COWPEA (Vigna unguiculata (L.) Walp) SEED QUALITY AND YIELD AS INFLUENCED BY MANIPULATING SOWING DATES FOR THE MANAGEMENT OF SCAB INDUCED BY Elsinoe phaseoli Jenkins, AT SAMARU, NORTH-WEST NIGERIA

ABSTRACT

Field experiment was conducted at the Institute for Agricultural Research (IAR) farm at Samaru, North-western Nigeria to investigate effects of sowing dates on incidence and severity of scab on yield and seed quality of cowpea. The varieties, SAMPEA 6, SAMPEA 8, SAMPEA 11, TVx 3236 and Ife brown were planted at three sowing dates, August 9, 19 and 29 in 2016. The design used was Randomized Complete Block Design (RCBD) with three replications. Parameters measured were disease incidence, disease severity and yield. Disease incidence and severity scores were taken at weekly intervals for ten weeks, and data was analysed using SAS statistical software. Cowpea sown on the first sowing date August 9, had the highest scab incidence, 48.89 %, 54.44 %, 36.67 %, 20 % and 13.33 % and severity, 25.11 %, 31.44 %, 18.33 %, 19.56 %, 18.11 % on leaves, stems, peduncles, flower cushions and pod, respectively while plants sown on the third sowing date, Aug 29, had no scab infection. The varieties, SAMPEA 11 and TVx 3236 had low incidence and severity and can be considered to be moderately resistant to scab. The yield of cowpea sown on August 9 was the highest (1,792.60 kg/ha), followed by that sown on Aug 29 (1,422.20 kg/ha) while cowpea sown on Aug 19, had the lowest (1,192.60 kg/ha). However, the yield of Aug 19 and 29, sowing dates were not significantly different from each other (P>0.05). The variety, SAMPEA 11 had highest yield (1,691.40 kg/ha) while in this study, TVx 3236 had the lowest (1,296.30 kg/ha); average potential yield of TVx 3236 had been put at 1,200 kg/ha. Path coefficient analysis showed that severity of scab on pod contributed highly to reduction in yield per hectare (18.94 %), followed by scab on flower cushion (16.42 %). Seed quality tests, germination and accelerated ageing were conducted at Legumes Pathology Laboratory, Department of Crop Protection, Ahmadu Bello University, Zaria while electrical conductivity test was at National Agricultural Seeds Council Laboratory, Sheda, Abuja. Seeds harvested from cowpea sown on August 29 sown cowpea had highest germination of 90 % and vigour, 82.07 % which was significantly different (P<0.05) from those sown on August 9 with lowest germination of 67.20 % and vigour of 64.47 %. Correlation coefficient analysis showed that scab severity on the different plant parts had negative and highly significant effects on viability and vigour of the seeds. Scab severity had a positive correlation with electrolyte leakage. Negative and significant correlation was observed between electrical conductivity and germination (r-0.330*). This was similarly observed with accelerated ageing tests (-0.224), though not significant. Path coefficient analyses showed that scab severity had effects on seed quality, causing a reduction of 73 % and 92.5 % on seed viability and vigour respectively. SAMPEA 11 and TVx 3236, despite its low yield which could be attributed to the small seed size, sown by second week of August should form an integral part for the management of scab in cowpea.

CHAPTER ONE

1.0         INTRODUCTION

Cowpea (Vigna unguiculata, L Walp) is a leguminous crop indigenous to Africa and a dicotyledonous plant belonging to the family Fabaceae (Schippers, 2002). It can be grown as sole crop, but more often planted alongside crops such as sorghum, maize or millet (Agbogidi, 2010a). More than 5.4 million tonnes of cowpeas are produced worldwide, with Africa being the highest producer with nearly 5.3 million tonnes annually (IITA, 2010). Nigeria, as the largest producer and consumer of cowpea, accounts for 61 % of production in Africa and 58 % worldwide, out of which Nigeria produced 5.1 million tonnes under cultivation area of 3.59 million / ha (NAERLS & FDAE, 2013).

Cowpea is one of the oldest crops known to man and an important food legume because of its nutritional value to man and usefulness as fodder to livestock (Agbogidi, 2010b; Davis et al., 1991). Its ability to replenish soil nitrogen makes it important in the modern crop farming system in rotation with other crops (Langyintuo et al., 2003). Also, its drought tolerance, relatively early maturity and nitrogen fixation characteristics fit very well to the tropical (humid) soils where moisture, erosion and low soil fertility is the major limiting factor in crop production (Hall, 2004). It serves as food security and at the same time can be combined with other recipe (Muoneke et al. 2012).

