799 AJCS 10(6):799-807 (2016) ISSN:1835-2707 DOI: 10.21475/ajcs.2016.10.06.p7303 Combining ability analysis of yield and late blight [Phytophthora infestans (Mont.) de Bary] resistance of potato germplasm in Rwanda Jean Baptiste Muhinyuza 1,2, 3* , Hussein Shimelis 1 , Rob Melis 1 , Julia Sibiya 1 and Magnifique Ndambe Nzaramba 4 1 University of KwaZulu-Natal, African Centre for Crop Improvement, Private Bag X01, Scottsville 3209, Pietermaritzburg, South Africa 2 Rwanda Agriculture Board, Southern Zone, P.O. Box 138 Huye, Rwanda 3 University of Rwanda, College of Agriculture, Animal Science and Veterinary Medicine (UR-CAVM) P.O. Box 210 Musanze, Rwanda 4 National Agricultural Export Development Board, P.O. Box 104 Kigali, Rwanda *Corresponding author: mujohnbapt25@ gmail.com Abstract Designed crosses among complementary parents and combining ability analysis are essential to recombine traits of economic importance and to select novel genotypes. The objective of this study was to estimate combining ability effects for yield and yield related traits and late blight resistance in potato. Crosses were performed using a 10 × 10 half diallel mating design to generate 45 F 1 s. Only 28 families with sufficient individuals and eight parents were field evaluated in experiments laid out in a 6 × 6 lattice design with two replications across two sites (Kinigi and Nyamagabe) in Rwanda. Late blight resistance was estimated using the relative area under the disease progress curve (rAUDPC: 100 % max). Furthermore, data on total tuber yield, total tuber number, and average tuber weight were collected and subjected to combining ability analyses. Results showed that additive gene action and non- additive gene action were present affecting yield and late blight resistance in potato. Additive gene action was predominant over non- additive gene action for both traits. All the families selected for further evaluation showed improved levels of late blight resistance and high yields. The study identified ten top families with high tuber yield and resistance to late blight for further evaluation and release. Keywords: Combining ability, gene action, late blight, potato, Rwanda. Abbreviations: rAUDPC: relative area under the disease progress curve. Introduction Potato (Solanum tuberosum L.) is a staple food crop grown in 149 countries across the world (Hijimans, 2001). It is one of the four most important food crops along with rice (Oryza sativa), wheat (Triticum aestivum L.), and maize (Zea mays L.) (Lang, 2001). Potato breeding towards development of new cultivars involves sexual recombination to generate genetic variation and select novel recombinants using agro- morphological traits (Acquaah, 2007). Segregating F 1 progeny from which to select superior clones contains suitable genetic constitution from its parents. It is important to understand the mode of gene action involved in the expression of important traits. Understanding gene action will help in selecting suitable parents and segregates in a breeding programme (Falconer and Mackay, 1996). Combining ability analysis is the basis for identification of the best parents and their crosses (Mondal and Hossain, 2006). The general combining ability (GCA) gives an indication on the average contribution of a parent to its progeny; it provides an estimation of the parental gametic contribution to its offspring by the mean performance of the progeny (Bradshaw and Mackay, 1994; Falconer and Mackay, 1996). The specific combining ability (SCA) is the deviation from the progeny mean from the expected on the basis of GCA (Bradshaw and Mackay, 1994). In this case, the performance of the progeny is either superior or inferior to the parents. Yield and late blight disease field resistance are quantitatively inherited (Falconer and Mackay, 1996). In quantitative inheritance many genes are involved, each contributing a small effect to the phenotypic expression of the character concerned. In the literature, it has been reported that both GCA and SCA effects are significant for potato yield and late blight resistance with slight differences in magnitude across experiments (Bradshaw and Mackay, 1994; Gopal et al., 2008). Potato genotypes with relatively high yield level and resistance to the late blight disease are being developed by the International Potato Centre (CIP) (Landeo et al., 1997). These genotypes are frequently introduced to developing countries including Rwanda to strengthen the existing potato genetic resources to boost productivity and control potato late blight disease. Subsequently, the national potato programme of the Rwanda Agriculture Board (RAB) is continuously evaluating and screening the CIP genetic stocks and locally adapted genotypes under target growing environmental conditions to identify clones with high yield and late blight resistance. Consequently, a number of best genotypes were identified to be used as parents in subsequent crosses and selection (Muhinyuza et al., 2014). The objectives of this study were to estimate combining ability