STUDIES ON SWEETPOTATO VIRUSES FROM THE MAJOR SWEETPOTATO GROWING AGRO-ECOLOGIES OF GHANA

ABSTRACT

Sweetpotato virus disease complex (SPVD) is the most destructive viral disease in Africa. It can cause yield loss up to 50%. In Ghana, not much work has been done on the identification and detection of sweetpotato viruses from the major sweetpotato growing agro-ecologies. A study was conducted to ascertain the incidence of sweetpotato viruses from the major sweetpotato producing areas and to ascertain the effects of sweetpotato virus diseases (SPVD) on the crop in Ghana. Sweetpotato viral disease samples were collected from all agro-ecologies in Ghana where the crop is grown and then preserved in the screenhouse for diagnostic purposes. Nitrocellulose membrane (NCM) enzyme-linked immunosorbent assay (ELISA), using specific virus antibodies and PCR techniques involving the use of specific and degenerate primers were used for the diagnostics. Virus diagnostics were done directly on virus-infected sweetpotato samples collected from the field and also on Ipomoea setosa indicator plants after they have been grafted with virus-infected sweetpotato collected from the various locations. In all, 127 samples were assayed. Effects of SPVD were assessed on three sweetpotato varieties, namely; ‘Dadanuei’, ‘Ligri’ and ‘Bohye,’ which are all varieties released by the CSIR-Crops Research Institute, Fumesua, Ghana. These were put under four levels of disease regimes; tissue culture cleaned and virus indexed planting materials, apparently healthy planting material collected from the field, virus infected planting material collected from field and artificially (using whiteflies) infected planting materials. There were four treatments and each treatment was repeated three times in a randomized complete block design (RCBD). Virus diagnostics, using NCM-ELISA, detected the following viruses; SPFMV (85.71%), SPCSV (16.67%), SPCaLV (6.35%), SPVG (4.76%), SPMSV (4.76%), SPCFV (1.57%) and CMV (3.97%). RT-PCR and PCR confirmed the detection of SPFMV and SPCSV as well as Begomoviruses in some of the samples. Several mixed infections were also detected in samples collected mostly from local varieties whilst the released varieties had mainly single virus infections. The study has also optimized serological detection (NCM-ELISA) and RT-PCR protocols for the effective diagnosis of sweetpotato virus isolates in Ghana. Across board, tissue culture cleaned virus-indexed planting materials of the three varieties produced the largest yield with a mean of 12.00 tons/ha, whilst artificially infected (whitefly inoculated) planting materials produced the least yield of 0.78 tons/ha. The study revealed planting tissue culture cleaned virus indexed planting materials can affect yield of the crop positively whilst it showed the usefulness in planting improved varieties, compared to the local varieties in term of virus infections.

