ABSTRACT
Viruses have been reported to be one of the factors hindering sweet potato production. Sweet potato feathery mottle virus and Sweet potato chlorotic stunt virus were first reported in Ibadan Nigeria. These were reported and found to be responsible for the severe Sweet potato virus disease (SPVD). These viruses had been reported to cause yield reductions of 78 % in field trials in Nigeria. Information on these viruses in Northern Nigeria and Kaduna State in particular where sweet potato is largely grown is not available. Therefore, field surveys were conducted to determine the occurrence, distribution and alternative hosts of viruses infecting sweet potatoes in six Local Government Areas of Kaduna State. In July 2016, survey was conducted on sweet potatoes and weed plants in the wet season. A second survey for weed hosts was done in January 2017 dry season. The Local Government Areas visited are Giwa (Sabon-Gida, Hayin-Safiu and Halkama), Igabi (Lambakau, Fanguruzan and Tumbau), Kachia (Angwan-Ayuba, Sakwai and Gwame), Kudan (Jaja, Angwan-Sako, Dumiga), Soba (Farin-Kasa, Tabasariki and Sambirni) and Zangon-Kataf (Lenak, Zonkwa and Samaru- Kataf). Eighteen (18) farms were surveyed. In each LGA, three farms were visited. In each farm, seven (7) sweet potato leaf samples were collected making a total of 126 samples for all the six Local Government. In both wet and dry season surveys, 3 weed plants was collected per farm making a total of 108 weed samples for all the six Local Government Areas. Sweet potato leaves and weeds with and without symptoms were analysed for detection of Sweet potato viruses. Triple Antibody Sandwich Enzyme-Linked Immunosorbent Assay (TAS- ELISA) was used for the detection of SPCSV while Double Antibody Sandwich Enzyme-Linked Immunosorbent Assay (DAS- ELISA) was used for the detection of SPFMV and SPV2. The three viruses were identified in single and mixed infection for the first time on sweet potatoes and weeds in Kaduna State Nigeria. Based on the result obtained
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in the laboratory analysis, Giwa LGA farms had a very high incidence of SPCSV (100%) while other viruses and their mixed infections were not detected. In Kudan LGA farms, percentage incidences of SPCSV was 70%, SPV2 (11%), SPFMV (4%), SPFMV + SPCSV (4%), SPCSV + SPV2 (11%) were identified, other mixed infections were zero. For Soba LGA farms, incidence of SPCSV was 72%, SPV2 (10%), SPCSV+SPV2 (29%) were detected. Igabi LGA farms recorded percentage incidence of SPCSV was 39%, SPFMV (22%), SPV2 (10%), SPFMV + SPCSV (11%), SPCSV + SPV2 (10%), SPFMV + SPV2 (4%), and triple infections of SPFMV +SPCSV + SPV2 (4%). Kachia LGA farms had incidences of SPFMV 62%, SPCSV (16%), SPV2 (9%), SPFMV + SPCSV (9%), SPFMV + SPV2 (4%). While Zangon-Kataf LGA farms recorded SPFMV incidence was (39%), SPCSV (31%), SPV2 (22%), SPFMV + SPCSV (8%) in percentage. The alternative host detected for SPFMV was the weed Morning glory, Ipomoea hederaceae L. of the family Convolvulaceae.
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TABLE OF CONTENTS
Title Page – – – – – – – – – i
Declaration – – – – – – – – – ii
Certification – – – – – – – – – iii
Dedication – – – – – – – – – iv
Acknowledgements – – – – – – – – v
Abstract – – – – – – – – – vi
Table of Contents – – – – – – – – vii
List of Tables – – – – – – – – – xi
List of Figures – – – – – – – – xii
List of Plates – – – – – – – – – xiii
List of Appendices – – – – – – – – xvi
List of Virus Abbreviations – – – – – – – xv
CHAPTER ONE
INTRODUCTION – – – – – – – – 1
1.1 Background to Study – – – – – – 1
1.2 Justification of the Study – – – – – – 3
1.3 Objectives of the Study – – – – – – 4
CHAPTER TWO
2.0 LITERATURE REVIEW – – – – – – 5
2.1 The Sweet Potato Plant – – – – – – 5
2.1.1 Origin and distribution of sweet potato – – – – 5
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2.1.2 Taxonomy and morphology of sweet potatoes – – – 5
2.1.3 Botanical description of sweet potatoes – – – – 6 2.1.4 Production of sweet potato – – – – – – 7
2.1.5 Cultivation of sweet potato – – – – – – 8
2.1.6 Nutritional composition of sweet potato – – – – 9
2.1.7 Uses of sweet potatoes – – – – – – 10
2.2 Production Constraints of Sweet Potato – – – – 10
2.3 Diseases and Insect Pests of Sweet Potato – – – – 11
2.4 Sweet Potato Infecting Viruses – – – – – 12
2.4.1 Sweet potato feathery mottle virus (SPFMV) – – – – 12
2.4.2 Sweet potato mild mottle virus (SPMMV) – – – – 14
2.4.3 Sweet potato chlorotic stunt virus (SPCSV) – – – – 14
