Genetic analyses of drought tolerance and resistance to late blight among potato genotypes.

Doctor of Philosophy in Plant Breeding. University of KwaZulu-Natal, Pietermaritzburg 2015.Potato is one of the valuable crops in Ethiopia serving as a source of food and income for smallholder farmers. About 70% of the country’s arable land is suitable for potato production. Despite the rapid growth of the total potato production in the country, productivity of the crop under the small-scale farming sector is estimated at 11 t ha-1, which is far below attainable yields of over 40 t ha-1. Recurrent drought and late blight disease are the most important constraints affecting productivity of potato in Ethiopia. Late blight disease of potato, caused by Phytophthora infestans, is widespread in most potato growing areas of Ethiopia causing yield losses reaching up to 100% in susceptible cultivars. Breeding for host tolerance and resistance could be the best option for managing recurrent droughts and controlling the disease, respectively. The overall goal of this study was, therefore, to contribute to improved food security in Ethiopia by developing potato cultivars with improved yields, late blight resistance and drought tolerance. The specific objectives of the study were to (i) select late blight resistant and high yielding potato clones under field conditions in the north-western parts of Ethiopia, (ii) determine combining ability and gene action controlling late blight resistance, yield and yield components and to identify promising potato genotypes as potential parents in a breeding programme, (iii) determine combining ability and gene action controlling yield, yield components and drought tolerance related traits among selected potato clones and to identify promising parents and crosses for cultivar development, and (iv) assess the level of genetic diversity among 18 selected potato clones using 23 simple sequence repeat (SSR) markers and to complement phenotypic selection for identification of suitable parents for breeding. These objectives were achieved based on four sets of experiments conducted in the north-western Ethiopia. Twenty-four selected potato clones, of which 17 from the B3C2 population acquired from the International Potato Centre (CIP) and seven widely grown released and farmers’ cultivars, were evaluated for late blight resistance and yield related traits at three locations in the north-western Ethiopia. A randomized complete block design was used with two replications. Results indicated significant variation among the genotypes for late blight resistance and yield related traits across the three locations. The following five clones were selected: 396029.25, 395017.229, 396004.263, 396034.103 and 395077.12, displaying high to moderate resistance to late blight and greater yield levels. Twelve parents selected from late blight resistant advanced population (B3C2) were crossed using North Carolina Design II. Eighteen F1 families derived from these crosses were evaluated at two late blight hotspot areas in Ethiopia. Combining ability and gene action were determined for late blight resistance under natural epidemics. Results showed that the general combining ability (GCA) effect was more important than the specific combining ability (SCA) effect for the relative area under disease progress curve (rAUDPC), total tuber yield, marketable tuber yield and average tuber weight, indicating that the expression of these traits was controlled by additive gene action. The parents with good GCA effects for late blight resistance were: 396264.14, 395109.34 and 396004.263. The first two contributed towards high tuber yield. Crosses from 396004.263 x 395017.229, 395096.2 x 396012.288 and 395109.7 x 396264.14 were best specific combiners for late blight resistance. Thirty-two potato families derived from crosses of two sets of 16 parents and 17 clones were field evaluated for yield and drought related traits in a 7 x7 lattice design with two replications under well-watered and managed drought stress conditions. Results revealed significant differences among genotypes for drought stress tolerance, growth, physiological and yield related traits. Significant GCA and SCA effects were detected among parents and crosses, respectively. The GCA effects were more important than the SCA effects for total tuber yield, marketable tuber yield, average tuber weight, plant height, chlorophyll content and groundcover, suggesting the predominance of additive over non-additive gene action for these traits under drought stress. The best general combiner parents for yield and drought tolerance were clones 395112.32, 396034.103 and 396012.288. The families with the best SCA effects for both tuber yield and drought tolerance were 395109.34 x 396041.102, 395096.2 x 396012.288, 395109.7 x 395017.14 and 396031.108 x 395017.14. Eighteen selected clones phenotyped for drought tolerance and late blight resistance were genotyped using 23 polymorphic SSR markers to determine genetic distance and to select suitable parents for breeding. Results showed the presence of wider genetic diversity among the tested clones. Pair-wise estimates of genetic similarity ranged from 0.26 to 0.52 with the mean of 0.35. Ninety-five alleles were amplified and polymorphic alleles per locus ranged from 3 to 7 with a mean of 5. The mean polymorphic information content (PIC) values, observed heterozygosity and expected heterozygosity were 0.62, 0.78 and 0.68, respectively. The genotypes were clustered into three distinct groups. The following clones were selected from each cluster: 396029.25 from cluster I, 396038.107, 396038.101 and 395112.32 from cluster II, and 395017.229 and 395109.34 from cluster III, for drought tolerance and late blight resistance breeding. In summary, the study demonstrated the existence of genetic variability among the tested clones for late blight resistance, drought tolerance and yield and related traits. The study identified promising potato genotypes with high combining ability for tuber yield, late blight resistance and drought tolerance. The selected parents and families will be further evaluated for release in the highlands of north-western Ethiopia or similar environments in sub-Saharan African countries