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Graduation date: 1993

Cowpea (Vigna unguiculata (L.) Walp is an indigenous food legume in Africa, which has great production potential, especially in areas with low agricultural resources. During surveys of cowpea fields in southern Africa, a new foliar disease was recorded. Alternaria cassiae Juriar&Khan was consistently isolated from diseased plant material. Pathogenicity was confirmed using Koch's Postulates. The effect of different culture media, temperature, light and wounding on the growth and sporulation of the fungus was studied. A. cassiae grew well and produced conidia abundantly when maintained on V8-agar at 25°C in a 12h UV-light/12h dark cycle. Sporulation was further enhanced by wounding the cultures. The pre-penetration and infection process of A. cassiae on cowpea leaves was studied by light and scanning electron microscopy. Conidia germinated within 2-3h post inoculation (hpi), forming multiple germ-tubes randomly that grew in any direction across the leaf surface. By 8hpi terminal or intercalary appressoria were formed above epidermal cells or over stomata. Occasionally germ-tubes entered stomata, without the formation of appressoria. Penetration of the plant surface, whether directly through the epidermis or indirectly through stomata was observed 72hpi. Following penetration bulbous primary hyphae were observed within the sub-stomatal cavities, secondary hyphae developed from the primary hyphae and grew within the intercellular spaces penetrating epidermis and mesophyll cells. A. cassiae is a necrotrophic fungus as the infection process is characterised by a destructive necrotrophic phase where plant cells became necrotic even prior to fungal penetration. Conidial morphology, types and development of the fungus were studied in vitro on different culture media and in vivo on cowpea leaves. A. cassiae produced a mixed population of three conidial types. Conidia were formed singly or in chains of 2-4 conidia. Conidia with long, filiform beaks and conidia with shorter beaks, converted into secondary conidiophores were more frequently produced than mature, beakless conidia on all the media, except on potato dextrose agar. Conidial body and beak sizes were variable when measured in culture and on cowpea leaves. Conidia produced in culture were larger, than those produced in vivo. Conidiophores emerged directly through the epidermis or stomata or were formed when hyphae growing on the leaf surface differentiated into conidiophores. Smooth, bud-like conidial initials were produced at the apex of conidiophores. Conidia matured and became elliptical to obvate and densely verrucose. Once a mature conidium had detached, a small pore was visible at the apex of the conidiophore. A. cassiae was shown to be seed-borne in cowpea. Six fungicides i.e. Benomyl, bitertanol, captab, mancozeb, propiconazole and triforine were evaluated for their efficacy in reducing mycelial growth of A. cassiae in vitro. All fungicides except benomyl proved to be effective. Cowpea seeds were artificially inoculated with A. cassiae and treated with all the fungicides except benomyl. Percentage germination and infection was determined in vitro. Percentage emergence, disease incidence, root and shoot lengths and abnormalities were determined in greenhouse trials. Only bitertanol at l.5x the recommended dosage significantly reduced percentage germination. All treatments except triforine l.0x and l.5x significantly decreased the percentage infection of artificially inoculated seeds. None of the treatments except bitertanol l.5x showed a difference in shoot and root length when compared to the control. Captab l.5x the recommended rate proved to be the best treatment over all. / Dissertation (MSc (Botany))--University of Pretoria, 2006. / Plant Science / unrestricted

Anthracnose of cowpea (Vigna unguiculata (L.) Walp) caused by Colletotrichum dematium (Pers. ex Fr) Grove has serious socio-economic implications. Subsistence farmers rely heavily on cowpea for protein and fodder; therefore, C. dematium poses a threat to production of this crop. The purpose of this study was to investigate the interaction between cowpea and C. dematium. Investigations involved characterising C. dematium field isolates using morphological and molecular techniques, infection studies, biochemical and histochemical analysis and determining factors that influence the severity of the fungus on the host. Random amplified microsatellite profiles of C. dematium grouped the isolates into eleven groups linked to morphological characteristics, pathogenicity and geographic origin. Infection studies indicated that C. dematium is a subcuticular intramural coloniser, that switches to destructive necrotrophy. Pulvinate acervuli were produced at 72 hours post inoculation over water-soaked lesions and complete necrosis of the host tissue occurred at 120 hours. The infection process was favoured by prolonged periods of high humidity and high temperatures, especially in cowpea plants between the ages six to nine-weeks-old. Investigations on the location and patterns of polyphenols in the cowpea seed coats indicated that brown coloured cowpea cultivars contained more soluble phenolic compounds than cream coloured cultivars and they were more resistant to C. dematium. / Thesis (PhD (Botany))--University of Pretoria, 2003. / Plant Science / unrestricted

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This item was digitized as part of the Million Books Project led by Carnegie Mellon University and supported by grants from the National Science Foundation (NSF). Cornell University coordinated the participation of land-grant and agricultural libraries in providing historical agricultural information for the digitization project; the University of Arizona Libraries, the College of Agriculture and Life Sciences, and the Office of Arid Lands Studies collaborated in the selection and provision of material for the digitization project.

