Blackleg: Scouting, identification and next steps

Around 60 per cent seed colour change (swath timing) is the best time to scout for blackleg as the basal cankers, which cause significant yield loss, are easy to see. Check fields – even if growing resistant (R) rated hybrids. An R rating is not immunity and does not tell much about if it will be effective against the races in a individual field.

To scout:

  • Pull up at least 50 plants in a W-pattern.
  • Clip with clippers at the base of stem/top of root (roughly half an inch into the root tissue) and look for blackened tissue inside the crown of the stem. The amount of infection present will help identify the level of risk and the best management practices for that field in following years.
  • Use the zero to five blackleg disease rating system to identify severity of the infection along with the incidence of the disease. Calculate an average for the field.
Blackleg rating from 0 (left) to 5 (right). Credit: Justine Cornelsen
Click to enlarge

Yield Loss. Updated yield loss models from the University of Alberta show that blackleg starts to rob yield at a disease severity rating of 2. Severity ratings over 1 start to see potential losses of 20 per cent per unit increase on the BL disease severity scale of 0-to-5 rating scale. Average severity ratings less than 0 to 1 have minimal losses. See the recent yield loss models study.

Blackleg that is severe enough to cause stem discolouration would have infected the plant very early in the season — cotyledon to two-leaf stages. This level of infection is not from disease that entered the plant recently from root maggot tunneling, for example. Root rot and foot rot might look similar to blackleg from the outside, but dry rot (base of stem turns woody) is more likely to be blackleg. Blackleg is an aerial disease which pushes up spores into the canopy to cause infection. This is the primary pathway for infection on new canola plants.

If not sure, get it tested. Suspected plant samples can be submitted to diagnostic labs to identify if blackleg species are present. Specific tests can also determine if Leptosphaeria maculans or Leptosphaeria biglobosa is the causal agent. L. maculans race identification testing is also available through private labs.

Quick comparison of blackleg, clubroot and sclerotinia stem rot

Canola stem diseases – a comparison

Next steps. If blackleg levels are rising, canola growers will want to make a plan to keep a lid on infection levels. Here are effective management steps:

  • Crop rotation. From a blackleg management standpoint, crop rotation allows for the decomposition of infected canola residue, reducing the spores available to infect the next canola crop. This could require a delay of up to three years between canola crops.
  • Resistant varieties. Grow B. napus varieties that carry at least a moderately resistant (MR) blackleg disease resistance rating, or better yet an ‘R’ rating. Resistance reduces blackleg infection to specific races of L. maculans, but does not mean the variety is immune to the disease.
  • Rotate varieties. Many genes are responsible for blackleg resistance in canola and at least 16 described virulent L. maculans races are currently found on the Prairies. Growing the same hybrid repeatedly on the same field will select for races of L. maculans that can overcome genetic resistance in that hybrid. Rotating varieties or resistance gene groups creates the opportunity to bring a mix of major resistance genes to the field over time, which can reduce selection pressure and improve durability. Quantitative resistance can provide effective support for major genes, but quantitative resistance is not standardized in the same way and we can’t know how much is in a cultivar.

Keep It Clean – Blackleg resources