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Tomato (Solanum lycopersicum L.) is the most important vegetable in Ghana in terms of area under cultivation and consumption, but production is challenged by lack of improved cultivars and the Tomato Yellow Leaf Curl Disease (TYLCD). In Ghana, tomato breeding programmes have focused on evaluation and screening of cultivars for fruit quality and resistance to the TYLCD but very little has been done to improve the crop through breeding. The objective of this study was to introgress TYLCD resistance genes into farmer preferred tomato cultivars. Tomato farmers were involved in the breeding programme through a Participatory Rural Appraisal in six tomato growing communities in the Ashanti, Brong Ahafo and Upper East Regions. Farmers identified TYLCD as the most important biotic stress and proposed that TYLCD resistance, high yield and shelf life must be prioritized in tomato improvement.
To identify tomato accessions with farmers’ preferred traits, diversity among 123 assembled germplasm was determined based on morphological traits valued by the fresh market and 348 SNP markers. The 123 accessions were evaluated in an augmented design with 11 accessions in each of the 11 blocks. The first five principal component explained 80% of the variation. Fruit shape, ribbing at peduncle end, fruit green shoulders, number of locules, growth type, shape at blossom end, fruit per plant, firmness, reproductive duration, yield and weight per fruit contributed to most of the variation. The accessions were grouped into three clusters with cluster I having 81 accessions, 37 accessions in cluster II and 1 accession in cluster III. A total of 338 SNP markers were polymorphic among 96 accessions. There was population overlap though major groupings was seen for accessions from PGRRI and UC Davis. The two most grown cultivars; Power Rano and Peto Mech were clustered with the improved accessions. To identify TYLCD resistant accessions, specific SCAR and SSR markers linked to the known TYLCD resistance genes were used to amplify the presence or absence of these genes in 21 tomato accessions. The accessions were further screened in TYLCD disease hot spot in a randomized complete block design at Akumadan in the Ashanti Region and Vea in the Upper East Region. The genes ty-5 and Ty-6 were discovered in accession GH9233 (Pimplifolium) and the Ty-6 gene was found in Pimpinellifolium x Wosowoso. Pimplifolium expressed high level of resistance to TYLCD at both Akumadan and Vea.
To identify lines with good general combining abilities and specific crosses that combine well for fruit quality and yield; a cross was made between 5 locally adapted cultivars with three exotic lines and Pimplifolium each carrying two of the six TYLCD resistance genes, following North Carolina II mating design. The generated 20 F1s were evaluated in the field in a randomized complete block design with three replications. GCA was significant for all traits studied. Peto Mech was a good combiner for fruit quality such as fruit length, pericarp thickness, firmness and hardness. Pimplifolium x Rano had the highest significant SCA estimate for fruit per plant. Lorry Tyre contributed positively to fresh tomato yield, having the highest yield per plant with AVTO1311 (252.75 g) and AVTO1219 (173.79 g). To confirm the presence of the TYLCD resistance genes, the specific SCAR and SSR markers were used to genotype the F1 plants. The observed heterosis for yield and fruit quality together with the TYLCD resistance offer opportunities for the development of new hybrids.
Twelve F2 populations from the F1s were studied for segregation pattern for fruit quality, yield and TYLCD resistance genes. Stem colour and fruit green shoulders followed the 3:1 segregation ratio but fruit quality traits and yield were quantitatively inherited. Screening the F3 progeny in a disease hotspot should unravel the segregation pattern of the TYLCD resistance genes.