Interactive Effect of Blackeye Cowpea Mosaic Virus and Cucumber Mosaic Virus on Vigna Unguiculata
CHAPTER ONE
Objectives of the study
The main objective of the study is to examine the interactive effect of Blackeye cowpea mosaic virus and Cucumber mosaic virus on Vigna unguiculata.
Specifically, the study sought to;
- Perception on cowpea virus diseases in some growing areas in Nigeria.
- Evaluate the physiological effect of two co-infecting viruses (Blackeye cowpea mosaic virus and Cucumber mosaic virus) on cowpea.
- Prove interaction through expression of symptoms.
CHAPTER TWO
LITERATURE REVIEW
Origin and taxonomy of cowpea
Cowpea is indigenous to Africa, with a probable centre of origin in the former Transvaal region, now Gauteng and Mpumalanga provinces, of South Africa due to the abundance of wild varieties in this region (Padulosi and Ng, 1997). Although some authors have suggested that cowpea originated in Asia, much of the published evidence suggested that it originated in Africa (Fery, 1990). Nevertheless, the centre of greatest diversity of cultivated cowpea is in the savannah regions of northern Guinea in West Africa (Ng, 1995). Ng and Marechal (1985) reported that germplasm accessions from Nigeria, Niger, Burkina Faso, and Nigeria show greater diversity than accessions from East Africa. This supports the theory that West Africa is the primary centre of cowpea domestication. Southeast Asia appears to be a secondary centre of cowpea diversity since significant genetic variability occurs on the subcontinent (Baudoin and Marechal, 1985).
Cowpea (V. unguiculata) is a diploid species (2n = 2x = 22), self-pollinated and belongs to the family Fabaceae (Padulosi and Ng, 1997). It is a dicotyledonous crop in the order Fabales, Family Fabaceae, subfamily Faboideae (Syn. Papillionoideae), tribe Phaseoleae, subtribe Phaseolinae, genus Vigna, and section Catiang (Verdcourt, 1970; Maréchal et al., 1978). The genus Vigna is pantropical and with differing reported number of species: 184 (Philips, 1957), 170 (Faris, 1965), between 150 and 170 (Summerfield and Roberts, 1985), 150 (Verdcourt, 1970), 154 (Steele, 1976) and about 84 of which 50 species are indigenous to Africa (Marechal et al., 1978). In addition to cowpea, other members include mungbean (V. radiata), adzuki bean (V. angularis), blackgram (V. mungo), and the bambara groundnut (V. subterranean). V. unguiculata subspecies unguiculata includes four cultigroups: unguiculata, biflora (or cylindrica), sesquipedalis, and textilis (Ng and Maréchal, 1985).
Biology and ecology of cowpea
Cowpea is a warm-season, annual, herbaceous legume with spreading growth habit and erect shoots up to 80 cm or more in height. Its leaves are glabrous and taproot is stout with laterals near soil surface. The roots have large nodules and the stems are usually procumbent, often tinged with purple. The first leaves above cotyledons are simple and opposite, and subsequent trifoliolate leaves are alternate. The terminal leaflet is often bigger and longer than the two asymmetrical laterals. Petioles are stout, grooved, 5 to 15 cm long, leaflets ovoid-rhombic, entire or slightly lobed, and apex acute. The leaflets are usually 6.5 to 16 cm long, and 4 to 11 cm wide and the lateral leaflets are oblique.
CHAPTER THREE
MATERIALS AND METHODS
Plant material
This research work was carried out between April and September 2019 during the wet season in the teaching and research farm of Federal University of Oye-Ekiti, Nigeria. A local variety of cowpea ‘Oloyin’ was used throughout the experiment. Four seeds per pot were planted in 5 kg sterilized loamy soil and were placed in an insect proof nylon wire mesh screen house of 12 m × 12 m × 11 m. Experiments were performed under temperature of 23 ℃ and 28 ℃ during the night and day, respectively.