TABLE OF CONTENT

ABSTRACT

CHAPTER ONE

1.0 INTRODUCTION

CHAPTER TWO

2.0 LITERATURE REVIEW

2.1 Botany of Sweetpotato

2.2 Economic Importance and Distribution of Sweetpotato

2.3 Sweetpotato Cultivation

2.4 Symptoms of Sweetpotato Virus infection

2.5 Sweetpotato Virus Disease (SPVD)

2.6 Types of Sweetpotato Viruses

2.6.1 Sweetpotato Feathery Mottle Virus (SPFMV)

2.6.2 Sweetpotato Chlorotic Stunt Virus (SPCSV)

2.5.3 Sweetpotato Mild Mottle Virus (SPMMV)

2.6.4 Sweetpotato Chlorotic Fleck Virus (SPCFV)

2.6.5 Sweetpotato Latent Virus (SPLV)

2.6.6 Sweetpotato Mild Speckling Virus (SPMSV)

2.6.7 Sweetpotato Caulimo-Like Virus (SPCaLV)

2.6.8 Sweetpotato Virus G

2.6.9 Sweetpotato Virus C-6

2.6.10 Cucumber Mosaic Virus (CMV)

2.6.11 Begomoviruses

2.7 Yield Loss Estimate due to Sweetpotato Virus Diseases

2.8 Methods of Detection for Sweetpotato Viruses

2.8.1 Grafting

2.7.2 Insect Transmission

2.7.2.1 Non-persistent transmission

2.7.2.2 Persistent transmission

2.7.3 Serological Detection

2.7.4 Polymerase Chain Reaction (PCR)

CHAPTER THREE

3.0 MATERIALS AND METHODS

3.1 Location of Experiments

3.2 Sweetpotato sample collections

3.3 Graft Inoculation

3.4 Identification of viral symptoms and determination of viral severity of the grafted I.

3.5 Detection of Sweetpotato Viruses with NCM-ELISA

3.6 Nucleic Acid Extraction

3.7 Nucleic Acid Quantification and Gel electrophoresis

3.8 Polymerase Chain Reaction (PCR) Amplification

3.9 Evaluation of sweetpotato for yield of sweetpotato in the field

3.10 Harvesting

3.11 Virus Incidence and Severity on sweetpotato in the field

3.12 Experimental Design and Data Analysis

CHAPTER FOUR

4.0 RESULTS

4.1 Disease Symptoms observed on Grafted I. setosa in the Screenhouse

4.3 Mean disease incidence and severity of sweetpotato varieties planted on the field

4.4 Detection of sweetpotato viruses using NCM-ELISA from grafted I. setosa and sweetpotato cultivars collected during the sample collection

4.5 Symptoms and viruses detected from the sweetpotato plants collected from the major growing areas

4.6 Nucleic acid-based detection of viruses from grafted I. setosa and sweetpotato varieties collected from the major growing areas

4.7 Viruses detected with NCM-ELISA, PCR and RT-PCR from samples planted in the field

4.8 Yield of tubers and foliage weight of sweetpotato varieties planted from tissue culture, field healthy, field infected and whitefly inoculated planting materials.

CHAPTER FIVE

5.0 DISCUSSION

5.1 Mean Incidence and Severity of Viruses on Sweetpotato

5.2 Detection of sweetpotato viruses with NCM-ELISA

5.3 Nucleic Acid Based Detection of Viruses

5.4 Evaluation of Viruses Detected from samples planted on the field

5.5 Assessment of yield reduction due to SPVD

CHAPTER SIX

6.0 CONCLUSIONS AND RECOMMENDATIONS

6.1 CONCLUSIONS

6.2 RECOMMENDATIONS

REFERENCES

CHAPTER ONE

1.0 INTRODUCTION

Sweetpotato (Ipomoea batatas L.) is a dicotyledonous, perennial plant that produces edible tuberous roots with lots of economic importance (Woolfe, 1992). According to FAOSTAT (2012), sweetpotato is the third most important vegetatively propagated crop in the world after Irish potato and cassava, with annual production of 126 million tons. Area harvested for sweetpotato in Ghana is estimated at 74,000 ha (FAOSTAT, 2012) which comes next to cassava and yam in order of importance among root crops.

Sweetpotato has a short growing period, is usually useful in crop rotations, helps in famine as a reserve crop, and grows well in marginal soils, producing large yields per unit area per unit time, and in some areas three harvests per year can be achieved (Karyeija et al., 1998). Because of its robust nature and wide flexibility, it can be grown in several agro ecological zones hence, providing a sustainable food supply when other crops fail (Jana, 1982). Nutritionally, the tuberous root is rich in dietary fibre (pectin, cellulose, hemi-cellulose and lignin), proteins, vitamins (B1 and B2, C and E), β-Carotene (beneficial in fighting vitamin A deficiency in youngsters beneath the age of five years and breast-feeding mothers), mineral contents (mainly K, Fe and Ca) and carbohydrates (Low et al., 1997).

The yellow and orange fleshed varieties represent the least expensive year-round source of dietary vitamin A available to deprived families in Africa (CIP, 1999). They are also used as animal feed and provide raw materials for alcohol production (Woolfe, 1992). Sweetpotato has high anthocyanin content which pigments are highly stable making the crop a healthier substitute to synthetic colouring elements in food. All these benefits make sweetpotato a high priority crop for food security (CIP, 2000).

