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Scouting and Management Options for Blackleg in Canola

By Alison Fransoo, PAg, Crops Extension Specialist, North Battleford

July 2021

The time to think about blackleg management starts right from developing seeding plans. Tightened canola rotations cause blackleg inoculum to build overtime in the field. By limiting the canola rotation in fields to every third year, it allows for stubble to break down and inoculum levels to decrease.

Cut canola stems showing blackleg symptoms
Cut canola stems showing blackleg symptoms

Scouting for blackleg at different times throughout the season will indicate what level is present in a particular field. The more aggressive fungus of the two blackleg causing fungi Leptosphaeria maculans can overwinter for many years on infected canola residue. Therefore, it is important to scout prior to planting to examine canola residue for the presence of pseudothecia, which produce ascospores that distribute blackleg to newly-planted canola. Scouting at the early stages (two to six leaf) to look for lesions with pycnidia is more difficult to identify the disease. The infection tends to enter through the cotyledons and grows through the stem in a downward direction. During mid-season flowering, infection moves downwards from the lower leaves to the stem base and the stem will display varying degrees of black. The simplest time to assess the disease level in a field is close to swathing time. However, it is important to scout at multiple stages since the earlier the infection occurs, the greater the yield loss.

An integrated management strategy is recommended to manage blackleg. This approach is long-term and involves several practices. To manage blackleg, you need to implement more than one strategy, consider the following:

  • Scout for blackleg regularly to help determine the effectiveness of your management plan.
  • Plant certified seed that is genetically resistant to blackleg. Make sure to use a R (resistant) or MR (moderately resistant) rated canola variety. Rotate cultivars by their major blackleg resistance gene so cultivars have a longer lifespan. If blackleg levels continue to increase in the field, consider getting a blackleg pathotype identification test done to determine if the blackleg resistance gene in the variety grown is the correct one.
  • Rotate to other crops. Maintaining a two or more year break between canola allows crop residue to break down and inoculum levels in the field to be drastically reduced.
  • Control weeds susceptible to blackleg to avoid further build up of blackleg in non-canola crop years. This includes volunteer canola and other weeds of the Brassica family.
  • Apply a fungicide for blackleg. A fungicide seed treatment on your canola seed protects plants from blackleg when they are most susceptible. If plant infection occurs earlier in season, effects on yield will be more significant. Foliar fungicides applied early, before the two leaf stage, will help to prevent yield losses, but a later foliar application will help to reduce the inoculum in the field.

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