2.4.4 Sweet potato virus disease (SPVD) – – – – – 15
2.4.5 Cucumber mosaic cucumovirus (CMV) – – – – 16
2.4.6 Sweet potato virus 2 (SPV2) – – – – – – 16
2.5 Management of Sweet Potato Viruses – – – – 17
2.6 Alternative Hosts of Sweet Potato Viruses – – – – 18
2.7 Detection of Sweet Potato Viruses – – – – – 19 CHAPTER THREE
3.0 MATERIALS AND METHODS – – – – – 20
3.1 Survey and sampling of sweet potato fields and alternative weed hosts
of sweet potato viruses – – – – – – 20
3.2 Laboratory Detection of Viruses in Sweet Potato Leaf Samples and
Weeds – – – – – – – – 23
3.2.1 Procedure for double antibody sandwich Enzyme-Linked Immunosorbent
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Assay (DASELISA) for sweet potato feathery mottle virus (SPFMV) and
Sweet potato virus (SPV2) detection – – – – – 23
3.2.2 Triple Antibody Sandwich Enzyme-Linked Immunosorbent Assay
(TAS ELISA) for sweet potato chlorotic stunt virus (SPCSV) detection 24
3.3 Data Analysis – – – – – – – – 25 CHAPTER FOUR
4.0 RESULTS – – – – – – – – 26
4.1 Incidence of Sweet Potato Viruses – – – – – 26
4.1.1 Virus incidence in Giwa, Kudan, Soba, Igabi, Kachia and Zango-Kataf 26
4.2 Distribution of the viruses infecting Sweet potatoes in Kaduna State 29
4.3 Weed Hosts of Viruses Infecting Sweet Potatoes in Kaduna State
During the 2016 Wet and 2017 Dry Seasons – – – 33
CHAPTER FIVE
5.0 DISCUSSION – – – – – – – 36
CHAPTER SIX
6.0 SUMMARY, CONCLUSIONS AND RECOMMENDATIONS – 40
6.1 Summary – – – – – – – – 41
6.2 Conclusions – – – – – – – – 41
6.3 Recommendations – – – – – – – 41
REFERENCES – – – – – – – 42
Appendix I: – – – – – – – – 51
Appendix II: – – – – – – – – 52
Appendix III: – – – – – – – 53
CHAPTER ONE
INTRODUCTION
1.1 Background to Study
Sweet potato (Ipomoea batatas (L.) Lam) is a dicotyledonous plant in the family Convolvulaceae. In this family, there are more than 1000 species with approximately 50 genera. Out of 400 species which make up the genus Ipomoea, only I. batatas is edible (Huaman, 1992). Common names are batata, sweet potatoes (tuberous morning glory in English), yam in the US, patates douce in French, Comote in Spanish etc. In Nigeria it is called “Dankali” in Hausa, “Duku” in Nupe, “Anama” in Yoruba, and “Nduku” in Igbo. It is cultivated mainly in the tropical and sub-tropical regions for its fibrous, edible, sweet and starchy tubers. It was first cultivated 5000 years ago in the tropical areas of Americas where it originated from and spread throughout the regions. It is wide spread in Europe, Asia, China, and Peru (Austin, 1988).
Huaman (1992) described sweet potato as an herbaceous perennial plant and as an annual plant by vegetative propagation using either storage roots or stem cutting. It is a prostrate plant with a vine system which extends horizontally with different types of growth habits which are either erect, semi-erect, spreading or very spreading. The root systems are fibrous for absorption of nutrient and water and storage root system for the synthesis of photosynthetic products. Depending on the variety, the tuber skin can be red, purple, brown, creamy or white. The stem is cylindrical in length and the leaves are simple and spiral in arrangements with colour varying from green-yellow, green and at times with purple pigments which are very high in anthocyanin (mostly the cyanidin type), some varieties do or do not produce flowers. Sweet potato is rich in carbohydrates, protein, minerals
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(potassium, manganese, zinc, iron, phosphorus and calcium) and vitamins (A, mostly orange pulp, C, K, B1, B2, B3, and B6). The vines contain 57-78 % water of fresh weight. The carbohydrate content of the tubers include starch (13-33 %), sucrose (2.6 – 6.0 %), reducing sugar (0.3 – 0.8%), minerals (0.8 -2.2%) protein (0.8 – 2.2 %) and cellulose (0.9-2%). Carotene content of tubers varies between 0 – 24 mg, ascorbic acid (vitamin C) ranges from 23 – 43 mg/100 g in fresh tuber (USDN, 2014). It can be boiled, fried, or roasted for consumption. The fodder is used for animal feed, and the flour can be used for baking in place of cassava or wheat flour. Industrially it is processed for production of starch, syrup, alcoholic beverages, protein enriched pulp, carotene feed, yeast silage and flour. Being a rich source of vitamin A, it is also used in the pharmaceutical industries (Zhang and Li, 2004; Kapinga et al., 2007). In Nigeria, sweet potato is one of the four major root and tuber crops after cassava, cocoyam and yam with production output of 3,464,135.00 metric tons (FAO, 2016).
