SCREENING OF GENERATED PARTIAL INBREDS FOR RESISTANCE TO CASSAVA BROWN STREAK DISEASE IN UGANDA By Kaweesi 1 T, Kawuki 1 R, Baguma 1 Y, Kyaligonza 1 V, Ferguson 2 M 1 National Crops Resources Research Institute 2 International Institute for Tropical Agriculture

Introduction The search for durable resistance to CBSD in cassava through conventional means is still a challenge in all CBSD affected areas This can be attributed to the high heterozygosity due to its outcrossing nature. As a result, the crop has accumulated a high genetic load that limits some of its traits from full expression especially traits controlled by recessive traits and quantitative trait loci. According to Kulembeka (2010), resistance to CBSD is quantitative and therefore is more controlled by additive genetic effects than dominant effects.

Introduction Cont’ns According to Walsh (2005), inbreeding allows “concentration” of desirable genes originally present in the elite clone. Inbreeding forces an average of half of the loci to become homozygous, the additive value in a generated inbeds is thus increased Against this premise, this study was initiated to generate new sources of resistance to both CBSV and UCBSV through inbreeding

Generation of partial inbreds at NaCRRI Eight cassava genotypes ( 0040, I00142, and 182/00661 from IITA and Namikonga, TZ/140, TZ/130 and kigoma Red from Tanzania) were selected as S 0 Seeds generated (table 1) were planted and seedlings were evaluated for 9 months then cloned for final evaluation Field evaluation of S 1 for CBSD was done using a severity score of 1-5 (Gondwe et al., 2002) Foliar data was take at 3, 5,7 and 9 month while root data taken at harvest (9 MAP) Wilcoxon ranking, ranksum and AUDPC were used in analysis of both foliar and roots data

Generation of S1 at NaCRRI Generation of seedling at NaCRRI Field establishment of seedling at NaCRRI

Data analysis DI – Ratio of diseased to total number of roots Rank-sum = Rank of DI + Rank of ISS Analysis of root data Analysis of foliar data Resistance Categorization

Results According to table 1, family , 182/00661 and 0040 were greatly affected by CMD and inbreeding depression causing a loss of 94.4%, 91.1% and 88.9% (Personal observation)

Determination of resistance levels of generated partial inbreds using rank sum and AUDPC

Frequency distribution of CBSD root necrosis reaction among family OO40, Tz/140, Namikonga and Frequency distribution within each family

Frequency distribution of CBSD root necrosis reaction among family , Tz/130, Kigoma Red and 182/ Frequency distribution within each family

Number of genotypes per family generated with root necrosis score 1 According to the graphs, the distribution among 0040, TZ/140, Namikonga and was more skewed on the left There was segregation among Kigoma Red and 182/00661 Family 0040, Tz/140 and Namikonga had the highest number of genotypes with root necrosis score 1 (92.8%, 90% and 82%) Family 182/00661 and Kigoma Red had the lowest number of genotypes with root necrosis sev score 1 (33.3% and 56.3%)

Comparison of frequency distribution based on root score 1 and Wilcoxon ranking

Challenges Male sterility among some genotypes Male sterility in Nachinyaya and kiroba limited generation of partial inbreds among these genotypes

Conclusions This study has demonstrated that inbreeding can be used in the generation of new sources of resistance to CBSD. The S 1 generated have been screened in a “hotspot” for both UCBSD and CBSD therefore can be used as parental genotypes in different breeding programs. This study has also demonstrated that Wilcoxon ranking, rank sum and AUDPC can be used together to combine foliar, root necrosis and disease incidence in assessment of resistance to CBSD Comparison of S 1 and S 0 in a Clonal evaluation trial is ongoing to evaluate the impact of inbreeding on CBSD resistance