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Genetic Resistance

Different levels of blackleg stem infection

Blackleg resistance has been available in commercial canola cultivars and varieties since the early 1990s, which has protected the crop against the predominant races of L. maculans. The use of a race-specific resistance strategy to manage this disease has been of great value in Canada, Australia and Europe. However, large-scale surveys in Europe and the more limited analysis of Canadian and Australian populations of the pathogen indicate that races with the ability to overcome most of the known specific resistance (R) genes can be found in many pathogen populations. In some cases the virulence of the local pathogen population changed in a matter of three years (two cycles of selection) to defeat the resistance. Stewardship of blackleg resistance and agronomic practices designed to control blackleg must be managed together.  [2],[4],[5],[6],[7]

Blackleg Testing Protocols and Ratings for Resistance

The Western Canada Canola/Rapeseed Recommending Committee (WCC/RRC) has established Blackleg Testing Protocols and Ratings to assess resistance. The resistance ratings currently available for commercial varieties are based on their field performance in terms of disease incidence and severity relative to check varieties in disease testing sites. Resistance reduces infection to certain races of L. maculans, but does not provide immunity. Normally the final resistance rating is based on trials from at least three locations each year in western Canada. [8]

The following scale is used to describe the level of resistance based on average severity ratings compared to the highly susceptible variety Westar:

R (Resistant) = <30 percent of the severity of Westar
MR (Moderately Resistant) = 30-49 percent of the severity of Westar
MS (Moderately Susceptible) = 50-69 percent of the severity of Westar
S (Susceptible) = 70-100 percent of the severity of Westar

The severity of blackleg infection is evaluated on a minimum of 100 plants averaged over four replicates at crop maturity. Individual plants are uprooted, cut through the basal part of the stem and scored on the basis of the amount of disease using the following 0-5 scale.

0 -  No diseased tissue visible in the cross-section.
1 - Diseased tissue occupies up to 25 percent of the cross-section.
2 - Diseased tissue occupies 26-50 percent of the cross-section.
3 - Diseased tissue occupies 51-75 percent of the cross-section.
4 - Diseased tissue occupies > 75 percent of the cross-section with little or no constriction of affected tissues.
5 - Diseased tissue occupies 100 percent of the cross-section with significant constriction of affected tissues; tissue dry and brittle; plant dead.

For the trial to be valid, the severity rating of Westar must score between 2.6 and 4.5.

Development of Commercial Varieties Resistant to Blackleg

Resistance to L. maculans in Brassica spp. is of two types: quantitative or major gene or adult plant resistance and qualitative or minor gener or race-specific resistance. Quantitative resistance is expressed at the adult plant stage as reduced development of necrotic tissue at the stem base compared to that found in susceptible varieties, and is believed to be due to many genes, each with a relatively small effect. Qualitative resistance in Brassica spp. is usually effective at the site of initial infection on the cotyledons and leaves and is controlled by race-specific resistance genes. A resistant response is presumed to result from recognition of the corresponding L. maculans avirulence genes in a gene-for-gene manner, according to the theory developed by Harold Flor in 1946. In Canada, all commonly grown varieties of B. napus canola and high erucic acid rapeseed carry moderate to high resistance to L. maculans, although the type of resistance or the specific genes for resistance present in a variety are not generally known.

Canola breeders likely use both quantitative and qualitative resistance to develop blackleg resistant varieties. Qualitative disease resistance results from the interactions between products of pathogen avirulence (Avr) genes, such as those from L. maculans that causes blackleg, and matching resistance (R) genes in the plant. Therefore, if a pathogen has an Avr gene and tries to infect a host plant with the corresponding R gene, the R gene recognizes the pathogen and causes the plant to initiate a defense or immune system response. Resistance only results if both the Avr gene in the pathogen and the R gene in the host are present, otherwise a susceptible reaction results and disease symptoms are observed.