The average yields of cowpea are generally low (Benue State Agricultural and Rural Development Authority (BNARDA, 2009). This can be attributed to many factors amongst which are drought, low soil fertility, parasitic weeds, insect pests, and diseases (Olufajo and Singh, 2002; Niringiye et al., 2005; Fawole et al., 2006). The major insect pests of cowpea include: the legume pod borer, Maruca vitrata Fabricius; the coreid pod-bugs, Clavigralla tomentosicollis Stal; the groundnut aphid, Aphis craccivora Koch; and thrips, Megalurothrips sjostedti Trybom and Sericothrips occipitalis Hood (Jackai, 1995; Tanzubil, 2000; Malgwi and Onu, 2004). Opareke et al. (2000) estimated yield loss in cowpea due to insect pests to be above 80%. Cowpea is also attacked by parasitic flowering plants such as Striga gesnerioides which causes important yield losses of between 30-80% (Muleba et al., 1997); the nematode Meloidogyne spp causes cowpea root knot (Sikora et al., 2005). The disease induced by fungi and bacteria include cowpea scab, Elsinoe phaseoli; cowpea wilt, Fusarium oxysporium f. sp. Lycopersici; and cowpea bacterial blight, Xanthomonas vignicola among others (Emechebe and Lagoke, 2002).

Scab, induced by Elsinoe phaseoli is a seed-borne disease that affects all above ground parts of the plant (leaves, petioles, stems, peduncles and pods) at all stages of growth (Emechebe, 1980; Iceduna, 1993; Mbong et al., 2010b), with yield losses of up to 80% and even total crop destruction under severe conditions reported in Nigeria (Emechebe, 1980; Mungo et al., 1995). Low seed viability and vigour in plants have been attributed to seed-borne fungi (Elias et al., 2004); infected seeds may fail to germinate, or transmit disease to the seedling and to the growing plant (Fakir et al., 2002). Nabakka (1997) reported germination reduction of up to 59.3% on cowpea seeds which was as a result of seed-borne fungi infection; Mew et al. (1994) reported that the quality of seeds account for at least 5–20% increase in productivity.

1.1         Justification of the Study

Cowpea is an important grain legume in Nigeria (FAO, 2012) because it serves as cheap and rich source of protein, minerals, and vitamins in diets (Davis et al., 1991) and the fodder is also useful as feed to livestock (Agbogidi, 2010b). The potential yield has been estimated to be between 1,500 – 2,000 kg/ha, though average obtainable yield

ranges between 200 – 400 kg/ha (Wakili, 2013). This is due to prevalence of abiotic and biotic factors which affect both yield and quality of the crop.

Scab, induced by Elsinoe phaseoli accounts for between 30% to 100% yield loss of the country’s total cowpea crop production (Mungo, 1996 and Mbong et al., 2010b) through reduction in yield components such as number of seeds per pod and number of pods per plant (Tumwegamire et al., 1998). The disease has a puckering effect on leaf lamina, reducing the photosynthetic surface area of affected leaves (Tumwegamire et al., 1998). The most destructive phase is on the flowering axis causing flower and pod abortion or even completely preventing flower formation (Emechebe, 1980). A most widely used method employed in the management of scab is application of mancozeb (Dithane M-45) (Iceduna, 1993). Excessive and indiscriminate use of fungicide has led to problems of resistance build up, fungicide residues, and environmental hazards. This has necessitated the search for alternative control method (Narayanasamy, 2008; Gurjar et al., 2012). One of such method is manipulation of sowing date which determines the ability of crops to escape infection by pathogens (Cook and Baker, 1996). Cowpeas have been reported to escape infection by Cercospora leaf spot (Akande et al., 2012) due to manipulation of the sowing dates.

Seed quality is a crucial determining factor of yield and quality of crop production; it has a significant effect on seed germination and seedling vigour (Wimalasekera, 2015). Seed-borne pathogenic fungi can greatly affect seed quality and cause diseases that impact seedling production and yield (El-Gali, 2015). The effect of seed-borne pathogens such as Macrophomina phaseolina on germination and seed vigour has been reported by Nabakka (1997) to cause 59.3 % germination reduction in cowpea; however, the effect of Elsinoe phaseoli on seed quality of cowpea has not been investigated.

The aim of this study is to relate the influence of sowing date on scab incidence and severity and provide farmers information on the possible date to sow in order to reduce scab infection and obtain high yield and good quality seed.

1.2         Objective of the Study

The objectives of this study are to:

1. Determine the effect of sowing dates on scab and yield of selected cowpea varieties.
2. Determine the effect of sowing dates and scab on seed quality of cowpea.

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