1. Evaluation of Namikonga-derived F1 cassava seedlings for resistance to Cassava Brown Streak Disease
V. Kyaligonza, R. Kawuki, M. Ferguson, Y. Baguma, T. Kaweesi , J. Orone, C. Omongo T. Alicai, A. Bua and P Vuzi GCP21-II Speke Resort Munyonyo-Kampala, Uganda June 2012

2. Introduction CBSD is a major threat to cassava production in Uganda
CBSD incidence and severity is high in CMD resistant varieties
Development of CBSD tolerant and or resistant cassava varieties is critical in boosting crop productivity
Screening germplasm in search for CBSD tolerance and or resistance
Namikonga has been identified as the best source of tolerance to CBSD (E Kanju & H Kulembeka, person communication)
The aim of this study is to generate four Namikonga-derived F1 families and assess their phenotypic response to CBSD under field conditions

3. Methodology Generation of Namikonga-derived F1 families Parents Parent
Sex
CBSD reaction
Namikonga
Male
Tolerant
MM96/4271
Female
Susceptible
TME 14
NASE 12
MH97/2961

4. Parental hybridization
TME 14 x Namikonga
MM96/4271 x Namikonga
NASE12 x Namikonga
MH97/2961 x Namikonga

5. Seeds from each female parent were bulked to form the respective F1 families

6. 1339 harvested F1 seeds were planted in a Nursery bed

7. SET comprising of 810 seedlings was established at Namulonge
Used the highly CBSD susceptible TME 204 as spreader rows
Shoot symptom data collected at 3, 5, 7 and 9 MAP while root necrosis data collected at harvest

8. CBSD foliar and root scoring scale4 1 2 3
1= no apparent symptoms,
2 = slight leaf chlorosis,
3 = severe leaf chlorosis, mild stem lesions,
4 =severe leaf chlorosis and severe stem lesions
5 = defoliation, severe stem lesions and dieback 5
1 = no apparent necrosis
2 = < 5 % of roots necrotic
3 = % of the roots necrotic
4 =10-25 % of the roots necrotic and mild root constriction
5=>25 % of the roots necrotic and severe root constriction

9. 488 genotypes advanced to Clonal evaluation trial
Single rows of 6 plants per genotype unreplicated

10. F1 seedlings evaluated at SET
Results
Table1: Generation of F1 families
Family
No. of Crosses
F1 Seeds generated
F1 Seedling s transplanted
F1 seedlings evaluated at SET
No. genotypes at CET
MM96/4271 x Namikonga
949
751
536
364
309
MH97/2961 x Namikonga
535
194 50 37 32
NASE x Namikonga
371
174 94 71 59
TME x Namikonga
410
220
130 99 88
Total
2265
1339
810
571
488

11. CBSD Foliar Severity and Incidence
Table 2: CBSD foliar severity and incidence
Family
No. of genotypes
CBSD Severity
Min Max Mean
Incidence (%)
MH96/4271 x Namikonga
183
± 0.04
58.5
MM97/2961 x Namikonga 11
± 0.19
63.3
TME 14 x Namikonga 58
± 0.07
69.0
NASE 12 x Namikonga 32
± 0.06
90.6

12. Mean CBSD foliar severity

13. Root severity Table 3: CBSD Root severity and incidence Family
No. of genotypes
CBSD root Severity
Min Max Mean
Incidence (%)
MM96/4271 x Namikonga
183
± 0.11
30.6
MH97/2961 x Namikonga 11
± 0.56
45.5
TME 14 x Namikonga 58
± 0.20
46.6
NASE 12 x Namikonga 32
± 0.19 25

14. Distribution of genotypes based on CBSD Root severity classes

15. Discussions
Hybridization efficiency varied among the parental combinations (lowest in MH97/2961 x Namikonga)
F1 progeny was highly segregating and expressed different CBSD symptom levels
Progeny of NASE 12 x Namikonga had the highest foliar incidence (90.6%) but the lowest root incidence (25%)
26.4% of the progeny were symptomless (no shoot and root symptoms)

16. Conclusion
Evaluation is based on single plant per genotype, this can be largely biased when evaluating traits of low heritability
For this reason, seedlings with a recommendable stem height were cloned for further evaluation
Given the high segregation observed in the F1 progeny, this makes the population suitable for genetic studies

17. THANK YOU FOR YOUR ATTENTION