Abstract:

Late blight, caused by Phytophthora infestans, is a major disease limiting potato yield and productivity in Uganda, especially in the highlands, and accounts for up to 70% of the yield losses and sometimes-even results in a total crop destruction. Breeding for host plant resistance is a sustainable approach to late blight control and management in Uganda, as other measures are expensive, ineffective or not easy to deploy. Additionally, the development of early maturing varieties are imperative to address the changing weather patterns and crop adaptation in different agroecologist overall goal of this study was to contribute to food security in Uganda through developing high yielding and early maturing potato genotypes that are resistant to the late blight disease. The specific objectives of the study were to: (1) document farmers’ knowledge of preferred traits in potential new varieties and their perspectives on late blight prevalence and severity, (2) phenotypically characterize potato genotypes in Uganda, (3) assess the genetic diversity among potato genotypes using SSR markers, (4) determine the yield response of potato genotypes to late blight disease in the tropical highlands of Uganda, (5) determine the combining ability effects for yield, yield related traits, resistance to Phytophthora infestansand early maturity in potato genotypes. A participatory rural appraisal (PRA) involving 577 individual farmers showed that farmers’ practices in potato production and disease management varied greatly. Most of the respondents used farm saved seed, while only 2% from Eastern and South-western Uganda obtained seed from research stations. The major pests were aphids and cutworms, while late blight and bacterial wilt were the prominent diseases. Commonly grown varieties were Rwangume and Victoria. The most preferred attributes in new varieties were high yield, resistance to late blight, early maturity and marketability, in that order. Late blight has been experienced by 98% of the farmers and 96% of these reported to have used fungicides to manage the disease. Phenotypic characterization of 48 potato genotypes in Kachwekano and Karengyere research stations showed significant sites differences (P < 0.01) among the genotype performances for all measured parameters. The cluster analysis using15 phenotypic traits grouped genotypes into three major clustersand the similarity distance ranged from 0.5 to 1.0. The mean tuber yield for the two sites was 29.8 t ha’1 and tuber yield was higher in Kachwekano than Karengyere. The best yielding genotypes were 396038.105 (54.5 t ha'1) and NAKPOT5 (50.9 t ha'1). Fifty two percent of the genotypes were high yielding (30 t ha'1 and above) and the most stable genotypes in terms of tuber yield were Rutuku, 395112.32, 395017.14 and 393220.54.Forty-eight genotypes, including advanced clones from populationB3C2of the International Potato Centre, commercial and farmers’ varieties, were evaluated under two environments for two seasons to determine their reaction to late blight in an 8 x 6 alpha lattice design with three replications. Genotypes showed significant differences in yield and resistance to blight. The most resistant genotypes were 395077.12 and 392657.8 with disease severity of 12% and 14%, respectively. The mean tuber yield under late blight infection was19.8 t ha'1 and the best yielding genotype across sites were 395112.32 (35.6 t ha'1). The following genotypes: 395112.32, 391919.3, 393220.54. 393077.54, 396038.107. 392657.8, Kinigi, 395014.17, NKRN59.58, NKRK19.17 and 395011.2 had high yield and high to medium resistance to Phytophthora infestans and thus were identified as promising parents for subsequent crosses. In addition, 20 selected tetraploid potato genotypes were characterized using 16 SSR markers to determine the pattern and level of genetic diversity amongst them to identify suitable parents for breeding purposes. The microsatellites showed considerable variation among genotypes and the 16 primer pairs amplified 64 alleles. The number of polymorphic alleles per locus ranged from 2 to 8, polymorphic information content (PIC) values from 0.0948 to 0.7832, while heterozygosity values ranged from 0.0997 to 0.805.A dendrogram was constructed using UPGMA clustering algorithm based on SSR data matrices, and this grouped the potato clones into three major clusters with a genetic distance of 1 .Oto 5.7.Combining ability effects for resistance to late blight, yield and yield related traits were determined using 12 potato genotypes were crossed in a North Carolina mating design ll(NCD ll)in two sets of six parents each to generate 18 families. Both additive and non-additive gene action-controlled yield and late blight resistance in potato. However, additive gene action was predominant over non additive for total tuber weight and late blight resistance. Broad-sense heritability estimates were 0.78 for total tuber weight and 0.68 for relative area under disease progress curve (rAUDPC). Parents Kinigi, 392657.8, 396034.103, 396038.107, 395011.2, NKRK19.17, NKRN59.58 and 395017.14 had good general combining ability (GCA) effects for both late blight disease resistance and yield related traits. Crosses 392657.8 x 395017.14 and 396038.107 x NKRN59.58 had the highest specific combining ability (SCA) effects for all the yield related traits, while families Kinigi x NKRK19.17and 392657.8 x NKRN59.41 had the lowest SCA effects for AUDPC.This study showed some evidence of maternal effects for rAUDPC (1.45) and ATW (1.56), although these were not significant at P5O.O5. The selected parents and families were the best candidates to develop improved potato varieties that combined both high yield and resistance to late blight. These will be subjected to further clonal evaluation before possible release. Combining ability effects for yield and yield related traits and earliness were also determined in a different set of 12 genotypes divided into two sets of six parents each and crossed in a North Carolina mating design II (NCDll)mating scheme to generate 18 families. Additive gene action predominantly controlled days to flowering, total number of tubers, total tuber weight and average tuber weight. Broad-sense heritability estimates were 0.70 for total tuber weight and 0.78 for days to flowering. Mean total tuber yield was 9.3 t ha’1, while the average number of days to flowering was 54. Parents Rwangume, 396038.107, 395011.2 and NKRK19.17 had desirable GCA effects for the number of days to flowering. For yield and related traits, parents 396038.107, 393077.54, Rwangume, NKRK19.17, Kimuri, and 392657.8 had desirable GCA effects. The selected parents had desirable attributes for high yield and early maturity and families will be subjected to further clonal evaluation and selection. Overall, this study documented the potato production constraints, farmers’ perceptions on late blight management and varietal attributes. It also identified potato genotypes with desirable combining ability for tuber yield, late blight resistance and early maturity. The nature of gene action controlling yield, late blight disease resistance and early maturity was also determined. The predominant control of additive genetic effects on most of the tested traits implies that advances in breeding can be made through population improvement methods and selection. Nevertheless, for traits with significant non- additive genetic effects, a hybridization breeding strategy could be employed to develop hybrids. In general, clones with these traits can be selected and evaluated further for eventual release as varieties in Uganda.