Viral diseases are a major limiting factor to cowpea production in many countries of Africa. In
Uganda, studies indicated that the cowpea aphid-borne mosaic virus (CABMV) is common and a
potential threat to cowpea production in the region. There have been no efforts to develop
cowpea cultivars with resistance to CABMV in Uganda. This work focused on the development of
cultivars resistant to CABMV. Production of cowpea in Uganda is constrained by several factors,
including a lack of awareness of diseases among the majority of farmers. A participatory rural
appraisal (PRA) was conducted to elicit farmers’ indigenous knowledge of cowpea production and
also to gain insight into their understanding of viral diseases affecting cowpea in Uganda. PRA
tools such as group discussions, transect walks, problem listing and ranking were used to
gather information. Insect pests, diseases, low yielding cultivars and the high cost of
pesticides were perceived to be the most important production constraints. Farmers were not
aware of the problem of virus diseases, but provided descriptive names of symptoms. Only three
cowpea cultivars (Ebelat, Ecirikukwai and Blackcowpea) were produced in the area. Seed size and
colour were seen as important traits in new varieties. Information about the occurrence,
distribution and identity of cowpea viruses is limited in Uganda. The objective of this study
was to identify the important cowpea virus diseases occurring naturally in the major cowpea
growing regions of Uganda. Surveys were conducted to determine the incidence and severity of
virus symptoms in four districts (Soroti, Kumi, Pallisa and Tororo) in 2004 and 2005. The
incidence ranged from 40.5 to 94.4% and severity ranged from 15.0 to 30.6% (for Kumi and
Pallisa districts, respectively) during the 2004 surveys. In 2005, the incidence ranged from
55.9 to 85.4% and severity ranged from 4.7 to 14.5% (for Tororo and Soroti districts,
respectively). The CABMV, cowpea mild mottle virus (CPMMV), cowpea severe mosaic virus (CPSMV)
and cucumber mosaic virus (CMV) were serologically detected by double antibody sandwich enzyme-
linked immunosorbent assay (DAS-ELISA). Fifty four improved cowpea genotypes were screened for
resistance to CABMV during the first season of 2004 at Serere Agricultural and Animal
Production Research Institute in Uganda. Further screening was conducted in the second season
of 2004 using 27 genotypes. The genotypes were planted in single rows between the rows of the
susceptible cultivar, Ebelat. This was to provide high pressure of aphid vector (Aphis
craccivora Koch) and CABMV inoculum. In addition, the test genotypes were artificially
inoculated with a CABMV extract on fully expanded primary leaves of fourteen day-old seedlings.
The CABMV incidence and severity was assessed. Disease severity was assessed on a 0-60% visual
estimation scale where 0 = with no symptoms and 60 = with severe symptoms. Serological analysis
was conducted using DAS-ELISA. Five genotypes showed good levels of resistance to CABMV, namely
MU-93, IT82D-889, IT82D-516-2, IT85F-2841 and SECOW-2W. These resistant lines were crossed with
three susceptible local landraces, namely Ebelat, Ecirikukwai and Blackcowpea in a North
Carolina II mating design. The F1, F2 and BC1F1 populations and the parents were evaluated in
the field to assess the response to CABMV and to study the inheritance of resistance to CABMV.
The general combining ability (GCA) and specific combining ability (SCA) effects were
significant, indicating that both additive and non-additive genetic factors are important in
determining the control of CABMV in cowpea. The proportions (%) of the sum of squares for
crosses attributable to GCA and SCA for CABMV severity were 51.4% for GCA due to females, 8.4%
for GCA due to males and 40.2% for the SCA. The narrow-sense heritability estimates, obtained
by regressing F1 on mid-parents was 0.87 and 0.84, F2 on F1 progenies 0.49 and 0.48, and F2
progenies on mid-parents 0.63 and 0.79, for AUDPC and final disease severity, respectively.
Single gene conditioned resistance in seven populations, but resistance was quantitatively
inherited and involved many genes in eight populations. Observation of transgressive
segregation and moderate to high heritability suggests a quantitative mode of gene action and
the importance of additive effects. The predominance of GCA variance, high heritability
estimates and observation of transgressive segregation suggested that resistance could be
improved by selection. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2007.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ukzn/oai:http://researchspace.ukzn.ac.za:10413/71 |
| Date | January 2007 |
| Creators | Orawu, Martin. |
| Contributors | Melis, Rob W. |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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