Breeding potato (Solanum tuberosum L.) for high yield and resistance to late blight in Rwanda.
Potato (Solanum tuberosum L.) is one of the most important food crops in the world, and in developing countries its production has increased over the last decade. Potato has a high content of carbohydrates as a source of energy, significant amounts of quality protein, and substantial amounts of vitamins, especially vitamin C. In Rwanda, it is grown throughout the country and its importance is expanding considerably. Despite the increase of both cultivated area and production, potato productivity remains low in Rwanda with a national mean yield of 13.6 t ha-1. Late blight disease caused by Phytophtora infestans (Mont.) Anton de Bary is one of the most important limiting factors to potato productivity in the country. The overall goal of this study was therefore to develop high yielding and late blight resistant potato cultivars in Rwanda. The specific objectives were; (i) to identify and analyse farmers‟ key constraints in potato production, and establish farmers‟ preferred traits to be included in cultivar development and variety selection in Rwanda, (ii) to determine yield response and late blight reaction of potato genotypes in Rwanda in order to identify suitable parents for breeding, (iii) to assess the genetic relationship and divergence among potato genotypes grown in Rwanda using SSR markers so as to identify suitable parents for crosses, (iv) to estimate combining ability effects for late blight resistance, yield and yield related traits and to estimate heterosis for yield in potato, and (v) to select the best potato clones for further evaluation and release. A participatory rural appraisal (PRA) study was conducted through a structured survey involving 144 households and 22 focus groups selected from Musanze, Gicumbi and Nyamagabe districts of Rwanda. The structured survey used a questionnaire administered to farmers to collect information on the importance of potatoes and other main crops. Focus group discussions used matrix scoring of key production constraints and pair-wise ranking of traits. The most important potato production constraints were lack of access to credit, lack of high yielding cultivars, insufficient clean seeds and late blight disease. High yield, disease resistance and high dry matter content were the most important attributes preferred by farmers. A total of 44 potato genotypes were evaluated under three environments (Kinigi, Rwerere and Nyamagabe) in Rwanda to select high yielding and late blight resistant parents. Experiments were laid out in an 11 x 4 alpha lattice design with two replications. Data were collected on late blight severity (%) based on the relative area under the disease progress curve (rAUDPC: 100% max), total tuber yield, marketable tuber weight and dry matter content. Genotypes had significant differences for blight resistance and yield levels among test locations. Eighteen genotypes (CIP 393371.58, CIP 393637.171, CIP 396033.102, CIP 395112.36, CIP 393280.57, CIP 393385.39, CIP 396026.103, CIP 393280.82, CIP 396036.201, CIP 393077.54, CIP 391047.34, CIP 39111.19, CIP 381381.13, Ngunda, Kigega, Kirundo, Nderera and Gikungu) were selected showing positive combinations of quantitative and qualitative traits such as late blight resistance, high tuber yield, dry matter content and productive flowers. Evaluation of genetic relationship and divergence of the 18 selected genotypes were conducted using 13 simple sequence repeat (SSR) markers for an efficient choice of parents for breeding. The 13 SSR primers identified 84 alleles across all genotypes. The number of alleles per locus ranged from 3 to 10 and the average was 6.5. The polymorphic information content (PIC) of loci ranged from 0.51 to 0.85 with an average of 0.71. Heterozygosity (He) varied from 0.59 to 0.86 with an average of 0.75. Significant positive correlations were detected between PIC and He (r= 0.99), PIC and number of alleles (r=0.76) and, He and number of alleles (r=0.80). The genetic distance between clones ranged from 0.44 to 0.93 and the average was 0.75. The SSR analysis provided five different genetic clusters of the potato clones useful for breeding. Cluster I consisted of clone CIP 393357.58 standing alone. Cluster II composed of six genotypes: CIP 393637.171, CIP 393385.39, CIP 396026.103 and CIP 395112.36, Nderera, and Gikungu. Cluster III allocated five clones: CIP 396033.102, CIP 393280.82, CIP 391047.34, CIP 396036.201 and CIP 393077.54. Cluster IV included three genotypes; 39111.19, 381381.13 and Kirundo, while cluster V consisted of two varieties Ngunda and Kigega. The genetic distance between clones ranged from 0.44 to 0.93. The shortest genetic distance (0.44) was found between Ngunda and Kigega whereas the highest distance at (0.93) was identified between clone CIP 393357.58 and Ngunda. Among the 18 genotypes, clone CIP 393357.58 was the least genetically related to the other genotypes. Overall, results showed that the thirteen microsatellite markers clearly distinguished all the eighteen potato genotypes. Nine genotypes CIP 393357.58, CIP 391047.34, CIP 393385.39, CIP 393280.82, CIP 396036.201, Gikungu, Ngunda, Kigega and Nderera were therefore identified as promising parents for subsequent crosses. The selected potato parents were crossed using a 10 x 10 half diallel mating design to generate 45 F1s. Only 28 families with sufficient individuals and the eight parents were 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 analyses. Results showed that across sites additive and non-additive gene action were present affecting yield and late blight resistance in potato. Additive was predominant over non-additive gene action for both traits. All the families and their F1 progenies selected for further evaluation had improved levels of late blight resistance, high yields and heterosis. The study identified ten top families (Gikungu x CIP 391047.34, Giukungu x CIP 393036.201, Kigega x CIP 393036.201, Kigega x CIP 393280.82, Gikungu x CIP 393385.39, CIP 393280.82 x CIP 391047.34, Nderera x CIP 393036.201, Ngunda x CIP 393280.82, Ngunda x CIP 391047.34, Gikungu x Ngunda) expressing high tuber yield and resistance to late blight. Moreover 58 and 46 promising clones were identified at Kinigi and Nyamagabe respectively for further clonal evaluation and variety release in Rwanda. Overall, the current study selected valuable potato genotypes with high combining ability for late blight resistance and tuber yield.
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