Despite the significance of cassava as food, feed and industrial root crop, little is known regarding the gene action determining root dry matter content (RDMC), fresh root yield, and tolerance to cassava mosaic disease (CMD), cassava green mite (CGM), and cassava mealy bug (CMB). Thus, a study was conducted to determine the general and specific combining abilities for disease, pest, RDMC, root yield and related traits by crossing 10 parents in a 6 × 4 line by tester design. The F1 progenies and their parents were assessed in-field in a randomized complete block design (RCBD) with three replicates. Findings implied sufficient genetic variability for all traits studied. Family TMEB419×IBA030305 had the highest RDMC of 35.47%, whilst family TMEB7×IBA0000203 had the least RDMC (23.87%). Genotypes IBA020588, IBA916132 and TMEB419 were the best parents for improvement of harvest index (HI) and RDMC due to its high positive and significant GCA effects. Genotype IBA000203 contributed the highest to increased plant height, whereas TMEB1, TMEB47 and ZAR010116 had significant negative GCA effects. ZAR010116 was the best tester for HI. Families TMEB778×ZAR010116 (34.23) and IBA020588×ZAR010116 (32.78) were the best performing families for mean RDMC, with parent ZAR010116, exhibiting the highest GCA effect for RDMC. Families TMEB419×ZAR000156, IBA916132×ZAR000156 and IBA020588×IBA000156 had low mean CMD scores of 1.1, 1.2 and 1.2, respectively. The preponderance of non-additive gene actions indicated that selection of superior plants should be postponed to later generation.
Key words: Cassava, genetic improvement, agronomic traits, combining ability, heritability, progeny performance.
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