69 / 84
67
June 2018
These lines/cultivars have mostly been imported from the United
States Department of Agriculture as well as CIMMYT, Mexico, as
they are the main sources of resistant germplasm collections in the
world. These acquired lines are multiplied, since limited quantities
of seeds may be imported. Then the lines are tested under artifi
cially inoculated field, laboratory and glasshouse conditions to con
firm the resistance, since our local
Fusarium
isolates may vary from
those isolates abroad.
For field-testing, a cocktail (five to seven isolates) of different
Fusarium graminearum
isolates is used during the extensive and
labour-intensive field screening, since
F. graminearum
is the most
isolated from infected samples. In fact, 85% of isolates isolated
from local infected samples, are those of
Fusarium graminearum
.
Fusarium graminearum
isolates from different irrigation areas are
used for artificial inoculations during flowering when the wheat plant
is most vulnerable.
Every year, more than 3 000 lines are screened to determine
Fusarium head blight resistance levels and those lines that have
showed reliable resistance over a two-year period over different
localities, are then considered worthy of inclusion in the breeding
programme. It is important to note, that more than 90% of all im
ported lines are not adapted/suited to South African conditions.
Therefore, they need to be planted and then secondarily selected,
based on agronomy including plant height, straw strength, general
appearance, tiller number, spike length and spikelet number before
they can be included into a pre-breeding programme (
Photo 1
).
There are currently nine phenotypically validated and molecularly
well-characterised resistant/moderate resistant entries, that have
been incorporated into the marker-assisted backcross (MAB) pro
gramme at ARC-Small Grain (
Table 2
). There are currently about
70 additional validated Fusarium head blight resistant sources,
which have not been characterised genetically.
Different crossing combinations with specific pedigrees and target
genotypes are screened and selected for marker-assisted selection
(MAS) during the seedling stage. DNA is extracted from each single
plant to conduct MAS of targeted Fusarium head blight resistance
genes/QTL, with an average of four to six linked markers used per
crossing combination.
Three to five susceptible high-yielding local cultivars are currently
used as recurrent parents for a total of 25 to 35 different pre-breed
ing combinations. The targeted markers are used to select for the
presence of a targeted Fusarium head blight resistance gene/QTL
per cross combination (
Photo 2
).
In the glasshouse, only the MAS selected lines are backcrossed
and/or topcrossed further, after being tested (
Photo 3
on page 69).
This increases the potential number and frequency of the resist
ance genes/QTL present/retained in each backcross line, as well as
maintains the good quality and yield characteristics that come with
the original recurrent parent.
The more targeted, accurate and stricter selection, possible with
the MAS on many different combinations, makes the develop
ment of a single durable resistant line much more efficient. This
marker-assisted programme screens on average 500 plants/lines to
1 000 plants/lines, annually.
On average, 5% to 15% of these lines are selected and retained for
the development of the next generation.
TYPE
DESCRIPTION
1
Resistance to initial infection
2
Resistance to the spread of the fungus within the plant
3
Resistance to kernel infection. The rates of seed infection can differ at a given level of resistance as measured by disease
severity
4
Tolerance to Fusarium head blight where tolerant wheat maintains yield despite of the presence of the disease
5
Resistance against toxin accumulation
Table 1: Types of resistance to scab.
CULTIVAR/LINE
REACTION
DESCRIPTION
Baisanyuehuang
Resistant
Resistant source
Catbird
Moderately resistant
Resistant source
Frontana
Moderately resistant
Moderate resistant source
Haiyanzhong
Resistant
Resistant source
Heyne
Resistant
Resistant source
Huangfangzhu
Resistant
Resistant source
Ning 7840
Resistant
Resistant source
Sumai 3
Moderately resistant
Resistant source
Wangshuibai
Moderately resistant
Resistant source
Table 2: The nine validated Fusarium head blight resistance sources used extensively in backcrosses.
2: An example of a gel containing developed BC
2
F
1
lines of a particular
crossing combination screened with Fusarium head blight marker A.
From the left, Lanes 1 and 25 DNA ladders, Lane 2 (recurrent parent
SA cultivar), Lane 3 (Fusarium head blight resistant source), Lane 4 to
24 developed material. Plants containing both parental fragments are
selected for the next generation.
2