Preventative management is your weapon for success

Canola forms an important part of producers’ crop rotation programs and the sustainable production of crops in the Western Cape. Blackleg (Leptosphaeria maculans / Phoma lingam) is one of the canola diseases that can lead to major yield losses. Where the fungus overcomes the resistance of cultivars, losses of 50% to 90% can occur.

Life cycle of blackleg

Various factors can contribute to the development of the disease. The main source of primary infection is canola residue. However, the inoculum can also be transmitted through seed (Diagram 1). The impact between seasons due to seed is small compared to the inoculum from infected stubble of previous years. Seasonal conditions play a major role in the level of infection that can be expected.

Rain and cool weather stimulate the development of spores, and moist conditions cause the release of blackleg spores from canola stubble present on the soil surface. Blackleg spores are mainly spread by wind, with the highest percentage within 500 meters of any canola stubble. Spores are released from the stubble until it has completely decomposed.

It can take up to four years for the stubble to break down (Diagram 2). The blackleg pathogen can infect the plant at any stage, but early seedling infection has the greatest chance of developing into crown cankers later in the season, which have the biggest impact on yield loss.

Diagram 1: The life cycle of blackleg.

Type of infection: leaf and crown canker

Leaf infection has almost no impact on yield, but it is at this stage that the pathogen grows from the lesion on the seedling leaf through the leaf petiole and down the stem to the crown of the seedling, later forming crown cankers (Figures 1, 2, and 3).

Diagram 2: Spore release over four seasons in Western Australia.

Blackleg upper canopy infection

If spores are released later in the season from the stubble, it leads to upper canopy infection (UCI). It is possible for blackleg to affect all parts of the canola plant. UCI is a collective term used to describe infection of the flowers, stems, pods, and branches.

UCI has increased in Australia over the past few years and may be associated with an earlier flowering period because cultivars are planted earlier and the crop undergoes faster development during warmer autumn and winter conditions. This infection has also been observed in South Africa.

The effect on yield depends on when the plant is infected and which part of the plant is affected. Flower loss due to infection of flowers or peduncles does not have a major effect on yield reduction, as the canola plant can compensate by producing more flowers. However, when the fungus infects associated branches, seed set and grain filling in the pods can be affected.

Infection of the pods or peduncles after pod formation can result in major harvest losses. Severe infections can cause stems and branches to break, leading to premature ripening and pod shattering. It can also make it difficult to determine the correct cutting stage, as there may be differences between infected plants and those that escaped the disease.

Fungicide control for blackleg crown canker

Fungicide control should be applied at the 4- to 6-leaf stage. The reason for early spraying is that infection can already occur early, which later leads to crown cankers.

Fungicide control of upper canopy infection

If UCI occurs, it has been found that fungicides used for Sclerotinia control can help reduce infection pressure. When the plant is about 30% in bloom, the application of Prosaro® and/or Aviator® Xpro™ for Sclerotinia can also provide protection against blackleg UCI.

Figure 3: Crown canker in canola that can lead to plant death.

Figure 4: Typical symptoms of upper canopy infection. Photos: Steve Marcroft.

Resistance management of chemical groups (from Australia) for blackleg and Sclerotinia

  • Group 3 – Demethylation inhibitor (DMI)
  • Group 7 + 3 – Succinate dehydrogenase inhibitor (SDHI)
  • Group 11 – Quinone outside inhibitor (QoI)
  • Group 11 + 3 – QoI + DMI

Important facts about fungicide use

  • Fungicides should preferably be used preventatively. If the disease is already established, optimal control cannot be expected and the risk of selecting for resistance increases. Sclerotinia applications should be done during the flowering period. These applications can place selection pressure on blackleg populations.
  • In high-pressure environments, integrated management should be applied to reduce fungicide resistance. This includes planting canola 500 meters away from the previous year’s stubble, using resistant cultivars (rotate cultivars with different resistance genes), using alternative fungicides from different groups, and applying stubble management.
  • The risk of developing fungicide resistance can be reduced by alternating fungicide groups for blackleg management as mixtures, combined formulations, or combined groups.
  • If a Group 7 seed treatment with foliar activity for blackleg is used (as indicated on the label), the fungicide applied at the 4- to 6-leaf stage should not be a Group 7 fungicide.
  • Do not use more than two Group 7 fungicides per season.
  • Do not apply more than two consecutive applications of Group 3 fungicides.
  • Do not use more than ONE Group 11 fungicide per season.
  • If a second Sclerotinia application is required, the second fungicide must be from a different fungicide group.