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Important Tips for Best Management

  • Blackleg is a serious disease of canola across all canola growing areas. It requires an integrated management strategy utilizing the best agronomic practices to minimize yield loss and maintain the effectiveness of genetic resistance in our varieties, our most valued management tool.
  • Scout crops for accurate identification of the proportion of plants affected by the disease, to estimate economic impact and assist blackleg management planning.
  • Select R rated resistant varieties. However, utilizing the same resistance genes repeatedly in the field over time can select for races of the fungus that are virulent against that resistance. The higher the frequency of growing the same variety, the greater the risk. Rotating varieties can help.
  • Increasing the number of years between canola crops in the rotation reduces incidence and severity of blackleg in fields. Maintain a minimum break of two or three years between canola crops, to protect against the breakdown of current blackleg resistance in varieties and allow effective long-term blackleg disease management.
  • Use certified seed and control weeds and volunteers.


About Blackleg

Click here to read the Blackleg Strategic Plan in Canada (PDF), December 2013

Overview

Blackleg disease of canola and oilseed rape is caused primarily by the fungus Leptosphaeria maculans. It is a serious disease of canola and can cause significant yield losses in susceptible varieties.

Blackleg, also known as stem canker or phoma stem canker, is actually attributed to a complex of two species, L. maculans and L. biglobosa. These species were previously referred to as highly and weakly virulent forms of L. maculans, respectively. L. biglobosa, a mild or weakly virulent species, is often associated with upper stem lesions and is widespread in western Canada. This species usually infects plants late in the season, rarely causing significant yield losses and is therefore considered a minor problem. [1],[2]

However, L. maculans is the virulent species causing blackleg that infects canola from the seedling stage onward. It progressively damages the crop as the season progresses by girdling stems and restricting moisture and nutrient uptake, leading to yield loss. L. maculans was first detected in 1975 in north east Saskatchewan and in 1987 in Ontario. Since then L. maculans has become widespread throughout western and eastern Canada.

In western Canada, yield losses up to 50% have been reported in individual fields. In the early 1990s, canola varieties with high levels of genetic resistance to the disease were introduced and cultural controls such as rotation were adopted by producers. These measures reduced the yield and quality losses associated with the disease but did not eliminate the pathogen. By the late 1990s, evidence of virulence changes in the pathogen was observed and in recent years producers have reported higher than expected disease severity in previously resistant varieties.

It is not unusual to observe blackleg symptoms in canola crops, even when resistant varieties are grown. It is important to be familiar with blackleg symptoms, the disease cycle, and to know the disease management practices that prevent yield and seed quality losses.

Disease Cycle

Fungus produces fruiting bodies: pseudothecia

Ascospores under the microscope

Stubble infected by pycnidiospores

The L. maculans fungus overwinters on infected canola residue.  The spores produced from diseased stubble, particularly the infected lower stem and upper root pieces, are the major source of the pathogen that contributes to widespread field infection resulting in yield loss. In the spring, the fungus produces fruiting bodies, called pseudothecia, on infected canola residue. Pseudothecia may continue to be produced on infected residue for several years, or until the infected residue breaks down.

Small microscopic sexual spores, called ascospores, are released from the pseudothecia and become airborne, resulting in long distance dispersal of the disease to newly planted canola crops. Ascospores are considered the primary inoculum. They initiate the disease and serve to spread new races of the pathogen. Ascospores are dispersed by wind and rain, and can be released from infested residue for at least three years. Ascospore release can begin as early as May, causing leaf lesions from the seedling stage onward.

During the growing season, the pathogen also produces another type of fruiting body called pycnidia that appear as pepper-like spots within lesions. From the pycnidia ooze masses of tiny spores called pycnidiospores. These spores are spread short distances by rain splash and wind and cause secondary infection within a crop. Infected stubble can continue to produce pycnidiospores for three to five years.

Seedling infection may arise from infected seed or from airborne and rain-splashed spores. Plants infected at this stage are usually the most severely affected and may be lost or stunted. Seed treatments of certified seed can control seed-borne blackleg infection, which will reduce the chance of new pathogen races being introduced to the field with the seed. However, seed treatments do not protect seedlings and adult plants from later infection by airborne spores.

Once the leaves are infected the fungus grows down the petiole into the stem, eventually leading to the most damaging phase of the disease, stem cankering, usually at ground level. This restricts moisture and nutrient movement in the plant. Flowering plants may lodge as the fungus girdles stem bases, often completely severing or cutting off the plant.

The earlier plants are infected, the greater the likelihood of the development of basal stem canker. Infection before the six-leaf stage is often associated with serious yield loss. Infections initiated after the six-leaf stage usually cause less damage than early infections. Less severely affected plants may survive but produce less seed of poor quality. Wounded stems from hail or insects may be infected directly from spores germinating in the wound.

Influence of Environment

Climate has an impact on blackleg incidence and severity. Regions with warm, humid conditions and frequent rain showers tend to have higher incidence and severity of blackleg. Warm, dry conditions slow disease development, while prolonged moist weather favours rapid spread and development. However, the disease is present in drier regions of the prairies and can be a problem even in dry years provided early season showers occur to disperse the spores for initial infection.

Infection of canola is due to ascospores and pycnidiospores of the pathogen, whose production and dispersal depends on environmental factors such as rain and wind. Therefore, the severity of blackleg disease can vary from region to region and year to year. Ascospores are responsible for the spread of new races of the pathogen as these are wind-borne and can move relatively long distances (at least 10 kms). Although most blackleg infection comes from canola residue within the same field, ascospores and even some pycnidiospores that become air-borne after rain-splash may land on canola or volunteer canola plants in nearby fields to begin new infections. In the management of blackleg disease, it is recommended that canola crops in western Canadian should be separated from fields that were planted to canola the previous year by at least 50 to 100 metres.

Research at the University of Manitoba found that peak ascospore and pycnidiospore dispersal was associated with rain events. Peak ascospore dispersal occurred several hours after rainfall of greater than 2 mm and persisted for approximately 3 days after such events. Peak pycnidiospore dispersal occurred during the same time as the rainfall. Spores are spread by wind, with more spores carried in the direction of prevailing winds. Pycnidiospores were found to travel further than first thought (45 metres) and may even move by wind. The research also found that on days without rain, air temperature and relative humidity (RH) had an effect on spore dispersal, with more ascospores and pycnidiospores trapped between 9 pm and 4 am, when temperatures were 13 to 18C and RH greater than 80%, than at other times of the day on days without rain. [3]


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.