blackleg symptoms, use garden clippers and slice through the stem at the base of the plant (at soil level) to confirm the presence of blackleg (see photo on the right below). Making this confirmation will help determine if the blackleg pressure resulted in yield loss. Remember, canola crop residue that hasn’t yet decayed is a source for new blackleg infections in the field so be sure to keep accurate scouting records of blackleg infection ratings.

Symptoms of Canola Affected by Blackleg

Crop Rotation

The longer length of time between canola crops allows the previous canola crop residue to degrade before another canola crop is planted in the field, limiting the source of new infection. The Canola Council recommends three years between canola crops for the decomposition of infected residue, to help with blackleg disease management.4

Blackleg Resistant Hybrids

While good crop management practices such as crop rotation and active scouting are the most valuable tools to help reduce the spread of blackleg, planting blackleg resistant canola is another tool you can use. Planting blackleg resistant canola limits yield losses caused by the aggressive disease. Canola is considered to have a R-rated resistance if it is less than 30% susceptible to blackleg when compared to Westar which is a fully susceptible variety.

DEKALB canola hybrids are selected from an elite breeding program that focuses on developing blackleg resistant canola. The DEKALB canola line-up is comprised of canola hybrids with very strong resistance ratings for blackleg as can be seen in the chart below.

DEKALB Canola Hybrid Blackleg Rating % of Westar
74-44 BL 15.4%
75-42 CR 25.5%
75-45 RR 29.9%
75-65 RR 28.7%
73-15 RR 30.4%

DEKALB® 74-44 BL offers enhanced multi-genic blackleg resistance, which means the blackleg protection is conferred by multiple resistance genes from more than one source. This multi-genic blackleg resistance is more durable in the field as the blackleg pathogen would need to overcome multiple genes before the resistance would be eroded in the field and yield losses from blackleg would be observed. However, it is important to note that this multi-genic blackleg resistance is not a silver bullet in terms of blackleg protection. There is always potential for the blackleg population in a field to shift due to the selection pressure exerted by the resistance genes in the canola hybrid, which means that rotation and crop scouting are still the most valuable tool for managing blackleg pressure in canola.

DEKALB® 74-44 BL

DEKALB® 74-44 BL is a broad-acre hybrid that is equipped with multi-genic blackleg resistance and offers outstanding yield potential. DEKALB® 74-44 BL also displayed a lower sclerotinia infection rating, on average, compared to Pioneer 45S54 in 2011-2016 Monsanto Field Scale Trials as well as higher yield.

In addition to its impressive disease package, DEKALB® 74-44 BL has consistent yield performance across all yield environments and has very good combining ease. These impressive features are why it has been the #1 selling Genuity® Roundup Ready® canola hybrid for four consecutive years.5


Finishing Big with DEKALB 74-44 BL Canola

Rotate Blackleg Resistance Genes

Field resistance to blackleg infection is based on both the non-specific background resistance to all blackleg, and the major resistance genes that provide very strong resistance to specific blackleg races.

There are at least 16 described virulent L. maculans races currently found on the Prairies.7 Growing the same hybrid repeatedly on the same field will select for races of blackleg that can overcome genetic resistance in that hybrid. Tight rotations of hybrids with the same blackleg resistance genes for more than two years will select for races of blackleg that can overcome genetic resistance in that hybrid and increase disease severity.6 Rotating varieties creates the opportunity to bring a mix of resistance genes to the field over time, which can reduce selection pressure and improve durability.7

Apply Fungicides if Necessary

Another option to consider is the use of a fungicide. In the right circumstances, fungicides can augment the blackleg protection offered in the canola hybrid under high blackleg pressure situations. However, the use of a fungicide will not replace the value offered inherently with blackleg resistant canola hybrids. As such, adopting this comprehensive approach to blackleg management is critical to preserving the genetic resistance tools which currently protect canola from blackleg and allow for high yielding and high-quality canola in Western Canada.