Genetic studies of aphids (Aphis craccivora Koch) resistance in cowpea

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August 2015
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Cowpea aphid (Aphis craccivora) is a serious pest in legume that causes significant grain yield losses. Chemical control measures are the most widely known form of control to this pest. However, breeding for resistance to this insect is very important because of its compatibility with other pest control methods; environment and eliminate dependence on environmentally toxic chemicals that resource poor subsistence farmers cannot afford. Breeding for cowpea aphid resistance was studied in crosses involving two resistant (Hewale and Asomdwee) and two susceptible (Asetenapa and Videza) genotypes. Crosses were made to generate direct and reciprocal F1 generations and direct and reciprocal F2 generations. All plants in each generation were evaluated for aphid resistance on the field using Randomized Complete Block Design. Each plant was infested with cowpea aphid two weeks after planting and the number of aphid colonies and damage scored between 0 – 5. The results showed most of the direct and reciprocal F1 generations to be resistant to the cowpea aphid and the F2 segregating generations were intermediate between the resistant and susceptible parents but were skewed towards the resistant parent. Results also showed that the resistance in Hewale and Asomdwee were controlled by a single gene and the F2 generation conformed to the 3 resistant and 1 susceptible after a χ2 test. Reciprocal differences were not detected in the crosses suggesting the absence of maternal effect. Negative heterosis over mid-parent was observed for aphid resistance score. It is possible to improve cowpea aphid resistance in susceptible cowpea lines in a hybridization programme.
A thesis submitted to the School of Graduate Studies, Kwame Nkrumah University of Science and Technology in partial fulfillment of the requirements for the award of the Degree of Master of Philosophy in Agronomy (Plant Breeding), 2015