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Blackleg Management

Managing blackleg disease of canola in western Canada requires an integrated management strategy including agronomic best practices and stewardship of blackleg resistance. Reliance on any one strategy alone, such as genetic resistance, is not a viable long-term strategy. The concerns and issues related to blackleg management go beyond yield, quality and resistance stewardship to international market barriers. Follow these steps to help control blackleg. [4]

Blackleg is a disease that can increase with some crop management choices, therefore assessing your risk is important. For example, growing canola in a tight rotation (canola more frequently than one in three years) will increase the risk for the development of blackleg and the risk of blackleg resistance being eroded or overcome.


Scout for Disease

Blackleg pseudothecia on canola stubble

Blackleg pod infection

Blackleg stem infection

It is important to accurately identify the disease, as there are other diseases that can be confused with blackleg (eg. sclerotinia, verticillium stripe, grey stem, root rots). 

Look for blackleg early in the season and just prior to crop maturity. Fields that are highly infected indicate that action should be taken to reduce the risk of blackleg. Highly infected fields indicate that the rotation may be too short, which results in the build up of large amounts of infected residue. It may also indicate that the resistance carried by the variety or varieties grown has broken down and different varieties should be selected in future years.

Crop Rotations

From a blackleg management standpoint, crop rotation allows for the decomposition of infected canola residue, reducing the spores available to infect the next canola crop. This could require a delay of up to three years between canola crops. Even where disease issues are less severe and genetic resistance is still working reasonably well, trends in yield and economic returns suggest canola should not be grown more often than one year in three. This is important not only for blackleg disease management, but also to maintain and protect the blackleg resistance present in canola varieties.

Crops should be isolated from infected fields, and the recommendation is that canola crops should be planted at least 50 to 100 m from each other in western Canada. Reducing the movement of ascospores and pycnidiospores between fields may be accomplished by maintaining an effective buffer distance between future canola fields and fields containing infected residue. Fields on which canola has been grown in the previous 2-3 years will likely contain significant quantities of canola residue that may harbour the pathogen until this residue fully decomposes.

Resistant Varieties

Grow B. napus varieties that carry at least an MR blackleg disease resistance rating, or better yet an R. Resistance reduces blackleg infection to specific races of L. maculans, but does not mean the variety is immune to the disease.

A rotation study conducted at Agriculture and Agri-Food Canada (AAFC) in Melfort and Scott, Saskatchewan from 2000-06 found that blackleg severity of canola was reduced significantly as the length of rotation increased from two to four years even when growing an "R" rated hybrid. Intensive rotations of every two or even three years duration will increase the risk that disease resistance will break down and that disease severity will increase over the longer term. [9]

Resistance Stewardship

As a result of the introduction of resistant varieties, changes in the races of the pathogen have been observed. New races have been isolated that can overcome the resistance of some Australian, European and Canadian canola varieties. Most of the resistance genes identified at the present time have been overcome in one or more continents.

The use of management strategies such as variety selection or rotation of resistance genes over time, in combination with good quantitative resistance and best agronomic practices such as crop isolation, and four-year rotations of canola crops to minimize the impact of these new strains, should provide the protection necessary for successful canola production. Stewardship of blackleg resistance and agronomic practices designed to control blackleg are recommended to be used together.  [2],[4],[5],[6]

Rotate Varieties

Many genes are responsible for blackleg resistance in canola and at least 16 described virulent L. maculans races are currently found on the Prairies. These races and populations are highly dynamic. While growers don't know which varieties have which resistance genes, growing the same variety repeatedly on the same field will select for races of L. maculans that can overcome genetic resistance in that variety. The variety Q2, for example, went from an "R" rating when it was originally launched to an "MS" or "S" rating in some geographic areas today because races of L. maculans virulent to this resistance carried by Q2 became more predominant. There are other examples of resistance genes failing because of changing races of L. maculans. Rotating varieties creates the opportunity to bring a mix of resistance genes to the field over time, which can reduce selection pressure and improve durability.

Certified Seed

With certified seed, each canola plant should have the same blackleg resistance genes and be equally resistant. With bin run seed from hybrids, this second (F2) generation seed may not have inherited the same mix of resistance genes and therefore may be more at risk to blackleg. As well, seed treatments on certified seed can control seed-borne blackleg infection, which will reduce the chance of blackleg being introduced to the field with the seed. But keep in mind; seed treatments do not protect seedlings and adult plants from later infection.

Weigh the Benefit of Fungicide

Propiconazole (Bumper, Pivot, Propel, Tilt), azoxystrobin (Quadris, Exempla) and pyraclostrobin (Headline, Priaxor, Quilt) are registered to control blackleg. They only have protectant activity and little or no eradicant activity, so ideally fungicides should be applied before blackleg symptoms are present. Quadris and Headline can be applied from the 2 to 6-leaf stage, while the propiconazoles can be applied from the 2-leaf to just prior to bolting. Consult the label or a current guide to crop protection for more information on registered products, rates, etc.

In the AAFC rotation study from Melfort and Scott, generally there was little or no reduction in blackleg severity or improved yield when applying a fungicide on "R" rated varieties. Even on highly susceptible varieties, fungicides typically only depressed incidence by 20 percent and severity ratings by about one severity point, which often did not translate into a significant or economical yield benefit. [9]

Control Weeds and Volunteers

Canola volunteers and related weed species such as wild mustard can host blackleg and other canola diseases in non-canola years. If not controlled, these volunteers and weeds act as a disease bridge, reducing the effectiveness of rotation and variety resistance for disease management. 

No Solid Benefit from Tillage or Burning

Recently published studies from AAFC at Melfort, Saskatchewan looked at tillage and burning as ways to manage canola diseases. Neither measure significantly reduced blackleg severity in canola in that region of the Prairies. The report concluded that differences in disease severity among tillage treatments were minimal, and the use of burning was ineffective. [10]

Consider Your Climate

Blackleg incidence and severity tend to be higher in regions that are humid and warm with frequent rain showers. Therefore tight rotations in areas with conditions conducive to disease development will tend to be less sustainable, from a blackleg management perspective, than tight rotations in areas that are less humid with lower rainfall.

Blackleg Spreads by Rain and Wind

While most blackleg infection comes from canola residue within the same field, blackleg ascospores can be dispersed by wind. Therefore, canola growing in fields that have never had canola can still be infected from infested fields immediately adjacent. In that case, infection tends to be highest in canola plants closest to this infested field.


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.