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not consistent from year to year or from loca-
tion to location. Li and colleagues (2008) used
a stem inoculation method under field condi-
tions to screen 93 genotypes of
B. napus
and
B. juncea
from China and Australia. They found
that most resistant accessions were from
B. napus
while most
B. juncea
accessions showed rela-
tively low levels of resistance to sclerotinia stem
rot. Detached leaf and stem inoculations were
used to test 68 accessions from six other Brassica
species, of which 47 belonged to
B. oleracea
and
its wild types such as
B. rupestris
,
B. incana
,
B.
insularis
, and
B. villosa
(Mei et al. 2011). These
B. oleracea
wild types showed higher levels of
resistance to sclerotinia stem rot, suggesting that
wild types of
B. oleracea
might be potent scle-
rotinia resistance sources in canola breeding. In
other research, cotyledons were inoculated with
a drop of macerated mycelium and this cotyledon
inoculation was used to test 32
B. napus
geno-
types (Garg et al. 2008). These authors identi-
fied a hypersensitive response and compared the
cotyledon testing data with the previously col-
lected field testing data, from which they deter-
mined that there was a significant correlation
between stem field testing and indoor cotyledon
inoculation.
All sclerotinia stem rot resistance screen-
ing results reported previously are a quantita-
tive assessment to illustrate that the resistance
is genetically controlled and can be adequately
measured. In reality, the resistance is not easily
or consistently identified. Environmental condi-
tions and inoculation methods play a major role
in the detection and quantification of sclerotinia
stem rot resistance. In general, stem inoculation
is commonly accepted as an effective and reli-
able method, which is therefore extensively used
in genomic analyses.
leagues (2003) used the detached-leaf and stem
inoculation methods to collect phenotypic data
from the F3 lines of a segregating population in
B. napus
. Three QTLs for resistance were identi-
fied using the leaf inoculation, and three QTL for
resistance were identified using the stem inoc-
ulation. However, none of the QTLs detected
using the detached-leaf technique overlapped
with the QTLs identified using the stem inoc-
ulation technique. Later, Zhao and colleagues
(2006) performed QTL mapping in two doubled
haploid (DH) line populations of
B. napus
using
two inoculation methods, petiole and stem, for
collecting phenotypic data. They aligned their
genetic maps with the commonly used genetic
map that has the linkage groups N1 to N19 with
their corresponding chromosomes (Parkin et al.
1995). In total, eight genomic regions or QTLs
in one population, and one region or QTL in
another population were identified in four eval-
uations with two inoculation methods. How-
ever, only one QTL on N16 was detected in
three evaluations and most other QTLs were
detected only once. With regard to the test-
ing methods, three QTLs - on N3, N12, and
N16 - were detected in one evaluation. These
QTLs explained relatively small portions (6%
to 22.7%) of phenotypic variation. In another
QTL-mapping report, stem inoculation under
field conditions was applied to phenotype a DH
population of
B. napus
over a period of four
years (Yin et al. 2010). Among all treatments,
17 QTLs were detected. Most of these QTLs
were mapped in only one treatment, while some
QTLs on N3, N10, and N12 were detected in
two to four treatments. The maximum pheno-
typic variation explained by a single QTL was
36%, but this QTL was detected only three times
in two of the four years, and the mapped position
of this QTL varied among different treatments
and years.
QTL mapping of sclerotinia stem rot resis-
tance suggested that the resistance is controlled
at least in part by genetic factors, while envi-
ronmental conditions play an important role in
the development of disease symptoms, which
QTL Mapping of Sclerotinia Stem
Rot Resistance
Since sclerotinia stem rot resistance is quantita-
tive, QTL mapping is commonly used to analyze
the genetic basis of resistance. Zhao and col-
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