ABSTRACT
The parasitic weed, Striga gesnerioides (Willd)
Vatke is one of the most important constraints to cowpea production in the dry
savanna (Derived Savanna, Southern Guinea Savanna and Northern Guinea Savanna)
of Northern Ghana. Yield losses due to S. gesnerioides range from 83 to
100%. No single method however, seems to be fully adequate in the
control of this parasite. Host plant resistance, appears to have merit in
effectively and economically controlling the parasite in that it is affordable
to farmers. The objective of this study was to introgress Striga
resistance into existing farmer-preferred cowpea varieties. Two resistant
genotypes IT99K-573-1-1 and GH3684 were crossed to two susceptible varieties
“Hewale” and “Asomdwee” respectively. The chi-square test was used to test the
goodness-of-fit of the observed ratios to the expected genetic ratio in F2
segregating populations. The results of the cross of genetic of inheritance
demonstrated 3R:1S ratio indicating single dominant gene action (monogenic
inheritance). The result of the inheritance study indicated that the
environment had great influence on a number of agronomic traits. The broad
sense heritability for susceptible and resistant were high (63% and 78%
respectively). Narrow sense heritability were low for some of the traits which
is an indication that environmental factors (Striga) influenced cowpea
production in this study. Three simple sequence repeat (SSR) markers SSR-1,
C42-2B and 61RM2 associated with Striga resistance were used to screen
93 F2 progenies. The study showed that the three markers had
discriminating power to distinguish between the resistant and susceptible
genotypes and with presence of bands in resistant genotypes. The allele
frequency for marker SSR-1 was 65% and 61RM2 was 73%, suggesting that these
markers are highly repeatable within the population. Yield loss due to Striga
infestation was estimated to be (78.22 to 87.17%). Other yield component
including pods per plant, 100 seed weight, fodder yield, pod length as well as
the number of seeds per pod of the susceptible genotypes were affected. There
was significant correlation between percentage yield reduction and percentage
reduction in various yield components indicating that Striga infestation
was responsible for the overall yield reduction. At present very limited
sources of Striga resistant varieties are available, therefore there is
the need to develop new Striga resistant cowpea varieties that meet
end-user preference. Promising lines will be screened with more Striga
resistant markers to determine their level of genetic status.
CHAPTER ONE
1.0 INTRODUCTION
Cowpea (Vigna unguiculata (L.)
Walp) is considered the most essential leguminous grain in the dry Savannas of
tropical Africa. It is also known as the black- eyed pea or southern pea and is
cultivated in a range of ecologies and cropping systems in the tropics. It
originated from the semi-arid areas of West Africa and has been cultivated for
human consumption for more than 4,000 years. (Tweneboah, 2000). The name cowpea
probably originated from the fact that the plant was an important source of hay
for cattle in the southern United States of America and in other parts of the
world (Timko et al. 2007). Some important local names for cowpea include
“Beng”in Dagari, “Ayi” in Ewe, and “caupi” in Brazil.
Cowpea is a member of the
Phaseoleae tribe of the Leguminosae family (Timko et al 2007). It plays a
critical role in the lives of millions of people in Africa and other parts of
the developing world, where it is a major source of dietary protein that
nutritionally complements staple low protein cereals and tuber crops. The high
protein content present a major advantage in the use of cowpea as nutritional
products, for infants and children and could compensate for the large
proportion of carbohydrate often ingested in African diets (Lambot, 2002).
In Ghana, cowpea is an important
source of vegetable protein and minerals for over 70% of the population and it
is the second most important grain legume after groundnut in terms of
production and utilisation (SRID-MOFA 2008). Notwithstanding its significance
as human food, cowpea fodder is an imperative source of animal feed (Tarawali et
al., 2002). Legume haulm provides an especially basic function in nourishing
livestock during the harmmattan season in various West African countries
(Tarawali et al., 1997, 2002, Tarawali 1997). Cowpea is a valuable and
dependable commodity
that generates income for farmers and helps to restore soil fertility for
succeeding cereal crops growing in rotation with it.
Cowpea is an important crop in
Ghana due to its contribution to national GDP, farmers incomes, food and
nutrition security and sustainable agriculture (CORAF/WECARD Cowpea Report,
2011). The per capita consumption of cowpea in Ghana is about 9kg each year
(Coulibaley et al., 2010). Ghana still import 3.380 metric tonnes of cowpea
grains which augment the country production of 219,300 metric tonnes in 2010
(Egbadzor et al., 2013).
Regardless of the significance of
black-eyed pea in West Africa, its production is still impeded by a myriad of
abiotic factors. Biotic components, for example, pests and diseases, and
parasitic weeds cause serious threat to cowpea production. The parasitic
angiosperm Striga gesnerioides (Willd) is one of the significant limitations to
cowpea cultivation particularly, in the Guinea Savanna agro-ecology. The
parasitic weed S. gesnerioides is an obligate root-parasitic blossoming plant
of the Scrophulariaceae family. Complete crop loss has been reported in
susceptible cowpea genotypes following severe S. gesnerioides infestation
(Muranaka et al., 2011). It is believed that the fast spread of this parasitic
weed and huge yield decrease would constitute an extreme danger to cowpea
production. For the resourced-poor farmers, developing S. gesnerioides-resistant
cowpea genotypes in blend with fitting management practices are most
conservative and effective choices to forestall yield loss brought on by this
parasite which seeds are found in plenitude in plagued fields. The utilization
of Striga - resistant cultivars lessens the parasite's seed multiplication and
this exhausts the Striga seed bank (Badu-Apraku and Lum, 2007; Haussmann et al.,
2004).
Over the years, the CSIR-Crops
Research Institute has released cowpea varieties which are being grown all over
the country. Examples of such varieties “Asomdwee” and “Hewale”.
These varieties are known to be early maturing, high yielding and farmer
preferred. They are also known to be adapted to Forest transition, Coastal and
Savanna agro-ecologies but are susceptible to S. gesnerioides. These varieties
are tolerant to other biotic and abiotic stresses and have consumer
acceptability. However, the cultivation of these two varieties is a problem in
the Savanna areas where S. gesnerioides is prevalent. The Savanna zones
including Derived Savanna, Southern Guinea Savanna and Northern Guinea Savanna
of Northern Ghana, constitute about 41% of Ghana’s landmass and major cowpea
growing areas. Therefore, there is the need to address this Striga problem by
developing resistant or tolerant varieties. This study sought to transfer Striga
resistance into the background of two existing farmer-preferred cowpea
varieties (“Asomdwe” and “Hewale”) using conventional and molecular breeding
tools.
The main objective of the study was
to introgress Striga resistance into two improved cowpea varieties using
molecular breeding tools.
Specific objectives were to:
determine gene action controlling Striga
gesnerioides resistance,
identify F2 progenies that may be
resistant to S. gesnerioides using SSR and SCAR markers associated with S.
gesnerioides resistance,
confirm Striga resistance in
selected lines through inoculation in pot experiments, and
determine the yield loss due to S.
geesnerioides in cowpea.
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