SANDRA ESI ODONKOR
DEVELOPMENT OF BACTERIAL WILT RESISTANT TOMATO (Solanum lycopersicum L.) IN GHANA
Abstract
Tomato (Solanum lycopersicum L.) is Ghana’s most important vegetable with regards to quantity consumed and land area cultivated. Albeit, tomato production is significantly affected by diseases, chiefly bacterial wilt disease and shortage of improved varieties. Breeding programmes have mostly focused on fruit quality and tomato yellow leaf curl (TYLCD) resistance while little to no work has been done to improve bacterial wilt resistance in the crop. The goal of this study was to introgress bacterial wilt resistance QTL in tomato varieties mostly preferred by Ghanaian farmers. Participatory Rural Appraisal was used to involve local tomato farmers from five tomato growing communities in the Upper East and Ashanti Regions. Farmers in the Upper East Region were mostly accustomed to the disease and identified it as the most important constraint to tomato production. They rated bacterial wilt resistance, yield and shelf life as the most important traits to consider when developing new tomato varieties. To determine R. solanacearum strains present on farmers’ fields, soil samples were collected from twenty-eight fields with a known incidence of the disease from seven communities in four tomato growing districts in the Upper East Region of Ghana and culture and molecular analysis using universal and phylotype-specific primers were carried out. Most (19) of the isolates showed the bacterium’s subunit flagellar filament, which helps with motility and virulence. Out of the four known phylotypes, phylotypes II (America) and III (Africa) were found present on farmers’ fields making this study the first report of phylotype II in Ghana. Additionally, variants of phylotypes I and IV were found on farmers’ fields. To identify tomato accessions with superior traits, a germplasm collection comprising 94 accessions was assembled. However, due to low seed count and low viability of certain lines only 44 accessions thrived and progressed through the experiment through to data analysis for diversity studies. Diversity among the germplasm collection was determined using morphological traits and 2201 single nucleotide polymorphism (SNP) markers. The accessions were evaluated in a completely randomized design with three replications in a greenhouse. The first five principal components explained 98% of the variation with reproductive days, days to maturity, days to first fruit set, fruit per plant, yield and brix contributing to most of the variation. The accessions were grouped into four clusters and an outlier when diversity was determined using morphological traits and three clusters using 2210 SNP markers. The two widely grown Ghanaian cultivars Peto Mech and Tropimech clustered together with both morphological characters and SNPs. To identify bacterial wilt resistant accessions among both local and exotic germplasm collected, genetic characterization using two fluorescent single sequence repeats (SSR) and two SNP markers coupled with greenhouse screening for disease response to an African R. solanacearum phylotype strain at hot temperatures were employed. Two resistant sources; Hawaii7996 and AVTO1314 were identified. Subsequent crosses between Hawaii7996 and three Ghanaian lines, Woso woso, Peto Mech and Tropimech, resulted in three F2 populations. After characterizing these F2 populations with the same genetic markers and greenhouse screening, 11 resistant lines with homozygous alleles for both QTL were selected. This study efficiently introgressed the two Bwr QTL into three Ghanaian lines. Tomato breeders can exploit these selected lines in breeding programmes to develop and release bacterial wilt-resistant tomato varieties.