Rethinking Blackleg in Canola

Earlier sowing of canola requires a change to Blackleg management, demanding a greater focus on controlling upper canopy infection.

Blackleg is a fungal disease that most commonly reduces canola yield through crown cankers that inhibit the flow of nutrients and water. However, in recent years the Blackleg fungus has been found infecting the vascular tissue of the stem, branches, flowers and pods, which has been termed upper canopy infection (UCI). The Blackleg pathogen that causes UCI is the same as that which causes crown cankers, however it occurs completely independently. Blackleg disease, both crown cankers and UCI have the greatest impact on yield in Decile 3-7 years as it is in these years when the plant is working hardest to extract water from the soil.

The release of Blackleg spores from previous years’ canola stubble is dependent on environmental conditions, with cool, moist conditions prompting spore release. Historically, these conditions are associated with the Autumn break and occur during May and early June. In recent years crops have been sown earlier, leading to rapid early growth and more advanced crops during this infection period.

Disease of the crown occurs only when Blackleg spores land on the leaves before the plant reaches the 6 leaf stage. Once the plant has 6 leaves the fungus is generally unable to reach and infect the crown. Spore release coinciding with later stages of crop development, has led to increased incidence of UCI. Stubble that is left standing is more likely to contribute to UCI, whereas stubble on the soil surface is more likely to contribute to crown canker.

Recent research into UCI has determined that yield loss has been up to 1t/ha. UCI causes yield loss in several ways, including reduced seed size, reduced seeds per pod, premature pod shatter and aborted pods. Additionally, seed sown from pods infected with Blackleg will also be infected, which may cause seedling blight and poor establishment. Unfortunately, many symptoms of UCI cannot be seen until after windrowing, which makes it difficult to detect early enough to control. Environmental conditions which are favourable for UCI are similar to Sclerotinia, cool moist conditions. However, Blackleg has a broader window of environmental conditions that favour infection and does not require the prolonged period of leaf wetness to infect the plant. Physical damage caused by hail or insects will also increase the susceptibility of canola to UCI.

Management decisions which can be made to mitigate the risk of Blackleg infection include:

  1. Growing cultivars which are genetically resistant to Blackleg infection

  2. Rotating varieties and resistance groups

  3. Growing canola as far away from previous years’ stubble as possible

  4. Sowing early with large, high vigour seeds

  5. Managing canola residue

  6. Seed, fertiliser and foliar applied fungicide

Canola has been bred for resistance to the Blackleg fungus, however each cultivar can contain different resistance genes and therefore genetic resistance varies between cultivars. There are two types of genetic resistance; major gene resistance and quantitative gene resistance. Major gene resistance is a single gene or combination of single genes that inhibits the Blackleg pathogen from infecting and causing disease in the plant. The Blackleg resistance group such as A or ABD and subsequent resistance ratings are determined by this major gene or group of genes. Major gene resistance is very effective, however can be overcome with high pressure over successive years. Currently groups D, H and F are the only genes which are still completely effective. Quantitative gene resistance utilises multiple genes to reduce the severity of crown canker but does not inhibit leaf lesions and UCI.

Blackleg is spread by wind borne spores that mature on previous years’ infected canola stubble. As the distance between current canola crops and previous years’ stubble increases, the Blackleg spore concentration will reduce. At a distance greater than 500 metres the risk of infection should be significantly reduced. As clients continue to expand and implement a blocked farming system, this has become easier to achieve. By sequencing crop rotations in a westerly direction this will further reduce the number of spores that can be spread by wind that generally blows from the west.

Early sowing can significantly reduce Blackleg infection, as the canola seedlings can reach the 6 leaf stage prior to spore showers starting. This may be the reason that despite widespread plantings of susceptible Group A varieties, limited crown cankers have been observed in recent years. However, as early sown canola flowers earlier, the upper canopy is at a much greater risk from spore showers during winter. This risk may need to be managed by applying foliar fungicide in high risk situations.

As all canola stubble, irrespective of age, can release Blackleg spores, the management of canola stubble can have a large impact on the quantity of spores released into subsequent canola crops. If stubble is in contact with the soil, microbial breakdown will occur faster than if the stubble is left standing, reducing the time period when spores can be released. Methods which can reduce canola stubble include Kelly chaining, harrowing, knocking down with a chain/cable/bar or using rotary harrows when sowing.

Seed and fertiliser applied fungicides, protect canola seedlings in the first few weeks when they are most vulnerable to spore showers and infection. Such fungicides have been a very effective tool in managing Blackleg infection. However, earlier sown canola crops often reach the 6 leaf stage, prior to the release of Blackleg spores, in which case, seed and fertiliser applied fungicides are of little value. Alternatively, a foliar fungicide could be applied prior to the 6 leaf stage, only if disease pressure is high and leaf lesions are present. This is a more reactionary strategy that may only need to be employed in a small percentage of years.

Given that most of the fungicides used to manage Blackleg are from the Triazole group, ongoing blanket application of these fungicides will increase fungicide resistance.

The following diagram outlines the risk and infection period for Blackleg in canola.