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Blackleg

Pest Profile: Blackleg

Blackleg is a serious disease known to cause up to 50% yield loss in canola fields. It can infect plants from the seedling stage onward, causing progressive damage as it develops cankers on the stem that restrict water and nutrient uptake.

The disease is often spread to crops through residues from infected plants, but wind or rain can also spread spores to new areas. Growers should be particularly cautious during warm, moist years or in regions that get a lot of rain, as these conditions favour rapid spread. However, the disease is found both in warm, humid regions and dry regions across Canada, so it’s important for growers to keep an eye out when growing susceptible plants.
Blackleg

 

IDENTIFICATION

The first symptoms appear on the cotyledons or leaves as round-to-irregular (1/2 – 1 inch) white to yellow-brown lesions that contain numerous small black dots (pycnidia). As the season progresses, the fungus may spread to the stem and crown of the plant, producing a canker that can girdle the stem. Severely infected plants ripen prematurely and have a black-to-grey discolouration at the base of the stem or crown.

INFECTION

  • Timing: Seedling stage—onwards 
    • Seedling infections are often the most severe.
    • Primarily enters through wounds caused by hail, rain and insects.
  • Infection on the leaves and spreads towards the stem which leads to stem cracking, leading to reduced water and nutrient movement within the plant.

Infections prior to 2 leaf stage result in most significant yield loss. 

DISEASE CYCLE

  • Over winters on infected canola residue.
  • The primary source of blackleg infection come from infected canola stubble/crop residue in the field. 
  • In spring, fruiting bodies; pseudothecia & pycnidia; develop.
    • Ascospores are released from the pseudothecia and infect plants at the seedling stage.
  • During the growing season, pycnidia is produce.
    • Black “pepper” like symptoms appear within the lesions. 
  • Pycnidiospores are produced by the pycnidia and spread via wind and rain splash.

Download our tech sheet here.

ENVIRONMENTAL INFLUENCES

  • Warm, humid, frequent showers favor blackleg infection and spread.
  • Drier weather leads to more persistence, as the plant material breaks down slower.
  • Wind borne spores can travel up to 10 km.

SCOUTING

  • Spring: Before seeding canola inspect the field for pycnidia (pepper specks) on canola residue or stubble.
    • The amount present will indicate the spore load in the field 
  • Fall: Inspect for stem cankers and stem cross section damage 
    • Collect 50 plants from across the field;  clip where the stem meets the root and rate 1-5 to determine severity of blackleg infection. 

Find the blackleg disease severity scale here under identifying blackleg>scouting tips.

CONTROL TIPS

  • Seed treatments effectively protect canola from the cotyledon to 1 leaf stage.
    • An effective tool against early seed and airborne blackleg spores.
    • Reduce seed borne spread of blackleg and prevent new pathogens introduction to the field.
  • Use fungicides as a preventative measure and apply early.
  • Maintain a good crop rotation that has at least 3 years between canola crops.
  • Plant less-susceptible varieties of canola. Most varieties are rated on a 1 - 5 scale, resistant to highly susceptible.
  • Avoid confusing blackleg leaf lesions (which are speckled) with downy mildew lesions (which are not).

REGISTERED FUNGICIDES

Product Active Ingredient Group Application Timing
BUMPER® 432 EC Propiconazole 3 Cotyledon – 6 leaf
MAXENTIS® Azoxystrobin & Prothioconazole 3 & 11 Cotyledon – 6 leaf

Always read and follow label directions.

References:
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