Bohran.3.3.3.Short Report

Defining Populations Of The L. maculans Pathogen In Test
Sites Used For Canola Blackleg Resistance Trials
Hossein Borhan, Agriculture and Agri-Food Canada, Saskatoon
Project Code: 3.3.3.
Final Report: April 2013
Blackleg caused by Leptosphaeria maculans (LM) is a major disease of canola
(Brassica napus) in Canada and world-wide. A solid understanding of the pathogenic
variability of L. maculans populations is vital to maintaining effective resistance in
future canola varieties. Researchers at Agriculture and Agri-Food Canada (AAFC) in
Saskatoon led a three-year study to produce a set of single blackleg-resistance gene B.
napus lines in a common susceptible background for the accurate pathotyping of L.
maculans isolates. From the study seven B. napus resistant (BC5S3) lines were
produced, and the final lines are expected to be 99.8% homozygous. This will prove to
be extremely valuable to the blackleg research community both in Canada and
worldwide. As well, results from the 2013 WCC Trials indicate that LepR1 and LepR2,
which were identified and distributed to breeders by AAFC Saskatoon, appear to offer
the most consistent level of protection to blackleg disease across Canadian locations.
Blackleg caused by Leptosphaeria maculans (LM) is a major disease of canola (Brassica
napus) in Canada and world-wide. A solid understanding of the pathogenic variability of
L. maculans populations is vital to maintaining effective resistance in future canola
varieties. Brassica lines that are currently used for pathotyping L. maculans races are
genetically poorly defined. These lines have mixed genetic backgrounds and often carry
more than one known R gene.
Researchers at Agriculture and Agri-Food Canada (AAFC) in Saskatoon led a three-year
study to produce a set of single blackleg-resistance gene B. napus lines in a common
susceptible background for the accurate pathotyping of L. maculans isolates as well as the
in-field monitoring of L. maculans populations in Western Canada. Seven B. napus
resistant donor lines containing the blackleg R-genes (Rlm1, Rlm2, Rlm3, Rlm4, LepR1,
LepR2 and LepR3) were crossed to the susceptible B. napus background (Topas
DH16516). This was followed for five generations before self-pollinating for three
generations (producing single resistant plant BC5S3 lines).
Four lines (Topas-Rlm1, Topas-Rlm2, Topas-LepR1 and Topas-LepR3) were increased in
tented field plots over the 2012 growing season. For the remaining three lines (TopasRlm3, Topas-Rlm4 and Topas-LepR2) off-season seed increases were completed in Chile.
The phenotypic responses of the seven lines are being rated and compared to canola
variety lines commonly used as control material to assess the value of the lines for
laboratory pathology assays. (See Figure 1).
Figure 1. Differential resistance response of Topas Introgression Lines allows for the accurate
determination of the avirulence gene profiles of L. maculans isolates.
From the study seven B. napus resistant (BC5S3) lines were produced: Topas-Rlm1,
Topas-Rlm2, Topas-Rlm3, Topas-Rlm4,Topas-LepR1, Topas-LepR2 & Topas-LepR3.
All seven lines, including susceptible parent line Topas DH16516 were included in all
eight WCC Blackleg Co-op Trials for 2013.
Each generation was screened for resistant progeny, both phenotypically using defined L.
maculans isolates and genotypically using linked molecular markers, before individual
plants were selected for use in producing the next generation. The results from the WCC
Trials indicate that LepR1 and LepR2, which were identified and distributed to breeders
by AAFC Saskatoon, appear to offer the most consistent level of protection to blackleg
disease across Canadian locations.
The final BC5S3 lines are expected to be 99.8% homozygous and contain less than 2% of
the donor parents’ alleles, effectively eliminating background variation between the lines
and the susceptible Topas control. Researchers are completing the final pathology testing
of each line; comparisons to other resistant cultivars commonly used as R-gene control
lines using defined L. maculans isolates, and the whole-genome characterisation of the
isolines using SNP marker arrays to define the introgressions.
The individual R-genes have been isolated in a common background, thereby eliminating
interference from other R-genes that may be present in other material. As well, some of
the genes produce a stronger, less variable phenotypic response to compatible L.
maculans isolates than either the donor parent line or other comparable control material.
This greatly reduces ambiguity in assessing phenotypic responses and will prove to be
extremely valuable to the blackleg research community both in Canada and worldwide. It
will also provide valuable insight into the effects of different genomic backgrounds on
the expression of R-gene phenotypes. A solid understanding of the pathogenic variability
of L. maculans populations is vital to maintaining effective resistance in future canola
varieties.