Abstract
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. Key words: Cassava green mite, cassava mealy bug, general combining ability, gene action, specific combining ability.
Highlights
Cassava (Manihot esculenta Crantz) is a primary staple food crop for more than 800 million people in the world (Lebot, 2009), in the developing countries where it is an essential energy source for many resource constrained people who face problems of food availability (Opeke, 1991; Axtell and Adams, 1993)
In Malawi, it is a staple food crop for almost 30 to 40% of the population in addition to providing income (Moyo et al, 1998). It is a multi-purpose root crop containing 25 to 35% starch; leaves are used as vegetables (Hahn, 1988; FAO, 1993; Moyo et al, 1998; Nweke et al, 2002); pellets from cassava are used in manufacturing animal feed; its starch is used as sweeteners, thickeners and used in the textile paper industry (Silvestre, 1989; Githunguri, 1991)
A crossing block was established at Chitedze Research Station in 2009 where seven parents (Mulola, 01/1313, Depwete, 01/1316, Silira, TMS4 (2)1425 and Maunjiri) were crossed in a diallel mating design based on the Griffing’s (1956) method three
Summary
Cassava (Manihot esculenta Crantz) is a primary staple food crop for more than 800 million people in the world (Lebot, 2009), in the developing countries where it is an essential energy source for many resource constrained people who face problems of food availability (Opeke, 1991; Axtell and Adams, 1993). In Malawi, it is a staple food crop for almost 30 to 40% of the population in addition to providing income (Moyo et al, 1998). It is a multi-purpose root crop containing 25 to 35% starch; leaves are used as vegetables (containing significant amount of protein, iron and other nutrients) (Hahn, 1988; FAO, 1993; Moyo et al, 1998; Nweke et al, 2002); pellets from cassava are used in manufacturing animal feed; its starch is used as sweeteners, thickeners (in food industries) and used in the textile paper industry (Silvestre, 1989; Githunguri, 1991).
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