Combining ability and mode of gene action in cassava for resistance to cassava green mite and cassava mealy bug in Malawi


Michael Malandula Chipeta, James M. Bokosi, Vincent W. Saka, and Ibrahim R. M. Benesi

The need to develop new improved varieties resistant to cassava green mite (CGM) and cassava mealy bug (CMB) as well as high yielding varieties requires knowledge on the mode of gene action controlling the expression of resistant traits to these pests. Therefore, a study was carried out to determine combining abilities and mode of gene action for resistance to CGM and CMB by crossing cassava genotypes in a 7 × 7 diallel mating design. The F1 progenies were evaluated in a seedling trial laid in randomised complete block design (RCBD) with two replicates. Results indicated highly significant (P<0.001) general combining ability (GCA) and specific combining ability (SCA) mean squares for CGM, CMB, average root number per plant and fresh root weight (kg per plant) which signified preponderance of significant variability existing in breeding material attributable to additive and non-additive gene effects. GCA variances for all the traits studied were lower than SCA variances which suggested that non-additive gene action was more important than additive gene action in influencing the traits. Mulola was the overall best general combiner for all the traits, followed by TMS4 (2)1425 and 01/1316. The best ranking hybrids were Mulola × 01/1313, Silira × TMS4(2)1425, Depwete × Silira, Mulola × Silira, Depwete × Maunjiri and 01/1316 × TMS4(2)1425. Overall, the results suggested that when designing breeding programs breeders should exploit both additive and non additive types of gene action.


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