Assessment of genetic diversity, combining ability, stability and farmer preference of cassava germplasm in Ghana

The aim of this study was to assess genetic diversity, combining ability, heterosis, heritability of traits, stability and farmer preference of cassava germplasm in Ghana in order to structure an effective breeding programme. The assessment of genetic diversity in a collection of local cassava genotypes using 33 SSR markers revealed a high overall heterozygosity. The SSR markers identified 63 duplicates in the collection. The heterotic pools were distinct for the savannah and the forest ecological zones and this was the basis for the selection of progenitors for the rest of the study. Seven and five cassava progenitors were crossed in a diallel design without reciprocals in the forest and savannah ecologies respectively. The progenitors and progenies were evaluated for CMD, CBB, yield and its components in two seasons in the forest and one season in the savannah zone. The GCA:SCA ratio indicated that the GCA was larger than SCA for average root number, indicating additive gene effects and a possibility for improvement by selection. SCA was larger than GCA for harvest index and fresh root yield, indicating environmental effects on the trait and confirms the non-additive effects mainly determining expression of root yield. The parental genotypes were selected from a particular region where CMD is the main economic constraint. Debor had the best GCA effect for cassava mosaic severity reaction; while Tuaka had the best GCA effect for fresh root weight. Debor x Kwasea was the best combination for CMD resistance. Debor and Kwasea were the best parents for resistance breeding to CMD. The predictability ratio varied between 0.58 and 0.84. CMD and root number had a ratio close to one for the savannah and forest zones, showing that these traits were highly heritable and influenced by additive gene action. Marker assisted selection was used to search for new sources of CMD resistance in the progenies and progenitors. Crosses among CIAT elite genotypes and landraces revealed progenitor Dabodabo as a putative source of new genes to CMD resistance.