Blackleg is one of the most serious diseases of canola, capable of causing significant yield losses of over 50% in susceptible fields. The more devastating form of the disease is caused by the fungus Leptosphaeria maculans. It overwinters on infected canola stubble and releases spores in the spring, which are then dispersed by wind and rain splash to newly planted canola crops. When plants are infected, the fungus grows within the plant’s vascular system causing a canker (lesion) that girdles the stem base, restricting water and nutrient flow within the plant and eventually causing the crop to lodge. Severe canker will sever the roots from the stem. The fungus may also attack pods and seeds.
Late season scouting
The most appropriate time to quantify the severity of blackleg infection in a crop is just before harvest. Scouting at this stage is necessary to ascertain whether a change in genetics and management is needed for the next season. A simple method is to clip 25 plants at the base of the stem/top of the root and look for blackened tissue inside the crown of the stem. Multiply the number of affected plants by 4 to get a percentage. If the incidence is over 30%, or if it has increased from the previous canola crop, then control strategies will have to be modified for the coming season. The Canola Council of Canada also provides a Blackleg Field Rating Scale (picture below) which rates each stem for percent discolouration caused by blackleg. A resistant variety should have a rating not worse than a “2”, otherwise that variety is no longer resistant to the blackleg races in that field.
The following strategies help to control and manage blackleg:
- Use of resistant varieties is the best defence against blackleg. However, using the same resistant variety consistently in the same field increases the risk of certain races of the fungus overcoming the genetic resistance in that variety. So, it is important to rotate varieties.
- Adopt a longer crop rotation (a 4-year rotation between canola crops is recommended) once blackleg has been detected in a field. An adequate crop rotation interval allows for the decomposition of infected canola residue, reduces disease pressure and prolongs the effectiveness of genetic resistance.
- Use disease-free seed and treat seed from high-risk areas with fungicide. Note thatseed treatment will help prevent the spread of blackleg from infected seed to seedling, but will not protect the seedlings from infection by airborne spores.
- Incorporate infected stubble after harvest.
- Control volunteer canola and wild mustard in other crops.
- A foliar fungicide can reduce symptoms of the disease and minimize yield loss if a susceptible variety is being grown.