Biology Reference
In-Depth Information
Substantial quantities of cowpea also are pro-
duced in similar agro-ecologies in South Amer-
ica, with Brazil being the second-largest produc-
ing country in the world after Nigeria. Although
of less importance today than in the recent
past, cowpea is also grown in the southern and
western parts of North America and is a 'tra-
ditional' crop of several indigenous tribes of
lower-elevation central Mexico. A form of cow-
pea (subspecies
sesquipedialis
) known as 'long
bean' or 'asparagus bean' is cultivated as a veg-
etable crop throughout East Asia for its long
fleshy green pods that can be one meter in length.
Long bean is fully fertile with other cultivated
and some wild subspecies of cowpea and com-
parisons of genetic maps show essential iden-
tity (Xu et al. 2010). Cowpea was documented
from before Roman times in Europe and the
presence of significant phenotypic and genotypic
diversity exists among landrace germplasm from
Italy (Tosti and Negri 2002) and other southern
European countries, suggesting a long history of
use in this region. As a general rule, cowpeas
are grown in hotter low-elevation equatorial and
subtropical areas, often being replaced by com-
mon bean (
Phaseolus vulgaris L.
) at altitudes
above 1,300 m, although cowpeas are grown
at altitudes up to 1,500 m in Kenya. Commer-
cial production of the crop extends as far north
as 40
◦
latitude in California, and experimen-
tal plantings of early maturing breeding lines
have been successful as far north as Minnesota
(45
◦
N latitude) in the United States (Davis
et al. 1986).
While cowpea is both responsive to favorable
growing conditions and tolerant to drought, high
temperatures, and poor soils (Hall 2004; Fery
1990), biotic stresses from pests and diseases
inflict heavy losses and are key factors under-
lying why on-farm cowpea yields of traditional
varieties in Africa are 3- to 5-fold lower than
potential yields (Ehlers and Hall 1997).
Development of cowpea cultivars that resist
or tolerate biotic stresses would result in dra-
matic yield improvements. Breeding resistant
cultivars is a particularly desirable strategy for
this crop because it is grown mostly by resource-
poor farmers, many of whom are women who
lack access to capital for application equipment,
pesticides, and protective wear, as well as to
expertise in the efficacious and safe use of these
products.
Breeding for resistance to biotic stresses has
been undertaken by the International Institute
for Tropical Agriculture (IITA) and by African
National Research Systems (NARS), in some
cases supported by the USAID Collaborative
Research Support Program (CRSP) and other
donors. These breeding programs have achieved
a number of successes in the last 20 years,
having bred cultivars resistant to some of the
key pests such as cowpea aphid, cowpea wee-
vil, the parasitic weed
Striga gesneroides,
bac-
terial blight, root-knot nematodes, and cowpea
aphid-borne mosaic virus (CABMV) (Hall et al.
1997; Hall et al. 2003; Timko et al. 2007) and
thereby contributing to raised on-farm yields.
These breeding efforts employed wholly con-
ventional breeding approaches and typically took
more than ten years from concept and crossing to
release.
Similar to breeding programs in other crops,
the need to employ sequential and repeated phe-
notypic evaluations and performance trials is
the most resource-intensive and time-consuming
aspect of the process. In many cases, these eval-
uations require complex, specialized conditions,
techniques and expertise to assess phenotypes
for selection. These evaluation resources are dif-
ficult to assemble and costly to operate, and in
many cases it is not possible to conduct a com-
plete breeding program consisting of teams of
scientists of all necessary disciplines because of
budget and manpower limitations for 'orphan'
crops (Delmer 2005) such as cowpea. Selec-
tion based on molecular markers linked to key
biotic resistance traits or quantitative trait loci
(QTL) can be used to reduce the phenotyp-
ing burden typically required through conven-
tional breeding efforts and thereby help speed the
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