Biology Reference
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leads to the challenges of collecting adequate
and consistent phenotypes with the same genetic
stocks. As described previously, most QTLs were
detected only once even if multiple inoculation
methods, evaluations, replicates, years, and loca-
tions were used. Moreover, most QTLs explained
relatively small portions of phenotypic varia-
tion, frequently less than 20%. Therefore, it is
very difficult to use the mapping information
to conduct marker-assisted selection in canola
breeding. In the future, it will be necessary to
optimize inoculation methods and screening con-
ditions to identify major QTLs consistently with
high proportions of explained phenotypic varia-
tion, which would lead to efficient and effective
marker-assisted selection to improve sclerotinia
stem rot resistance.
analyze clubroot resistance in a cross of broccoli
and cauliflower.
Availability of Clubroot Resistant
Sources
Clubroot is a devastating disease in Brassica
crop production when the cultivars in production
are susceptible. This disease causes heavy losses
in canola production in European countries and
Chinese cabbage production in East Asia. This
disease was discovered a few years ago in a sin-
gle canola field in Alberta, Canada; however, it
spread quickly from the original infection site
to surrounding areas and has now spread to
Saskatchewan, the neighboring province. Cur-
rently, most Canadian canola cultivars do not
contain clubroot-resistance genes and are there-
fore highly susceptible to this disease. Conse-
quently, clubroot disease poses a major threat to
canola production in the Canadian prairies.
Clubroot resistant cultivars are considered
the most feasible solution for controlling this
disease effectively. To develop clubroot resis-
tant Brassica crops, knowledge of available
genetic sources of resistance is critical. Knowl-
edge of the inheritance of the clubroot-resistance
genes is also essential. The range of available
clubroot-resistance genes and their inheritance
will influence what strategies are used to com-
bine different clubroot-resistance genes. There
is genetic complexity in both disease resistance
in the host and virulence in the pathogen (Some
et al. 1996), suggesting that the development of
durable clubroot-resistant Brassica crops will be
challenging.
Clubroot resistance in
B. oleracea
,
B. rapa
,
B. napus
and other Brassica species has been
extensively screened and tested. For example,
Tjallingii (1965) tested several Brassica species,
radish, and
Sinapis alba
in 11 locations where
infected Brassica crops occurred. In this inten-
sive testing, most turnip and radish accessions
were resistant, while cauliflower, cabbage,
B.
napus
accessions, and
S. alba
accessions were
susceptible in most cases. In another report,
B.
Clubroot
In
B. napus
, most canola and rapeseed acces-
sions are highly susceptible to
P. brassicae
while
in one subspecies - Swedes, or rutabaga -
most accessions are resistant to clubroot dis-
ease. Recently some canola varieties such as
'Mendel' and 'Tosca' were bred.
P. brassi-
cae
displayed extremely diverse pathogenicity
that was reported in many reports (Williams
1966; Voorrips and Visser 1993; Voorrips et al.
1997). To establish a standard classification of
pathogen isolates, the European Clubroot Differ-
ential (ECD) series, including a total of fifteen
accessions, five each of
B. rapa
,
B. napus,
and
B.
oleracea
, respectively, were tested in 299 inde-
pendent experiments, with data collected from
236 tests used to set up 894 ECD triplet codes.
The triplet codes were also used to distinguish
pathogen isolates in several other publications
(Laurens and Thomas 1993; Voorrips and Visser
1993; Voorrips et al. 1997). In contrast, in other
research, pathogen isolates were classified into
race 1 to 9 according to the interaction between
host and pathogen (Williams 1966). Ayers and
Lelacheur (1972) used race 2 and 3 to study the
genetic basis of clubroot resistance in rutabaga
and Figdore and colleagues (1993) used race 7 to
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