Kutcher: A large-scale survey of races of L. maculans occurring on canola in western Canada

Date: April 2011
Term: n/a
Status: Completed
Researcher(s): Randy Kutcher, D. Cross, K. Nabetani, and C.L. Kirkham, Agriculture and Agri-Food Canada, Melfort, SK
SaskCanola Investment: n/a
Total Project Cost: n/a
Funding Partners: ACPC

Project Summary

In a previous study (Project Code: CARP2006-02) AAFC researchers determined the race structure of Leptosphaeria maculans, the species responsible for the most damaging symptoms of blackleg of canola in Canada. In this project, the specific objective was to conduct a large-scale survey of isolates of L. maculans to better understand the pathogen’s population structure. The results indicated considerable variation in the frequency of avirulence (Avr) alleles of L. maculans among locations in western Canada. This knowledge, coupled with knowledge of R genes present in current canola varieties, should be useful to provide recommendations on variety selection to canola growers based on frequencies of avirulence genes in the pathogen populations (races) that exist in their area.

Genetic resistance has been used effectively to control blackleg of oilseed rape or canola (Brassica napus) in western Canada. Specific resistance genes are an effective means of disease control when the pathogen population is mainly avirulent on the cultivated varieties carrying the corresponding resistance (R) gene. For example, there are specific pathogen avirulence (Avr) genes, such as those from L. maculans that cause blackleg, and matching resistant (R) genes in canola, which have been used to develop blackleg resistant canola varieties. However, host genetic resistance may be rapidly overcome by the pathogen if the Avr genes mutate to virulence and therefore develop into new races, as has been found in several canola growing regions around the world.

A previous study (Project Code: CARP2006-02) determined the race structure of Leptosphaeria maculans, the species responsible for the most damaging symptoms of blackleg. Isolates of L. maculans from western Canada were classified by the avirulence (Avr) genes they carry, which corresponds to specific R genes in B. napus. In a random collection of L. maculans from western Canada, researchers identified 16 races based on the analysis of 10 avirulence genes in the pathogen, with considerable variation in the frequency of avirulence alleles among these 10 genes.

In this project, the specific objective was to conduct a large-scale survey of isolates of L. maculans in western Canada to improve our understanding of the pathogen’s population structure. Knowledge of the structure of the pathogen population provides the information needed to develop blackleg resistance management strategies for varieties dependent on specific R genes. These strategies include adhering to crop rotations long enough to allow breakdown of the previous crop residue that harbours the pathogen, and therefore destruction of the pathogen itself, rotation of varieties with different specific R genes, or combining race-specific resistance with race non-specific resistance.

In this study, the frequency of avirulence (Avr) alleles at ten avirulence loci in over 800 isolates of the pathogen were determined from a canola trap using a variety carrying no specific resistance genes (cv. Westar) that became infected with L. maculans. These samples were collected from nine locations, three in each of Alberta (Camrose, Vegreville, and Vermillion), Saskatchewan (Scott, Melfort, and Indian Head) and Manitoba (Brandon, Carberry, and Plum Coulee). The results indicated considerable variation in the frequency of avirulence alleles of L. maculans among locations in western Canada. Knowledge of avirulence allele frequency and the race structure of L. maculans in canola producing regions of western Canada will be crucial to develop strategies to maintain the efficacy of resistance genes.

The study showed the frequency of avirulence alleles AvrLm1, AvrLm3, AvrLm4, AvrLm7, AvrLm9, AvrLmLepR1 and AvrLmLepR2 varied from 1 to 53% depending on the loci and the geographic location (See Figure 1). Three avirulence alleles were observed to be at high frequency in this study: AvrLm2, AvrLm6 and AvrLepR3 (77 to 94%). Therefore corresponding R genes (Rlm2, Rlm6 and RlmLepR3) could therefore be good candidates for use in variety development to condition resistance to L. maculans in western Canada.

Figure 1. Frequency (%) of avirulence alleles in the L. maculans population of western Canada (all locations combined, number of isolates determined for each Avr gene is indicated in brackets).

At eight locations ≥ 80% of the isolates carried the avirulence allele AvrLm2, except Plum Coulee, MB at 34%. The avirulence allele at AvrLm6 was present in all isolates tested from eight locations, but only in 48% of isolates at Plum Coulee, MB. Avirulence allele AvrLmLepR3 was present at a high frequency at Vegreville, AB; all SK locations and Plum Coulee, MB, but a lower frequency (48 – 61%) at Vermillion, AB, and Brandon and Carberry, MB.

This knowledge, coupled with knowledge of R genes present in current canola varieties, should be useful to provide recommendations on variety selection to canola growers based on current frequencies of avirulence gene in the pathogen populations (races) that exist in their area. This improved understanding of the race structure of L. maculans in western Canada is beneficial in the development of improved blackleg disease resistance strategies that might prolong the effectiveness of specific resistance genes, and will improve management of this disease through other means as well, such as reinforcing to producers and the industry, the importance of crop rotation.

Scientific Publications

Cross, D.J., Kirkham, C.L. and Kutcher, H.R. 2009. Determining the race structure of Leptosphaeria maculans in western Canada. Proceedings of the Soils & Crops Workshop, February 25-26, 2009, University of Saskatchewan, Saskatoon, SK [CD_ROM]

Kutcher, H.R., Balesdent, M.H., Rimmer, S.R., Rouxel, T., Chevre, A.M., Delourme, R. and Brun, H. 2009. Identifying races of Leptosphaeria maculans, cause of blackleg disease of canola in western Canada. Proceedings of FarmTech 2009, Edmonton, AB. January 28-30.

Cross, D.J., Kirkham, C.L. and Kutcher, H.R. 2009. Determining the race structure of Leptosphaeria maculans in western Canada. Proceedings of the Soils & Crops Workshop, February 25-26, 2009, University of Saskatchewan, Saskatoon, SK [CD_ROM].

Kutcher, H.R., and Yu, F. 2009. Blackleg revisited – races and resistance. Proceedings of the Manitoba Agronomists Conference. December 14-16. Published on-line at http://www.umanitoba.ca/afs/agronomists_conf/2009/proceedings2009.html

Kutcher, H.R., Yu, F., Brun, H. 2010. Using knowledge of genetic interactions between Brassica napus and Leptosphaeria maculans to manage specific resistance of blackleg of oilseed rape. Can. J. Plant Pathol. 32 (1): 29-34.

Dokken-Bouchard, F.L., Bassendowski, K.A., Boyle, T., Cowell, L.E., Gugel, R.K., Ippolito, J., Kirkham, C.L., Kutcher, H.R., Lewchuk, Z., Miller, S.G., Morrall, R.A.A., Phelps, S., Schemenauer, I., Sommerfeld, S., and Vakulabharanam, V. 2010. Survey of canola diseases in Saskatchewan, 2009. Canadian Plant Disease Survey 90: 127-129.

Kutcher, H.R., Balesdent, M.H., Rimmer, Rouxel, T., S.R., Delourme, R., Chèvre, A.M., and Brun, H. 2010. Frequency of avirulence genes among isolates of Leptosphaeria maculans in western Canada. Can. J. Plant Pathol. 32 (1): 77-85.

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