PHENOTYPING OF Striga gesnerioides (Willd.) AND Megalurothrips sjostedti (Trybom) RESISTANCE IN COWPEA (Vigna unguiculata (L.) Walp.) POPULATION IN NORTHERN GHANA

ABSTRACT

Parasitic weed Striga gesnerioides (Willd.) and flower Thrips (Megalurothrips sjostedti Trybom) are among the major constraints of cowpea (Vigna unguiculata (L.) Walp.) production. The single approaches used for their control appears to be highly insufficient. Host-Plant Resistance seems to have merit in efficiently and economically controlling these pests. The objectives of this study were to evaluate recombinant inbred lines developed between Striga and Thrips resistant parents, IT97K 499-35 and Sanzi respectively by Single Seed Descent (SSD), for Striga and Thrips resistance in Northern Ghana. The study also evaluated the promising Striga gesnerioides resistant lines for yield loss assessment. Studies involved field and pot screening under artificial inoculation. Twenty-seven (27) RILs out of the 251 RILs screened were completely resistant to Striga gesnerioides. The level of Thrips infestation was very low (0 to 11 flower Thrips per plot) making it difficult to rank the genotypes into the categories (resistant and susceptible). The damage index (scores) were therefore not recorded due to the total absence of flower Thrips in a good number of plots. The percentage reduction in the grain yield and dry biomass among the RILs was lower in the resistant RILs (0.55% to 3.08% and 1.11 to 7.7% respectively) than the susceptible ones (28.45% to 58.88% and 47.29% to 61.71% respectively). The negative effect of Striga infestation on cowpea grain yield and dry biomass can then be reduced when resistant genotypes are used.

CHAPTER ONE

BACKGROUND OF THE STUDY

1.1 INTRODUCTION

Cowpea is an important crop in the semi-arid tropics including parts of Asia, Africa, Southern Europe, Southern United States, Central and South America (Singh, 2005; Timko et al., 2007a). It is highly adapted to the warm and sparse rainfall climates of the Sahelian and Sudanian zone in Africa (Hall et al., 2002; Hall, 2004). The total production area of the crop was estimated around 12 million hectares with West Africa accounting for about 10 million hectares (FAO, 2016).

Cowpea contains high-quality protein (Langyintuo et al., 2003). According to Ohler et al. (1996), the grain and dried foliage contain about 23-25% of protein by weight. Its fodder is used for livestock feed and also to improve soil fertility by its ability to fix nitrogen in the soil. Cowpea production is affected by many constraints. Currently, cowpea yields are estimated around 300 to 500 kg ha-1 on farmer’s field in the Savannahs of Sub Saharan Africa (SSA) while its yield potential is up to 3000 kg ha-1 in optimum growing conditions (Tanzubil et al., 2008).

Cowpea production is influenced by both biotic and abiotic constraints. The constraints to cowpea production include weeds infestation such as Striga gesnerioides and Alectra vogelii (Parker and Riches, 1993) and low soil fertility (Asare, 2012). Other factors limiting yield include its susceptibility to several bacterial, fungal, and viral diseases and various insect pests (Singh, 2005; Timko et al., 2007a). The most common insect pests that cause injury to cowpea are aphids (Aphis craccivora), flower bud Thrips (Megalurothrips sjostedti), Maruca pod borer (Maruca vitrata), pod sucking bugs (Clavigralla spp., Riptortus spp.), and the storage weevil Callosobruchus maculatus (Caswel, 1981).

The flower bud Thrips is the most economically important cowpea pest at the flower initiation and flowering stage that cause yield losses between 20% and 70% depending on infestation level (Ngakou et al., 2008). Nevertheless, a severe infestation can result in complete grain yield loss (Singh and Allen, 1980). The damage to cowpea flower as a result of Thrips is characterized by a distortion, a malformation of the floral parts, flower bud abscission and non-elongation of peduncles. Apart from the direct damage caused by Thrips, Ullman et al. (1997) reported that Thrips are vectors for a number of pathogens that they transmit mechanically from plant to plant. They are known to be vectors of some bacterial (Bailey, 1935), fungal (Farrar and Davis, 1991) and viral (Garcia et al., 2000) diseases. Several Thrips species, all belonging to the family of Thripidae are able to transmit plant viruses (Ullman et al., 1997) which are “prunus necrotic ringspot ilavirus” (Greber et al., 1991), “tobacco streak ilavirus” (Sdoodee and Teakle, 1993), “soybean mosaic sobemovirus” (Hardy and Teakle, 1992), and the most important “tomato spotted wilt virus” (Marchoug et al., 1991). Thrips are the only known transmitters of tospoviruses which belong to the Bunyaviridae (Ullman et al., 1997).

Apart from insect pests that are harmful to cowpea, parasitic plants are also a major constraint to today’s agriculture with most crop species being potential hosts (Westwood et al., 2010). Out of about 30 Striga species which have been identified, Striga gesnerioides is the only Striga species that is virulent to dicots (Mohamed and Musselman, 2008). S. gesnerioides is a major limitation to cowpea production in Africa (Timko et al., 2007b), causing considerable yield losses (Aggarwal and Ouédraogo, 1989).

The extent of the damage in cowpea is due to the close interaction between the host and the parasitic weed. Crop yield losses due to S. gesnerioides may be up to 70 % depending on the extent of damage and level of infestation (Alonge et al., 2004). On susceptible cultivars, yield losses can reach up to 100 % when S. gesnerioides population is over 10 emerged shoots per plant (Kamara et al., 2008). Omoigui et al. (2009) reported that yield losses caused by Striga in dry savannah of SSA are estimated in millions of tons annually and the prevalence of Striga infested soils is steadily increasing.

Methods including improved cultural practices and the use of chemicals to control S. gesnerioides are available but most of them are ineffective whilst others are not affordable for small-scale farmers of SSA (Singh et al., 1997; Timko et al., 2007).

In general, S. gesnerioides control is difficult to achieve due to the close association with its host (Lane et al., 1997) and because each plant produces a large number of seeds which remain viable in soil up to 20 years. The use of resistant cultivars appears to be therefore, a generally acceptable, effective, economically sound and environmentally safe method to control this parasite (Timko et al., 2007).

Host plant resistance (HPR) can also be used to control Thrips and reduce or eliminate the use of environmentally toxic chemicals (Jackai and Adalla, 1997). Insect resistant cowpea varieties can thus help to sustain the productivity of cowpea by resource-poor farmers (Jackai and Adalla, 1997).

The significance of resistance and its durability for plant production in all countries especially in developing countries justifies that breeding for resistance be given top priority worldwide (Shaner, 1981).

The development of resistant cowpea cultivars to multiple pests would have a significant impact on yield and food availability and nutritional status in many regions. It will positively influence seed production and yield without the use or reduce used of insecticides.

The main objective of this study was therefore to identify Striga and Thrips resistant lines for Thrips and Striga resistance in Northern Ghana The specific objectives were to:

Evaluate the field performance of 251 Recombinant Inbred Lines (RILs) under

Evaluate  the  performance  of  251  Recombinant  Inbred  Lines  (RILs)  under

Assess yield loss of promising Striga gesnerioides resistant lines under artificial infestation.

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Item Type: Ghanaian Topic  |  Size: 105 pages  |  Chapters: 1-5

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