Biology Reference
In-Depth Information
Muchero and colleagues (2011) identified nine
quantitative trait loci (QTLs) that accounted for
between 6.1 and 40.0% of the phenotypic vari-
ance (R
2
). Maturity effects on the expression of
resistance were indicated by the co-location of
Mac-6
and
Mac-7
QTLs with maturity-related
senescence QTLs
Mat-2
and
Mat-1
, respectively.
The multigenic nature of resistance and inter-
action with crop maturity will complicate the
use of these markers in MAB, but may lead to
greater precision in identifying resistant individ-
uals in segregating populations. As breeding tar-
gets, combinations of pairs of QTLs, such as
Mac-2
and
Mac-5
, can provide significant pro-
tection from this disease (Muchero et al. 2011).
Efforts are under way to combine QTL for
Macrophomina resistance and drought tolerance
for cowpea protection in rain-fed, drought-prone
production areas (Muchero et al. 2011).
Cowpea rust caused by
Uromyces vignae
is a
severe problem in humid ecologies across Asia
and Africa that is best managed by the use
of resistant varieties (Emechebe and Shoyinka
1985). Li and colleagues (2007) reported the
identification of an amplified fragment length
polymorphism (AFLP) marker converted to a
sequence characterized amplified region (SCAR)
marker for resistance to
Uromyces vignae
in
China.
Fusarium oxysporum
f.sp.
tracheiphilum
(Fot) is a soil-borne fungal pathogen that causes
vascular wilt disease in cowpea. Fot race 3 is one
of the major pathogens affecting cowpea produc-
tion in the subtropical regions where the crop is
grown. Resistance to Fot race 3 was studied in the
RIL population 'California Blackeye 27' (resis-
tant) x 24-125B-1 (susceptible). Biparental map-
ping identified a Fot race 3 resistance locus, Fot3-
1, which spanned 3.56 cM on linkage group 1 of
the CB27
tified using other cowpea mapping populations
developed from crosses between resistant and
susceptible parents (Pottorff et al. 2012b).
Cowpea is attacked by a wide spectrum of
other fungal diseases. Those of major impor-
tance include anthracnose (
Colletotrichum lin-
demuthianum
(Sacc. & Magnus) Lams.-Scrib.),
cercospora leaf spot (
Cercospora canescens
Ellis & G. Martin), web blight (
Rhizoctonia
solani
Kuhn), ascochyta (
Ascochyta phaseolo-
rum
Sacc.), brown blotch (
Colletotrichum cap-
sici
(Syd.) Butler and Bisby and
C. truncatum
(Schwein.) Andrus & W. D. Moore), and pow-
dery mildew (Emechebe and Shoyinka 1985).
Provision of molecular markers for resistance to
these diseases should be an important goal for
cowpea breeding programs in areas where these
organisms are problematic.
Markers for Resistance to Viruses
Development of virus-resistant cultivars is an
important objective of several cowpea breed-
ing programs in Africa and in Brazil. The
main seed-borne viruses affecting cowpea pro-
duction include cowpea aphid-borne mosaic
virus (CABMV), blackeye cowpea mosaic virus
(BlCMV), cucumber mosaic virus (CMV), cow-
pea mosaic virus (CPMV), cowpea severe
mosaic virus (CPSMV), southern bean mosaic
virus (SBMV), and cowpea mottle virus (CMV).
Cowpea golden mosaic virus (CGMV) and cow-
pea chlorotic mottle virus (CCMV) are important
non-seed-borne viruses. The seed-borne viruses
are particularly damaging because they spread
via infected seed. Seed-borne infections can
result in early season spread of the virus to neigh-
boring plants. Sources of resistance to all of these
viruses have been identified, and cultivars with
resistance to several viruses have been developed
at IITA (Singh 1993). The presence in Senegal
and Nigeria of different strains of CABMV that
have overcome resistance complicates breed-
ing efforts, but breeding lines have been devel-
oped that are resistant to multiple strains of this
virus as well as to other viruses. IT86D-880,
24-125B-1 genetic map. A marker-
trait association narrowed the resistance locus
to a 1.2 cM region and identified SNP marker
1_1107 as co-segregating with Fot3-1 resistance
(Pottorff et al. 2012a). Two QTLs governing
resistance to Fot race 4, which attacks cowpeas
with Fot race 3 resistance, have also been iden-
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