Diagnosing blackleg in canola

Blackleg is the most common and serious disease of canola in Western Australia. It is a sexually reproducing fungus that will overcome cultivar resistance genes.

Infection can spread from cotyledon of seedling leaf and pinch off the hypocotyl
Pale round to irregular leaf lesions,often with a dark rim and black fruiting bodies
Dark cracked stem canker that causes plants to lodge and die prematurely
Annual monitoring is important to test whether resistance is breaking down

What to look for

    Paddock

  • The most obvious symptom is plant death with lodging and premature death after stem extension.
  • Plants may be randomly distributed, or more affected in moister areas, or adjacent to canola stubble.

    Plant

  • Infection at the seedling stage also causes symptoms seen at later growth stages.
  • Cotyledons develop grey circular spots with pinhead-sized black fruiting bodies then die prematurely.
  • Infection spread through the plant to the hypocotyl that pinches off causing plant death, or ring-bark the stem causing the top to snap off
  • Leaves have round to irregular creamish-grey lesions with black fruiting bodies. These also occur on varieties with adult blackleg resistance.
  • After stem extension, internal blackening at at the base of the stem causes yield loss without external signs. With very severe infection dark cracked cankers from internal infection cause plants to lodge with premature death. and shrivelled seed.
  • White or grey, round to linear lesions with black specks and a dark rim may occur on the surface of the stem.
  • Pale lesions with a purplish rim on petioles cause pod death or similar lesions on pods that cause seed death under the lesion.

What else could it be

Condition Similarities Differences
Diagnosing damping off in canola Sudden seedling death Blackleg attacks the hypocotyl at ground level, whereas damping off pinches the roots or hypcotyl below ground. Damping off often occurs in patches
Diagnosing false wireworm Sudden seedling death Chewing damage on stem, petioles, leaves
Diagnosing vegetable beetle damage Sudden seedling death Chewing damage on stem, petioles, leaves
Diagnosing white leaf spot in canola Greyish leaf lesions Black lesions contain dark fruiting bodies
Diagnosing Group I herbicide damage in canola Cracked stem canker at ground level after stem extension The canker is pale not blackened. Above ground plant parts are twisted and distorted.

Where did it come from?

  • Blackleg survives as black fruiting bodies on canola stubble that release large quantities of airborne spores after autumn and winter rainfall
  • Within two weeks of spores landing on canola cotyledons and young leaves, clearly visible off-white coloured lesions develop fruiting bodies that release rain-splashed spores.
  • Once the lesion has formed, the fungus grows within the plant’s vascular system to the crown where it causes the crown of the plant to rot, and the plant may snap off and die.
  • Less severe infection will result in a cracked canker at ground level that restricts water and nutrient flow within the plant, causing it to lodge and mature prematurely.
  • Blackleg symptoms have also been found in the plant roots, that causes the entire plant to die prematurely.

Management strategies

Seed dressing fungicide
Seed dressing fungicide
important
Resistant varieties
Resistant varieties
  • Foliar fungicide at the 6 leaf stage can slightly reduce loss, but is not generally economic, particularly in the absence of fungicidal seed treatments.
  • Fungicide seed treatments, are effective but can not totally prevent yield loss.
  • Relying only on fungicides to control blackleg poses a high risk of fungicide resistance. Choose a cultivar with adequate blackleg resistance for your region.
  • If your monitoring has identified yield loss and you have grown the same cultivar for three years or more, choose a cultivar from a different resistance group.
  • Never sow your canola crop into last year’s canola stubble

How can it be monitored?

  • As blackleg is capable of overcoming genetic resistance, it is important to monitor crops and manage according to guidelines in the current version of the GRDC blackleg management guide.
  • Sample crops annually anytime from the end of flowering to windrowing (swathing).
  • Pull 60 randomly chosen stalks out of the ground, cut off the roots with a pair of secateurs and, using the reference photos in the GRDC blackleg management guide, estimate the amount of disease in the stem cross-section. Yield loss occurs when more than half the cross-section is discoloured.

Where to go for expert help

DDLS Seed Testing and Certification
+61 (0)8 9368 3721
Page last updated: Thursday, 16 April 2015 - 1:20pm