Phytophthora vignae, causal agent of stem and root rot of cowpea (Vigna unguiculata), was reported for the first time in Sri Lanka. The pathogen was found in cowpea field soils from 3 of 5 geographic regions sampled. Only one site however, had plants exhibiting disease symptoms. Of the eight cowpea varieties grown in Sri Lanka, four were shown to be relatively resistant; all other legumes inoculated were completely resistant. Two morphologic and physiologic races of P. vignae were identified among the 24 isolates recovered, based on differential pathogenicity on cowpea varieties. Bacteria isolated from field soils, and other known bacterial biocontrol agents, inhibited P. vignae in culture, but only three Sri Lankan isolates considerably suppressed the disease in greenhouse tests. Volatile substances produced by most bacteria inhibited mycelial growth and sporangial production by P. vignae. The increased pH of the exposed medium suggested the involvement of ammonia. Volatile inhibitors were produced by these bacteria in soil, but only with added substrate; Strain DF-3101 also reduced oospore germination in soil. Cowpea plants inoculated with the VA mycorrhizal (VAM) fungus Glomus intraradices in P. vignae-infested soil were larger than non-mycorrhizal plants, but only at low levels of the pathogen. VAM colonization was reduced at high levels of the pathogen, and root infection by the pathogen was reduced by VAM. The fungicides metalaxyl, fosetyl-Al, Banrot, and Manzate-200DF reduced in vitro mycelial growth, but at different concentrations. Sporangia formation and germination, and oogonia formation by P. vignae, was reduced significantly by metalaxyl and fosetyl-Al. In greenhouse tests, metalaxyl, even at low concentrations, reduced disease; Fosetyl-Al was effective at high concentrations; Manzate-200DF was effective as a soil drench but not as a foliar spray; Banrot effectively reduced disease at 50 mg a.i./L. Exposure of a bacterial biocontrol agent to these fungicides in vitro did not affect its capacity to subsequently produce volatile inhibitors, but exposure to 10 ug/ml of metalaxyl and 50 ug/ml of Manzate-200DF reduced its capacity to subsequently inhibit mycelial growth of P. vignae. / Graduation date: 1991

In Burkina Faso, the existence of different races of Striga gesnerioides (Willd.) Vatke, with apparent variable aggressiveness on cowpea (Vigna unguiculata (L.) Walp) renders the breeding task very complex. Therefore, a number of studies was carried out from 2006 to 2009 in field, pot and ‘’in-vitro’’ to identify new sources of resistance to three prevailing Striga races, SR 1, SR 5 and a newly occurring Striga race named SR Kp and to understand the genetic pattern of the underlying resistance of cowpea germplasm to Striga races found in Burkina Faso. To achieve these objectives, the following investigations were initiated: (i) a participatory rural appraisal (PRA), a participatory variety selection (PVS) and grain quality survey were implemented to identify cowpea breeding priorities for Burkina Faso Striga hot-spots; (ii) the identification of sources of resistance in Burkina Faso germplasm, using three prevailing Striga races of S. gesnerioides as sources of inoculum; (iii) the identification of the mechanisms of resistance underlying the resistance to Striga in such genotypes; (iv) a study of combining abilities of selected parents through a diallel cross; (v) a study of the segregation patterns in crosses involving resistant and susceptible sources and a study of the allelic relationships between different resistance sources. The participatory studies conducted in 2007 and 2008 over three districts in Striga hotspots; there was no effective control method against Striga at farmers’ level. These investigations highlighted the importance of cowpea across all sites. Rain decline over time, low input use coupled with a poor extension system were the major constraints mentioned by farmers. Differential reactions of genotype KVx61-1 for Striga resistance suggested that different Striga races were prevailing in different areas. Farmers’ preferred traits in cowpea genotypes were oriented towards grain quality such as big sized grain, white seed colour and rough texture of cowpea grain, except in Northern-Burkina Faso, where farmers preferred brown-coloured grain for food. Cowpea was also seen as an income generating crop. An evaluation of 108 genotypes was done in 2007 in the field (rainy season) and in pots (off-season) for Striga resistance assessments. The screening trials enabled the identification of sources of resistance to S. gesnerioides. Genotypes KVx771-10, IT93K- 693-2, KVx775-33-2, Melakh and IT81D-994 are potential sources of resistance to all three Striga races with acceptable yield. Landraces were susceptible and late-maturing whilst most wild species were resistant but with unwanted shattering traits. A combining ability study for Striga resistance parameters conducted in pots and a resistance mechanism study conducted ‘’in-vitro’’ were performed using F1 populations from a 10 x 10 diallel cross. The general combining ability (GCA) effects were significant for the resistance parameters Striga emergence date (DSE), Striga height above soil (SH), cowpea grain weight (CGW), hundred grain weight (HGW) for all Striga races involved and Striga vigour (SVIG) for SR 5 and SR Kp. The pot-screening showed that, regardless of the SR used as inoculum, the additive genes were important in conferring Striga resistance for parameters DSE, SH, CGW and HGW. The selection of parents could therefore result in breeding advance. Complete dominance, partial, over-dominance and non-allelic interactions (epistasis or failure of some assumptions) were present for some parameters. The ‘’in-vitro’’ screening showed that additive genes were important, with high narrow sense heritability values for the resistance mechanisms Striga seed germination frequency (GR) for SR 1 and SR Kp, the frequency of Striga radicle necrosis before the penetration in cowpea rootlet (NBP) for SR 5, the frequency of Striga radicle necrosis after the penetration in cowpea rootlet (NAP) for SR 1 and SR Kp and the susceptibility ‘’in-vitro’’ (SIV) for SR 5 and SR Kp. The selection of parents can be useful in accumulating the genes for Striga resistance mechanisms in progenies. The F2 populations derived from crosses between Striga-resistant x susceptible genotypes were evaluated in Striga infested benches in 2008 and 2009. The segregation patterns suggest that single dominant genes govern Striga resistance. The test for allelism showed that two non-allelic genes were responsible for the resistance to S. gesnerioides in cowpea. A new Striga resistance gene seems to be involved in genotype KVx771-10 resistance to S. gesnerioides, which confers resistance to all studied Striga races. Gene 994-Rsg in genotype IT81D-994 which confers Striga resistance to SR 1 and gene Rsg 3 also conferring Striga resistance to SR 1 segregated differently for the resistance to SR 5 suggesting that they were different but both confer resistance to SR 5.

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.