"Making a comback" - Blackleg on canola

• 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).

1. 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.
2. 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.
3. 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.
4. 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.
5. 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.
6. 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 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.