Research at AAFC Melfort, Saskatchewan and the University of Manitoba have isolated and identified new virulent races of L. maculans, threatening the current blackleg resistance available in current varieties. A recent study of western Canadian isolates indicated variation for eight of the ten Avr genes tested in a sample of 96 L. maculans isolates. In total, 16 races of L. maculans were identified based on ten Avr genes, with seven races accounting for 90% of the isolates. There were similarities in the frequency of various Avr genes between the European and the Canadian studies, but also major differences.  Unfortunately, surveys in Europe, Canada and analysis of Australian isolates of the pathogen have identified races of L. maculans with the ability to overcome most of these R genes. [2],[4],[5],[6],[7]

Using a specific set of races of L. maculans, the specific R genes in varieties or breeding material of B. napus and other Brassica spp can be determined. Knowledge of host R genes and pathogen Avr genes is beneficial to the development of blackleg disease resistance strategies that might prolong the effectiveness of specific resistance genes.  Knowledge of the frequency of Avr genes in the pathogen population across the Canadian prairies and in other oilseed rape growing regions of the world will allow pathologists, breeders and farmers to consider strategies to manage specific resistance.

R-gene Rotation

Current labels for blackleg are based on field ratings of blackleg in comparison to a susceptible variety (Westar) for blackleg. Typically only Resistant “R” (0-29.9% of Westar) and Moderately Resistant “MR” (30-49.9% of Westar) are on the market. In recent years, some growers have noticed increased blackleg severity in their “R” and “MR” varieties. This is where the addition of more detailed labels will help to provide more accurate information.

Using the same disease resistance genetics over and over causes a shift in pathogen population, which can then overcome the resistance in our varieties – similar to herbicide resistance in weeds. Knowing the resistance genetics used in previous years will allow growers to rotate to a different resistance gene and reduce the blackleg infection within a field. As many as 10 new blackleg resistance labels will be applied to varieties in the coming years. They will use these letters  A, B, C, D, E₁, E₂, F, G, H, X to identify major resistance genes present.

How to use the labels

Blackleg management starts with scouting and identifying the disease on the previous canola crop’s stubble. If a grower has a rotation break longer than two years without canola, and has been growing “R” rated varieties, the risk of severe blackleg infection is minimal. But if a growers uses a tighter canola rotation and the disease is present and/or severe, the new additional label will come into play.

Picking an “R” rated variety with at least one different resistance gene group than what was used previously will help to provide protection from the aggressive, new virulent blackleg race(s) within the field. In those cases where the disease is not present/evident in the field, then changing resistance groups would not be necessary.

The most effective means for reducing blackleg disease is reducing inoculum or spore production in a field. This is accomplished by lengthening the break between canola crops in a field. But spores can also migrate from adjacent fields. Under these circumstances, using a canola variety with a different resistance gene can be very beneficial in protecting the field and reducing blackleg disease.

In conclusion, the most important step is to scout for disease. If you do not have blackleg, choose varieties with the highest probability of profitability and reduced production risk on your farm. If you do have blackleg disease that is increasing, use the tools available to manage and reduce the disease. Switching to varieties with a different resistance gene(s) is one tool.

Here are some examples of how the new label will work:

  1. Variety alpha: R (BC)
    The traditional “R” rating means average field performance of blackleg resistance was below 30% of Westar, the susceptible check. The additional “(BC)” designation means the variety contains the resistance genes Rlm2 and Rlm3.

  2. Variety beta: MR (A)
    The traditional “MR” means average field performance of blackleg resistance was 30-49.9% of Westar check. The additional “(A)” means it contains the resistance gene LepR3 or Rlm1.

  3. Variety Charlie: R (CX)
    As an “R” rated variety, average field performance of blackleg resistance was below 30% of Westar check. “(CX)” means it contains the resistance gene Rlm3 and an unidentified major resistance gene.


Reminder: The use of blackleg R-gene labels is a voluntarily process for seed companies to include on their varieties. The industry is committed to providing the best genetics and advice to producers, so it will take some time to effectively deploy and incorporate these labels. Talk with your Canola Council of Canada agronomist for more information.


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.