Biology Reference
In-Depth Information
with historical phenotype information, can result
in the discovery of haplotypes associated with
traits. These genotyping profiles can also lead
to the discovery of duplicates and help uncover
hidden, accumulated 'off-types' in breeding pro-
grams (Lucas et al. 2013). In work conducted by
the Tropical Legumes Project,
incidence and severity vary with the amount and
distribution of rainfall (Williams 1975). Sep-
toria leaf spot (
Septoria vignae
P. Henn and
S. vignicola
Vasat Rao, S. Kozopolzanii Niko-
lajeva), scab (
Sphaceloma
sp.), brown blotch
(
Colletotrichum capsici
(Syd.) Butler and Bisby
and
C. truncatum
(Schwein.) Andrus & W. D.
Moore), and cercospora leaf-spot are important
in the Guinea savanna, while cercospora leaf-
spot, bacterial blight, and ashy-stem blight (or
charcoal rot) caused by
Macrophomina phase-
olina
(Tassi.) Goid. are important in the Sudan
savanna, and ashy stem blight in the Sahel. Scab,
cercospora leaf spot, powdery mildew (
Erysiphe
polygoni
DC.), Fusarium wilt, and ashy stem
blight are significant diseases in Brazil (Lin and
Rios 1985). To date, molecular markers have
been developed only for a few of these diseases,
and this section will focus on those with near-
term prospects for MAS.
Bacterial blight is a major disease of cow-
pea affecting most production areas. Severe out-
breaks occur regularly and can completely dev-
astate crops when environmental conditions are
conducive to disease development. Earlier inher-
itance studies suggested resistance was simply
inherited (Patel 1985). More recently molecu-
lar markers have become available that could
be valuable for MAB. Agbicodo and colleagues
(2010) identified three SNP-based QTLs,
CoBB-
1
,
CoBB-2,
and
CoBB-3,
on linkage groups LG3,
LG5, and LG9, respectively (Lucas et al. 2011),
which co-segregated with resistance to two Nige-
rian strains of bacterial blight,
Xav18
and
Xav19
.
CoBB-1
,
CoBB-2,
and
CoBB-3
explained 22.1,
17.4, and 10% of the genetic variation in the RIL
discovery population. SNP markers linked to the
QTL include 1_0853 on LG3, 1_0037 on LG5,
and 1_1202 on LG9. Validation of these markers
in an actual breeding program and across multi-
ple populations remains to be done.
In semi-arid agro-ecologies, the soil-borne
fungus
Macrophomina phaseolina
(Tassi.) Goid.
causes ashy-stem blight of cowpea (known in
soybean as charcoal rot). This ubiquitous dis-
ease is exacerbated under drought conditions.
600 cultivars,
breeding lines, and accessions were genotyped
with the 1,536 Illumina GoldenGate Assay. This
SNP fingerprinting information was combined
with map coordinates from the consensus map
described earlier, providing a rich resource for
breeding and haplotype analysis, as discussed in
the following sections.
∼
BioticStresses and Genetic
Marker Resources
Until recently, finding trait-linked molecular
markers for cowpea breeding was slow and labo-
rious. Now, with the availability of the high-
throughput SNP genotyping platform and the
consensus genetic map, coupled with exten-
sive phenotyping of RIL populations and pan-
els of germplasm within the last five years,
SNP markers or QTL have been identified
for a range of biotic stress-resistance traits
and examples are presented in Table 10.2.
Among these are trait determinants for resis-
tance to foliar thrips (
Thrips tabaci
) (Lucas et al.
2012), flower thrips (
Megalurothrips sjostedti
)
Trybom (M. Lucas et al., unpublished data),
ashy stem blight (
Macrophomina phaseolina
)
(Muchero et al. 2011), bacterial blight (
Xan-
thomonas axonopodis
pv.
vignicola
) (Agbicodo
et al. 2010), root-knot nematodes (
Meloidogyne
spp.) (Roberts et al. unpublished data), and to
Fusarium wilt (
Fusarium oxysporium
Schlech-
tend.:Fr. f. sp.
tracheiphilum
(E. F. Smith) W.
C. Snyder & Hansen) (Pottorff et al. 2012a;
2012b). Additional information on these biotic
stresses and their associated SNP markers is
presented
in
Table
10.2
and
the
following
sections.
Several bacterial and fungal diseases can sig-
nificantly affect cowpea production, and their
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