Collection and characterization of local cassava germplasm from the Brong - Ahafo Region
About 200 local cassava germplasm were collected from four districts viz. Asunafo, Wenchi, Nkoranza and Dormaa in the Brong - Ahafo region of Ghana. The germplasm were assembled at Wenchi in the same region and characterized based on a number of morphological and agronomic characteristics. A cluster analysis was undertaken to ascertain the relationship between the accessions. Molecular characterization using Rapid Amplified Polymorphic DNAs (RAPDs) was applied to further characterise a representative sample of 50 accessions from the different clusters. The results indicated that certain characters were predominant which gave the indication that over the years, farmers have selected their varieties based on certain important criteria which included the following. (i) For vegetative characters, farmers had selected varieties with purple petiole colours. (ii) To suit their intercropping farming systems, farmers had selected for types that had only two or three levels of branching. (iii) The root characteristics that appeared to have been selected for by farmers included: Cultivars that spread their tubers horizontally just beneath the soil surface and possessed root stalk or “neck”, root tubers with pink colour of outer surface of root cortex, smooth rather than constricted root tubers with acceptable cooking quality. (iv) Disease resistance did not appear to have been seriously considered in the selection process by farmers. The cluster analysis produced five clear and easily identifiable groups at the first stage of classification based on the morphological data. There were some relationship between the clusters and the geographical distribution of the germplasm. Accessions collected from the forest ecozone (Dormaa and Asunafo districts) dominated in Clusters III, IV and V, while those from the forest-savanna transition ecozone dominated in Cluster I. A phylogenetic tree (dendrogram) generated using UPGMA hierarchical cluster based on the genetic similarity estimates produced six clusters at the 67% Jaccard similarity coefficient level from the molecular characterisation. Accessions from the forest ecozone dominated in Cluster II, III and V. Clusters I, IV and VI consisted of one accession each. With the exception of AFISIAFI, all the other three remaining IITA improved varieties including the mutant TEKBANKYE, which also has IITA background appeared again in the same cluster (ie. Cluster II) at the second level of classification. ABASAFITAA and TEKBANKYE were in sub-group b1, GBLEMODUADE in subgroup b2 of Cluster II, while AFISIAFI was the only variety that constituted Cluste. This study has shown that germplasm collection and characterization, firstly, leads to identification of promising cultivars farmers have selected for over the years. Secondly, it reveals farmers’ selection indices which assist breeders in designing future breeding programmes to address the needs of farmers. The study has also indicated the benefit of characterising germplasm at the morphological level before proceeding to further characterise at the molecular level.
A thesis submitted to the School of Graduate Studies, Kwame Nkrumah University of Science and Technology in partial fulfilment of the requirements for the award of Master of Science degree in Plant Breeding, 2001