Blackleg was once a devastating disease that commanded the respect of all canola producers in Western Canada 20 to 30 years ago. The disease received less attention after the introduction of resistant canola cultivars that essentially eliminated the disease risks, especially when sound crop rotation practices were employed with the resistant cultivars. However, blackleg disease is now making a comeback across the Canadian prairies and once again getting lots of attention from those involved or interested in canola production.
Blackleg is a disease of the cabbage family and occurs on crops such as mustard and canola. It is caused by two species of fungi. One species, Leptosphaeria maculans, is a highly virulent, or aggressive, pathogen of canola that can cause serious yield losses in susceptible canola crops. It infects canola seedlings early in the season and colonizes plants through the summer, eventually causing a dry rot at the base of the stem that cuts off water and nutrient flows. The diseased plants ripen prematurely, have reduced seed yield and may even be killed. The other fungus, Leptosphaeria biglobosa, causes mild infections that do not lead to significant economic losses and is therefore less of a concern to canola producers.
The virulent or aggressive blackleg fungus was first detected in Canada in 1975 in east central Saskatchewan, but today both blackleg fungi occur everywhere canola is grown in western Canada. In the 1980's and 1990's it was estimated that the aggressive blackleg fungus caused over $500 million in losses in Saskatchewan alone (www.canolacouncil.org). Today blackleg is still one of the most commonly found diseases in canola fields on the Canadian prairies, but it can be successfully managed by scouting/evaluating risks, and employing the necessary management tools.
Growers can scout for evidence of blackleg risk at three key times:
- Spring and summer – look for dirty-white lesions with small "pepper-like" spots on cotyledons, foliage and stems (Figure 1)
- August and September – look for dry basal stem cankers and blackening of the internal stem tissues just prior to swathing (Figure 2)
- Next year (or the year after canola was harvested) – look on the remaining canola stubble/residue for fungal spore producing structures, called pseudothecia (Figure 3).
Taking time to carefully develop a blackleg management plan is important for canola producers because of the explosive nature of the disease and the serious economic losses that could result from a severe outbreak. Additionally, multiple management tools need to be incorporated into a blackleg management plan because blackleg cannot be controlled with just one management tool. The blackleg fungus is highly adaptable to most management tools and therefore control of the disease will rapidly decline when only one tool is used. For example, the blackleg fungus can rapidly become insensitive to many fungicides, and it has the potential to overcome the genetic resistance of the host canola plant very quickly. A sound blackleg management plan would include some of the following components:
- Rotate away from crops that are susceptible to blackleg for 3 or 4 years. This is the foundational management tool for blackleg management upon which the consistent success of all other tools is rooted. The blackleg fungus does not survive very long in soil, so once the canola residues have decayed and broken down, the fungus has nowhere to live, and the pathogen populations quickly decline. This process of canola residue breakdown takes 3 to 4 years. Once the populations of the blackleg fungus have declined, then other management tools will be able to successfully, and consistently, control blackleg disease. If crop rotation options are limited, then rotation of canola varieties, foliar fungicides, tillage and weed management will become increasingly important. However, without adequate crop rotation, the consistency of the performance of these other tools may be compromised. The blackleg risk on a farm or field can be estimated using the information given in Table 2. Where growers have a high risk in categories 1-3, a fungicide application should be seriously considered.
- Use resistant, or moderately resistant, canola cultivars. Avoid susceptible or moderately susceptible cultivars. Using resistant cultivars, in conjunction with rotation, will successfully control blackleg in most canola producing areas of western Canada. Keep in mind that the population structure of the blackleg fungus will adapt to host resistances over time, therefore rotating canola resistance genes will also be an important part of disease management. Canola seed company representatives and canola agronomists may be able to assist canola growers in making educated choices about what canola cultivars could be used to "mix up" the resistance genes that the blackleg fungus is trying to adapt to. It's like shuffling a deck of cards so that you don't play the same card each year. When the same card is played repeatedly, the fungus quickly adapts to the resistance gene the resistance breaks down. As mentioned in #1, mixing up (or rotating) the resistance genes employed on a farm is even more important when crop rotation options are limited.
- Foliar fungicides can be an important tool to help reduce the blackleg populations in a field, or region, and can reduce disease severity and avoid yield penalties due to early blackleg infections. There are a number of fungicides registered to control blackleg on canola. The fungicide application targets the early infections of the aggressive/virulent fungus which become very damaging and yield-reducing. These damaging infections are initiated at the cotyledon to bolting stage in canola; therefore the fungicides must be applied early in the season, at the 3-5 leaf stage, to prevent these infections. Table 1 lists the available fungicides, their application timings and other properties associated with their use. A fungicide application may not be a sufficient compensation for a lack of crop rotation or employment of effective genetic resistance.
- Blackleg can be transmitted on seed. Even though the transmission of blackleg on seed is likely very low, seed infection could initiate disease early, or introduce a new race of the blackleg pathogen onto your farm. Therefore high quality, disease-free, with a registered seed treatments applied, should be used to establish a healthy canola crop. Do not use bin-run grain as seed, especially if it was harvested from a region that had any fields with high levels of blackleg disease.
- Tillage practices can, in theory, affect the rate of decomposition of canola stubble, and also prevent the release of spores from the pseudothecia the year after the canola was harvested. However, mixed results have been obtained when burying or burning canola stubble have been performed in attempts to control blackleg. Ploughing to bury canola stubble in the top 12 cm (5 in) could be considered as a potential management strategy on farms where blackleg is a serious and recurring problem, however it should not be considered as a foundational management principle and it should not be expected to provide consistent, effective management of blackleg.
- Weed control in important for managing canola diseases, because the susceptible weeds can provide a reservoir of disease that carries inoculum through the non-host rotation period. If there are ample weeds from the Brassica family (volunteer canola or mustard, wild mustard, shepherds purse, stinkweed) then the rotation period will be ineffective.
Blackleg quick facts:
- Blackleg was the most commonly occurring disease in Alberta canola fields in 2012
- The blackleg fungus causes disease lesions on leaves and stems, but it is the unseen growth of the fungus inside the stem that causes almost all of the yield loss
- Blackleg caused crop failures in some Alberta fields in 2012
- Both the virulent and weakly virulent species of blackleg occur in western Canada, while only the weakly virulent species occurs in China. This situation has caused some trade barriers for Canadian canola supplies destined for Asia.
- Blackleg incidence and severity has been slowly increasing over the past few years as crop rotations have become shorter, and less diverse, due to the increasing canola production
- Spraying fungicides to control blackleg must be done prior to scouting for in-crop disease symptoms.
- Evaluating blackleg risk can be done using canola stubble from the previous year, and even in an adjacent field.
|Table 1. Fungicides registered for use against blackleg on canola|
|Product||Active Ingredient||Fungicide Group||Stage||Rate (mL/acre)||Water (L/acre)|
|Headline||Pyraclostrobin||11||2-6 leaf stage||120 – 160||40|
|Quadris||Azoxystrobin||11||2-6 left stage||200||40|
|Table 2. Assessment of blackleg risk|
|Category||High Risk||Low Risk|
|Scouting||No scouting||Quantify blacklet at end of season and in-crop scouting|
|Crop rotation||2 year or less rotation||3 or 4 year rotation|
|Variety rotation||Same variety in tight rotation||Use a different variety each time|
|Fungicide||No fungicide use||Use Headline or Quadris|
|Seed source||Bin run seed||Certified treated seed|
|Weed control||Poor control or brassica weeds||Good weed control|