For the reason that sweetpotato has vast genetic diversity (Zhang et al., 1998) and the accompanying diversity in phenotypic and morphological traits, the crop has great potential for further development to accommodate specific uses. However, its production is beset with abiotic and biotic limitations (Geddes, 1990).

Among the biotic stresses, viral diseases are the second most important constraint. This comes after the sweet potato weevil (Qaim, 1999), causing extensive losses worldwide (CIP, 2000). Virus complexes influence disease symptom severity thereby affecting yield losses considerably.

Sweetpotato virus disease (SPVD) is the most alarming complex condition of sweetpotato viruses caused by co-infection of Sweetpotato chlorotic stunt virus (SPCSV) and Sweetpotato feathery mottle virus (SPFMV). SPCSV is whitefly-borne and transmitted in a semi-persistent manner while SPFMV is aphid-borne and transmitted in a non-persistent manner. The combined infection of the two viruses causes the most severe disease of sweetpotato in Africa (Karyeija et al., 2000; Gibson and Aritua, 2002; Mukasa et al., 2003; Cuellar et al., 2008). SPVD can cause yield reduction as high as 56-98% in Africa (Gibson et al., 1998a) and in numerous countries throughout the world (Clark and Moyer, 1988; Carey et al., 1999)

The costs of viral infections are not only restricted to decrease in crop yield, but also undermine the efforts in genetic improvement for yield and quality, since farmers normally abandon susceptible but otherwise high yielding varieties (Aldrich, 1963) which are also rich in starch and vitamin A. Also, the existence of lone virus infections may compromise the usage of farmer-saved vines, especially in regions where insect vectors are predominant. The reason being that single virus-infected vines can become sources of inocula for vector spreads, leading to mixed infections of different viruses.

Under field conditions, sweetpotato frequently develops complexes of mixture of viral diseases of up to three viruses and in rare situations, four viruses which reduce the quality of planting materials (Mukasa et al., 2003).

In Ghana, not much work has been done in the identification and characterization of sweetpotato viruses in the major growing areas. Sweetpotato production is only popular and restricted to a few ecologies where leaves and roots are mostly consumed as staple. However, in these areas, farmers normally grow landraces and, in some cases, improved varieties which are susceptible to viruses.

Vine cuttings from mature crops are used to establish new fields, which are potential sources of infection in the newly planted fields. Even for the new improved varieties that have been produced over the years and adopted by farmers, not much has been done to preserve the true-to-type virus-tested foundation seed stocks.

Virus-tested varieties, produced from tissue culture plants that have been confirmed virus-free, have actually been found free of these viruses. Planting diseased vine cuttings or storage roots is the greatest collective source of sweetpotato viruses, but clean planting material can rapidly be re-infected by some viruses, particularly those spread by aphids and whiteflies. In Ghana, almost 70% of the crop is propagated from vines chosen and kept by farmers or bartered and traded locally.

Sweetpotato cultivars increasingly lose their resistance over time after release and are often replaced within 20 years. It is believed that this leads to virus accumulation in the propagating material.

Virus complexes rank second to weevils in causing yield reduction in sweetpotato. However, it is important to know the extent of yield losses caused by sweetpotato viruses in Ghana to guide breeders in the development of resistant cultivars. Similarly, information on sweetpotato viruses and their detection with effective methods can enhance the management of the SPVD. The convenience of accurate viral investigative procedures and practice of providing virus-indexed clean planting material for farmers can improve productivity in farmers’ fields. It is, therefore, important to know the different types of sweetpotato viruses and their distribution in the different ecologies so that management strategies can be implemented against them.

The main objective of this study was to optimize sweetpotato virus detection protocols, detect sweetpotato viruses in the important growing areas and estimate their effects on yield. The specific objectives were to:

i. detect  different  sweetpotato viruses  in the target ecologies,

ii. optimize the effectiveness of sweetpotato virus detection protocols for the screening of sweetpotato virus isolates in Ghana, and

iii. estimate the effect of SPVD infection on yield.

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Item Type: Ghanaian Topic  |  Size: 95 pages  |  Chapters: 1-5

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