1. COMBINING ABILITY AND MODE OF GENE ACTION IN CASSAVA FOR RESISTANCE TO CASSAVA GREEN MITE AND CASSAVA MEALY BUG.
Michael M. Chipeta, J.M. Bokosi, V.W. Saka & I.R.M. Benesi
(University of Malawi, Bunda Collge of Agriculture)

2. PRESENTATION OUTLINE Introduction Objectives Materials and Methods
importance of cassava
production constraints
Objectives
Materials and Methods
Results and Discussion
Conclusions and Recommendation

3. Introduction
Cassava (Manihot esculanta Crantz) is a primary staple food crop for more than 800 million people in the world (Lebot, 2009)
In Malawi- a staple food crop for almost 30-40% of the population (Moyo et al., 1998)
Contains 25 to 35 % starch; leaves used as vegetables (Nweke et al., 2002; Moyo et al., 1998; FAO, 1993; Hahn, 1988)
Pellets from cassava are used in manufacturing animal feed
Industrial crop-its starch used in food industries and in the textile paper industry (Githunguri, 1991; Silvestre, 1989)

4. Introduction
Cassava production is constrained with a number of factors which include insect pests.
Among the arthropod pests reportedly causing crop losses in Africa are cassava green mite (CGM) & cassava mealy bug (CMB) (Bellotti, 2002; IITA, 1990).
Fresh root yields reductions due to CGM attack ranges from 15% (resistant cultivars) to 73% (susceptible cultivars ) (Bellotti, 2002).
Yield loss due to CMB on susceptible cultivars can reach as high as 88% (Bellotti et al., 1987).

5. Introduction
Cassava being predominantly grown in traditional farming systems, pest control is often a low priority.
Continual use of pesticides is not feasible for low income small scale farmers & adverse effects on environment
Therefore, host plant resistance has been advocated for the control of pests and reduce their populations below economic injury levels

6. Objectives
In Malawi, virtually no genetic studies on cassava have been done to determine CA & the mode of gene action controlling the expression of CGM and CMB traits
The objective of this study was to estimate CA and mode of gene action controlling the expression of CGM and CMB traits in cassava.

7. Materials and Methods Plant materials
A total of 21 F1 crosses and 21 reciprocals derived from a 7×7 diallel mating design were the genetic material used in the study.
Seedling nursery
Seeds sown in Nov & seedlings transplanted in Jan. Chitedze Research Station (Malawi)
spacing of 90 cm × 50 cm. Using RCBD & 2 reps.
Parents (Mature stems 25 cm long) were also planted at the same spacing but 20 plants per genotype of each parent.
No irrigation and fertiliser were applied at this stage.

8. Materials and Methods
Individual plants in each F1 cross and seven parental stands were assessed for their reaction to CGM and CMB due to natural infection at 3, 6, 9 and 12 MAP.
The assessment was based on the standard 5 point scoring scale for CGM and CMB.
where 1 indicates no obvious symptom and 5 indicates severe symptoms (IITA, 1990).
At harvest, 12 MAP, the individual plants were assessed for their number of storage roots per plant and root weight (Kg/plant)

9. Materials and Methods
Data was analysed for combining ability variances, estimates of genetic parameters and heritability and phenotypic correlations using Agrobase 2000.
CMB severity indices were transformed [square root (x+3/8)] before performing the analysis.
GCA effects and SCA effects were estimated using Griffing (1956) model 1 of method 3

10. Results and Discussion

11. Table 1. Mean square values and estimates of genetic components in cassava for CGM, CMB, ARN and FRW

12. Table 2. Mean and GCA effects of genotypes for CGM, CMB, root number and root weight.  

13. Table 3. Mean and SCA effects of the crosses for CGM, CMB, Root number and Root weight (Kg plant-1)

14. Table 3. Continued

15. Severity symptoms
Cassava green mite
Cassava mealy bug

16. Conclusions and Recommendation
The best rated general combiners based on GCA effects were Mulola for CGM, CMB, average root number, fresh root weight; 01/1316 and TMS4(2)1425 for CGM and average root number.
Therefore, a multiple crossing programme involving these parents is recommended to select superior genotypes with desired traits.
The best ranking hybrids based on significant mean performance and SCA effects were Mulola × 01/1313, Silira × TMS4 (2)1425, Depwete × Silira, Mulola × Silira, Depwete × Maunjiri and 01/1316 × TMS4 (2)1425.

17. Conclusions and Recommendation
The results suggest that both additive and non-additive gene actions should be considered when designing a breeding program.
The study has also demonstrated that cytoplasmic effects are important in the genetic control of the traits under investigation.
Therefore, the choice of the female parent to be included in the breeding program is critical.

18. THANK YOU VERY MUCH FOR YOUR ATTENTION