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Identifying Blackleg

Pycnidia found on four-leaf canola

Figure 6: Pycnidia starting the blackleg infection on a canola leaf

Figure 7: Blackening stem lesions

Different levels of blackleg stem infection

Symptoms

Disease lesions may occur on cotyledons, leaves, stems and pods. Leaf spots are dirty white, round to irregularly shaped, and usually dotted with numerous small, black pycnidia (the pepper-like spore-bearing structures Figure 6). Pycnidia appear as tiny round specks that may be seen more easily with the aid of a hand lens. Under moist weather conditions, a viscous pink liquid carrying the pycnidiospores oozes from the pycnidia. Canola plants are susceptible to blackleg infection throughout the lifecycle.

On stems, blackleg lesions can be quite variable, but are usually found at the base of the stem or at points of leaf attachment. Stem cankers, which occur most frequently if infection of plants happens before the six-leaf stage, are associated with serious yield loss. Stem lesions may be up to several inches in length, and are usually white or grey with a dark border. Numerous pycnidia form in the centre of the lesion (Figure 7). Basal stem lesions may also appear as a general blackening at the base, again with numerous pycnidia, with stem tissue constricted or pinched at the soil surface.

Lesions may also be observed where physical damage has occurred. Damage due to hail, insect feeding, tire tracks, etc. may provide an open wound where the pathogen can successfully establish, resulting in infection and lesion development.

Severe infection usually results in a dry rot or canker at the base of the stem. If basal infection begins early, stem cankers may appear from flowering onwards. As the season progresses, cankers penetrate, deepen and may girdle stem bases, often completely severing the plant. By mid-July, plants ripen prematurely and may lodge. Less severely affected plants remain standing but have restricted moisture and nutrient flow. Slicing through the canker or stem base perpendicular to the stem should reveal black and/or brown necrotic discolouration in the interior of the stem of plants affected with virulent blackleg.

Pseudothecia, which are usually slightly larger than pycnidia, may form on the canola residue within the basal stem cankers. This occurs as a result of crossing of alternate mating types and takes place in autumn or the spring of the following year.

Pods and seeds may also be infected, although this is uncommon, especially in resistant varieties. Infected pods ripen and shatter easily at harvest, resulting in seed loss. The seed formed close to a pod lesion may be sunken or shrivelled and pale grey. 

The earlier pod infection occurs, the less likely it is that viable seed will be produced as the fungus infects the seed coat and embryo.

The weakly virulent species L. biglobosa usually infects plants later in the growing season as the crop matures, resulting in shallow stem lesions but rarely forming cankers that girdle the stem and cause yield loss.

Scouting Tips

Scouting is an important management tool for blackleg management. There are three main scouting periods for blackleg disease during the growing season:

1. Prior to planting:

  • examine canola residue to see if pseudothecia are present


2. Vegetative stage (3 to 6 leaf)

  • scout using a 'w- pattern' through field, starting at field edge. Focus on edges where new canola residue may be present in adjacent field.
  • examine at least 50 plants for the presence of lesions. If lesions are found on more than 10% of plants, then there is a risk of significant disease development.


3. At swathing:

  • best time to scout for the disease as the basal cankers, which cause significant yield loss, are easy to see
  • pull up at least 50 plants in a w-pattern as described above
  • clip with clippers at the base of stem/top of root and look for blackened tissue inside the crown of the stem. The amount of infection present will help identify the level of risk and the best management practices for that field in following years
  • use the 0-5 blackleg disease rating system to identify severity of the infection along with the incidence of the disease


References

[1] Fitt, B.D.L. ,H. Brun, M.J. Barbetti and S.R. Rimmer. 2006. World-wide importance of phoma stem canker (Leptosphaeria maculans and L. biglobosa) on oilseed rape (Brassica napus). European Journal of Plant Pathology, 114: 3-15.

[2] Kutcher, H.R., F. Yu, and H. Brun. 2010. 'Improving blackleg disease management of Brassica napus from knowledge of genetic interactions with Leptosphaeria maculans', Canadian Journal of Plant Pathology, 32: 1, 29 -34.

[3] Guo, X.W. and W.G.D. Fernando. 2005. Seasonal and diurnal patterns of spore dispersal by Leptosphaeria maculans from canola stubble in relation to environmental conditions. Plant Dis. 89:97-104.

[4] Kutcher, HR, WGD Fernando, TK Turkington and DL McLaren 2011. 'Best Management Practices for Blackleg Disease of Canola. Prairie Soils & Crops Journal. Volume 4.2011.

[5] Kutcher, H. R., M. H. Balesdent, S. R. Rimmer, T. Rouxel, A. M. Chèvre, R. Delourme, and H. Brun. 2010. 'Frequency of avirulence genes in Leptosphaeria maculans in Western Canada', Canadian Journal of Plant Pathology, 32: 1, 77 - 85.

[6] Fernando, W.G.D. 2010. Managing Blackleg Resistance Breakdown and Trade Barriers through Blackleg Resistance Stewardship in Canola. MB Agronomists Conference, December. University of Manitoba.

[7] Van de Wouw, Anton Cozijnsen, Jo Rayner and Barbara Howlett. 2009. Monitoring of virulence in Australian populations of the blackleg fungus. School of Botany, University of Melbourne.

[8] 2011. Procedures of the Western Canada Canola/Rapeseed Recommending Committee Incorporated for the Evaluation and Recommendation for Registration of Canola/Rapeseed Candidate Cultivars in Western Canada. Appendix B: Disease Testing Protocols, 14.

[9] Smith, E.G., M.L. Favret, S.A. Brandt and H.R. Kutcher. 2008. Economics of Shorting Canola Rotations. Agriculture and Agri-Food Canada. Poster.

[10] Kutcher, H.R. and S.S. Malhi. 2010. "Residue burning and tillage effects on diseases and yield of barley (Hordeum vulgare) and canola (Brassica napus)." Soil & Tillage Research, 109 (3), pp. 153-160.