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 18-22 June 2012

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

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

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

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

1339 harvested F1 seeds were planted in a Nursery bed

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

CBSD foliar and root scoring scale 4 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 = 5 - 10 % 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

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

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 12 x Namikonga 371 174 94 71 59 TME 14 x Namikonga 410 220 130 99 88 Total 2265 1339 810 571 488

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 1 3 1.62 ± 0.04 58.5 MM97/2961 x Namikonga 11 1 3 1.73 ± 0.19 63.3 TME 14 x Namikonga 58 1 3 1.74 ± 0.07 69.0 NASE 12 x Namikonga 32 1 3 1.94 ± 0.06 90.6

Mean CBSD foliar severity

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 1 5 1.82 ± 0.11 30.6 MH97/2961 x Namikonga 11 1 5 2.55 ± 0.56 45.5 TME 14 x Namikonga 58 1 5 2.12 ± 0.20 46.6 NASE 12 x Namikonga 32 1 5 1.50 ± 0.19 25

Distribution of genotypes based on CBSD Root severity classes

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)

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

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