Sweet potato is reported to be infected by viruses which can either be in single or mixed infection with the dual infections of Sweet potato feathery mottle virus (SPFMV) and Sweet potato chlorotic stunt virus (SPCSV) the most commonly detected. These viruses are mostly spread by the use of vine cutting from infected mother plant, through plant to plant by sap sucking aphids and white fly (Stathers et al., 2005). Some of the viruses reported to infect sweet potatoes include Sweet potato feathery mottle virus (SPFMV) infect the crop worldwide (Moyer and Salazar, 1989), Sweet potato chlorotic stunt virus (SPCSV) (Karyeija et al., 2000; Gibson and Aritua, 2002), Sweet potato virus G (SPVG) (Colinet et al., 1994), Sweet potato mild mottle ipomovirus (SPMMV) (Hollings et al., 1976), Sweet potato chlorotic fleck virus (SPCFV), Sweet potato latent virus (SPLV), Sweet potato
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caulimo-like virus (SPCaLV), Cucumber mosaic virus (CMV) (Cohen et al., 1988), Sweet potato leaf curl virus (SPLCV), Sweet potato ring spot virus (SPRSV), Sweet potato virus Y (SPVY) (Ateka et al., 2004) and a host of many others. The dual infections of SPFMV and SPCSV was first reported in Ibadan Nigeria by (Schaefer and Terry, 1976) and later by winter et al., 1992). It is reported to be responsible for the severe Sweet potato virus disease (SPVD). Gibson et al. (1988) reported SPVD to be the major viral disease of sweet potato in East Africa. Also co-infections of SPFMV, SPCSV and SPMMV were detected in Argentina and shown to cause Sweet potato chlorotic dwarf disease (SPCDD) (Di feo et al., 2000). Sweet potato mild mottle virus (SPMMV) and Sweet potato chlorotic stunt virus (SPCSV) are transmitted by whitefly predominantly Bemisia tabaci, while Sweet potato feathery mottle virus (SPFMV) and the related Sweet potato virus 2 (SPV2) are transmitted by Aphids (Schaefer and Terry, 1976; Clark and Moyer, 1988). Symptoms of SPVD include stunting of plants with small leaves often distorted, narrow strap-like and crinkled, with a chlorotic, mosaic and or vein clearing symptoms but it varies with cultivar. Affected sweet potato plants generally appear pale. Yield losses of up to 90% have been reported in plants infected with SPVD (Gutierrez et al., 2003; Hahn, 1976; Ngeve, 1990).
1.2 Justification of the Study
Sweet potato is one of the world’s most important food crops. Orange-fleshed sweet potatoes are particularly nutritious, ranking highest in nutrient contents of all vegetables for vitamins A and C, iron, copper, calcium, and fiber (CIP, 2014). It can be boiled or steamed, baked or fried and used for human food and animal feed as the root contains 16% of starch and 4% of sugar. It is used for the production of industrial starch, syrup and alcohol (USDN, 2014). Yield obtained per hectare in Kaduna State is approximately 12,000,000 Mt/Annum
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(NAERLS, 2014). Abdulkareem et al. (2015) reported (5.5-7.4 t/ha) of sweet potatoes produced in Zaria Local Government Area of Kaduna State. Average yield of 3,400,000 Mt /Annum is recorded in Nigeria and 77,375,000 Mt /Annum in China (FAO STAT, 2016). Viruses, however have been implicated to be one of the limiting factors in sweet potato production. SPFMV and SPCSV were first reported in Ibadan Nigeria by (Schaefer and Terry, 1976) and later by winter et al. (1992). These viruses had been reported to cause yield reductions of 78 % in field trials in Nigeria (Hahn, 1979). In Uganda, yield loss of 14-52% was recorded for SPCSV in single infection while in mixed infection with SPFMV, 60- 95% yield loss was recorded (Gibson et al., 1998). Research on viruses infecting sweet potatoes and its reservoir hosts have not been documented in Kaduna State being one of the major producing States in Nigeria. There is therefore the need to have information on the incidence and distribution of viruses infecting sweet potatoes and to determine the alternative hosts which may serve as reservoir hosts of these viruses. This study was therefore conceived with the following objectives.
1.3 Objectives of the Study
The objectives of this research are to determine:
i. the incidence and distribution of viruses infecting sweet potatoes in Kaduna State.
ii. the alternative hosts of the viruses.
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