Inoculation
Black eye cowpea mosaic virus (BlCMV) from genus Potyvirus actually recognized as strain of Bean common mosaic virus (BCMV) and Cucumber mosaic virus (CMV) from genus Comovirus were sourced from the virology laboratory of International Institute for Tropical Agriculture (IITA) Ibadan, Nigeria and maintained in healthy cowpea plant separately through mechanical inoculation. Cowpea plants were mechanically inoculated with CMV, BlCMV, and co-infected by CMV and BlCMV. Cowpea plant mechanically inoculated with potassium phosphate buffer was mock inoculation.
Each virus was transmitted to the cowpea plants by mechanical rub inoculation (Fajinmi, 1995). Virus infected plant tissues were ground in phosphate buffer solution [50 mmol·L-1 potassium phosphate buffer (pH 7.0), containing 10 mmol·L-1 sodium sulphite] at a ratio of 1 g of tissue to 10 mL of buffer. Carborundum was dusted on the leaves of the cowpea plant prior to inoculation. The plant sap extract containing the viruses were stroked on the cowpea plant leaves once without repeating any stroke, for each potted plant. The procedure was then repeated for each treatment. The combined inoculum of CMV and BlCMV consisting of similar amounts of tissue were ground together in the buffer and used for inoculating healthy plant. Also inoculum consisting of each virus alone was used in inoculating another sets of healthy plants. All inoculums including the buffer, mortars, and pestles were maintained on ice during inoculations. Treatments (CMV, BlCMV, CMV+ BlCMV, and mock-inoculated) were arranged in a randomized complete block design with five potted plants of each treatment per replication (i.e., a row of five pots) and four replications per treatment. Four experiments were performed and the 20 cowpea potted plants per treatment were mechanically inoculated onto leaves in each case.
CHAPTER FOUR
RESULTS AND DISCUSSION
Results
Virus interaction based on plant height and fresh weight
The difference in plant stem height and fresh weight one day prior to inoculation and 25 days post inoculation (dpi) in each treatment were compared. The difference in stem height and aboveground fresh weight of plants co-infected with CMV and BlCMV were significantly less than that of plants inoculated with either virus alone (P < 0.05) (Table 4.1). Likewise cowpea plants inoculated with either virus alone or with CMV and BlCMV mixed infection had less stem height and above ground fresh weight than mockinoculated controls (Table 4.1). Based on the equation Cexp = A + B – (AB/100), the response of mixed infection of CMV and BlCMV were more than expected, confirming interactive effect on both measured parameters (Table 4.1).
CHAPTER FIVE
CONCLUSION AND RECOMMENDATIONS
Conclusion
The study revealed interactive effect of Blackeye cowpea mosaic virus and Cucumber mosaic virus on Vigna unguiculata. Farmers’ activities which included high cropping density as a result of haphazard sowing methods, recycling of seeds from season to season, closeness of fields to each other with different planting and pesticide application periods as well as preference for and cultivation of susceptible cowpea cultivars, increased the incidence and severity of cowpea viruses.
BCMV-BICMV was found to be seedborne from infected cowpea fields. The number of multiple viral infections detected and the distribution of viruses observed in this study, suggest imminent greater losses in cowpea production in the country as multiple virus infections are usually associated with higher disease severity and yield reduction.
The interaction of BlCMV and CMV manifested through the disease effects on the host growth parameters compared with the effects of each virus alone. The plant height was relatively affected through the interaction compared with each virus alone.
One of the biggest setbacks for CMV and BlCMV infection is the continuous propagation of infected plants and production of plants with asymptomatic infections. Plants that exhibited high disease severity showed physiological disorder which could result in impaired growth, poor plant performance, low yield and subsequent death. Further research needs to be carried out on possible transmission of both viruses through seeds and functional control methods. Use of resistant varieties is however advocated.
Recommendations
- Mere identification of these viruses is not enough. Sequencing should be done for specific identification of the disease-causing agents.
- The use of resistant cultivars is the most effective method of managing cowpea viral diseases. There is the need therefore to study and identify virus resistant genes in cowpea varieties to enable breeders incorporate into preferred but susceptible varieties present in the surveyed areas.
- Also, it is necessary to produce virus-free seeds in order to prevent farmers from sowing infected seed stock to reduce infection.
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