Overcoming blackleg disease in canola through establishment of quantitative resistance

Term: 4 years, ending January 2023
Status: Complete
Researcher(s): Hossein Borhan, Isobel Parkin, Kevin Rozwadowski, AAFC
SaskCanola Investment: $396,000
Total Project Cost: $396,000
Funding Partners: N/A

Grower Benefits

• A major quantitative trait locus (QTL) “BL-QTL1” that corresponded to both growth chamber and field resistance to blackleg was identified on the Brassica napus (canola) chromosome A08. This major adult plant resistance (APR) gene should provide durability against the blackleg pathogen Leptosphaeria maculans (Lm), particularly when combined with race specific (qualitative) resistance genes.

• BL-QTL1 restored a partial quantitative resistance and reduced the development of stem lesions when expressed in highly susceptible B. napus cv Westar.

• Kompetitive allele specific PCR (KASP) markers against the BL-QTL1 gene were developed, which could be used in marker assisted selection (MAS) by canola breeders.

Project Summary

Non-race specific resistance against blackleg disease of Brassica napus canola, known as adult plant resistance (APR), is a quantitative trait controlled by multiple genes. The APR trait is highly durable against the blackleg pathogen Leptosphaeria maculans (Lm), although the nature of causative APR genes is not known.

Researchers took advantage of the well-defined population made by crossing the B. napus cv Castle with APR to blackleg and the B. napus susceptible cv Topas to identify the QTL for blackleg resistance (BL-QTL). They developed a growth chamber based APR screening method and tested the progenies of a Castle X Topas population for APR against blackleg. A quantitative disease resistance gene and a major QTL locus candidate, named BL-QTL1 located on chromosome A08, was identified. This gene was the only QTL identified under the growth chamber APR assay and was also the most significant QTL under field condition.

Building on previous APR genotyping efforts, the objectives of this four-year project were to identify and accurately map quantitative trait loci (QTL) controlling APR, clone APR genes, develop and test APR gene specific markers and understand the function and downstream pathways of APR genes. Under growth chamber conditions, accurate mapping of QTL combined with RNA-sequencing of individual progenies during response to L. maculans infection resulted in identification of several QTLs. Combining the mapping and gene expression data helped to identify several candidate genes with a high probability of controlling the APR response.

Researchers also evaluated BL-QTL performance under field conditions, and tested 53 spring type B. napus lines with a potentially functional BL-QTL1 gene in a blackleg field nursery in Alberta, comparing 25 plants for each line in randomized replicated blocks. B. napus Castle and Westar, with and without APR respectively, were used as control. At the end of growing season stem lesions were scored on a 0 to 5 scale.  Eleven lines showed APR to the same level or better than Castle, while in 18 lines the size of stem lesions was equal to or less than 60% of the lesion size recorded for Westar.

As a result of the project, the discovery of quantitative resistance genes against blackleg, including the significant Bl-QTL1 gene, is an important first step in understanding the molecular mechanism of QTL resistance. The KASP marker developed could be used in MAS by canola breeders. Going forward researchers will continue to investigate how to successfully apply the outcome of this research in designing canola cultivars with durable resistance to blackleg.