Peng: Analysis and Monitoring of Leptosphaeria Maculans Race Dynamics in Western Canada for Effective Use of Cultivar Resistance in Management of Blackleg on Canola
Date: July 2017
Term: 4 years
Status: Completed
Researcher(s): Gary Peng, Fengqun Yu, Agriculture and Agri-Food Canada, Saskatoon SK, Dilantha Fernando, University of Manitoba, Sheau-Fang Hwang, Alberta Agriculture and Forestry; Stephen Strelkov, University of Alberta
SaskCanola Investment: n/a
Total Project Cost: n/a
Funding Partners: ACPC, MCGA, WGRF, ACIDF, ARDI and seed companies
Project Summary
For many years, the main blackleg management strategies in western Canada were variety resistance and crop rotation. However, with tighter crop rotations in most of the canola growing regions, there have been increasing reports of blackleg, with a sudden spike reported in 2012. In 2013, a four-year project was initiated to analyze/monitor the Avr profile in the pathogen population and identify new pathogen races capable of overcoming specific R genes. Over the four years of the study, the pathogen population changed noticeably across the Prairies, showing that the major blackleg resistance (R) genes Rlm1 and Rlm3, found commonly in our canola cultivars, are no longer effective due to the generally low presence of AvrLm1 and AvrLm3 in the pathogen population. In cases where severe blackleg damage occurs in a field, continued good practice is to consider a longer crop rotation option as the first line of defense and use a variety carrying effective R gene(s). An additional 5 years of funding starting in 2017 will support continued monitoring of pathogen populations.
For many years, the main blackleg management strategies in western Canada were variety resistance and crop rotation. However, with tighter crop rotations in most canola growing regions, there have been increasing reports of blackleg in past few years, as well as new pathogen races reported as early as 2002. In 2012, a large number of fields with resistant (R) or moderately resistant (MR) cultivars were found with severe blackleg, especially in southern Manitoba and parts of Alberta. This sudden spike of blackleg was suspected to be a result of the proliferation of new virulent pathogen races that can overcome the R genes in canola cultivars, resulting in resistance breakdown.
In 2013, a four-year project was initiated to analyze/monitor the Avr profile using Westar trap plots for regional pictures of Avr frequencies in the pathogen population. Previous research starting in 2007 had begun to study and monitor blackleg pathogen populations across western Canada. In this project, researchers also wanted to help identify new pathogen races capable of overcoming specific R genes. The information from this study can help the judicious deployment of specific R genes in canola cultivars for effective control of blackleg.
Between 2012 and 2015, up to 35 Westar trap plots were seeded each year in all major canola production areas on the prairies. At early crop maturity, diseased stems were sampled from each plot, pathogens isolated and tested on 14 differential brassica varieties/lines carrying known R genes to determine the presence/absence of specific Avr genes in pathogen. Plants carrying an R gene would resist the infection only when the pathogen isolate carried the corresponding Avr gene.
The number of pathogen isolates tested each year varied slightly depending on the blackleg occurrence in different regions, but overall the number ranged from 202 to 626 isolates each year. A total of 372 L. maculans isolates collected from commercial canola fields in Alberta, Saskatchewan and Manitoba during 2012 and 2013 were categorized into 90 races based on the Avr genes carried by each isolate. Of these, 22 were more common, each accounting for >1% of the pathogen population.
Over the four years of the study, the pathogen population changed noticeably across the Prairies. Between 2012 and 2015, the study showed that the pathogen races carrying AvrLm2, AvrLm4, AvrLm6,(8) and/or AvrLm7 were found at high frequencies, while AvrLm1, AvrLm3 and AvrLm9 were generally low or absent. This information is highly useful to canola breeders in deploying R genes effective against the current population of pathogen. Since only Rlm1 and/or Rlm3 were found commonly in Canadian cultivars, it is likely that the major-gene resistance to blackleg is currently lacking on the prairies due to the generally low presence of AvrLm1 and AvrLm3 in the pathogen population. The general resistance by many canola cultivars is likely through other mechanisms, especially the race nonspecific or quantitative resistance.
A 0-9 scale used for Avr testing on differential hosts based on cotyledon inoculation. Interactions below 5 is considered resistant.
Based on these study results, researchers expect that introducing Rlm4, Rlm6 or Rlm7 into canola varieties can enhance the resistance performance against blackleg in most regions on the prairies by targeting the most prevalent Avr genes in the pathogen population, especially when these R genes can be combined with a superior quantitative resistance background which has been shown in many current varieties already.
Researchers note that because the pathogen race structure can change substantially from one field to another, as shown in this study, growers should use caution when using this big-picture information to guide canola cultivar rotation based on specific R genes in a given field. In the cases where severe blackleg damage occurs in a field, it would be a good practice to consider a longer crop rotation option as the first line of defense. To use a canola cultivar effective against the pathogen population in a specific field, it would be prudent to assess prominent Avr types in the field for maximum efficacy.
The approval of an additional 5 years of funding starting in 2017 will provide researchers with the support to continue tracking any further change of the pathogen population and provide up to date Avr profile on the prairies. This research will also support the potential introduction of new R genes and R-gene labeling practices over the next few years in western Canada.
Scientific publications.
Peng G, Fernando WGD, Lange R, Kutcher HR 2014. Blackleg of canola -new management strategies against an old disease in western Canada (Abstr.). Can. J. Plant Pathol 36:289.
Harker KN, O’Donovan JT, Turkington TK, Blackshaw RE, Lupway NZ, Smith EG, Dosdall LM, Hall LM, Kutcher HR, Peng G. 2015. Canola rotation frequency impacts canola yield and associated pest species. Can J. Plant Sci. 95: 9-20.
K. N. Harker1, J. T. O’Donovan1, T. K. Turkington1, R. E. Blackshaw2, N. Z. Lupwayi2, E. G. Smith2, L. M. Dosdall3, L. M. Hall3, H. R. Kutcher4, Willenborg CJ, Peng G, Irvine RB, Mohr R. 2015. Canola cultivar mixtures and rotations do not mitigate the negative impacts of continuous canola. Can J Plant Sci. 95: 1085-1099.
Zhang XH, Peng G, Kutcher HR, Balesdent MH, Delourme R, Fernando WGD. 2016. Breakdown of Rlm3 resistance in the Brassica napus - Leptosphaeria maculans pathosystem in western Canada. Eur J Plant Pathol. 145:659–674.
Liban SH, Cross DJ, Fernando WGD, Kutcher HR, Peng G. 2016. Race structure and frequency of avirulence genes in the western Canadian Leptosphaeria maculans pathogen population. Plant Pathol 65:1161–1169