SCREENING OF GENERATED PARTIAL INBREDS FOR RESISTANCE TO CASSAVA BROWN STREAK DISEASE

IN UGANDA

By Kaweesi1T, Kawuki1 R, Baguma1 Y, Kyaligonza1 V,

Ferguson2 M1National Crops Resources Research Institute

2International 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, 130040 and 182/00661 from IITA and

Namikonga, TZ/140, TZ/130 and kigoma Red from Tanzania) were selected as S0

• Seeds generated (table 1) were planted and seedlings were evaluated for 9 months then cloned for final evaluation

• Field evaluation of S1 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

-3 -2 -1 Grand mean 1 2 3

HR R MR MS S HS

Fig 1: Resistance categories of partial inbreds as determined by deviation (d) from the grand mean

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 Table 1: Number of S1 seeds generated, seedling established and clones generated

S0 Seeds generated Seedling Planted clones generated clones established

182/00661 79 40 24 7

Kigoma 60 40 20 11

TZ/130 123 90 79 67

I00142 396 280 160 22

130040 353 200 104 56

Namikonga 123 60 46 29

TZ/140 25 17 11 8

0040 418 200 100 46

According to table 1, family 100142, 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

Table 2: Relative resistance of generated cassava partial inbreds to cassava brown

streak disease as determined by rank sum method and AUDPC

S1 DI Rdi ISS RISS RS d RC AUDPC d2 RC

182/00661 5 8 1.87 7 15 1.63 MS 284.3 1.34 MS

Kigoma 0.36 7 2.14 8 15 0.99 MS 243 0.54 MS

TZ/130 0.19 6 1.48 6 12 0.30 MS 237.6 0.27 MS

I00142 0.17 4.5 1.4 5 9.5 0.06 MS 205.4 0.26 MS

130040 0.17 4.5 1.22 3 7.5 0.01 MS 202.4 -0.12 MR

Namikonga 0.08 2 1.23 4 6 -0.34 MR 196.7 -0.81 MR

TZ/140 0.1 3 1.1 2 5 -0.72 MR 180 -97 HR

0040 0.03 1 1.09 1 2 -1.1 R 186.5 -1.14 R

DI – Disease incidence ISS – Index of symptom severity d – Deviation from grand mean of the rank sum RC –

Resistance category MS – Moderately susceptible MR – Moderately resistant R – Resistant HR – Highly

resistant d2 – Deviation from the grand mean of the rank score for AUDPC RS -Rank sum Rdi - Rank score of

disease incidence RISS - Rank score of index for symptom severity

Frequency distribution of CBSD root necrosis reaction among family OO40, Tz/140, Namikonga and 130040

Frequency distribution within each family

Frequency distribution of CBSD root necrosis reaction among family 100142, Tz/130, Kigoma Red and 182/00661.

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 130040 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

Table 3: Comparison between frequency distribution based on the number of genotypes with root

necrosis score 1 and Wilcoxon ranking

S0 No* No of S1 with score 1(%) Ranking Wilcoxon Ranking* RC

0040 83 92.8 1 1 R

TZ/140 10 90 2 2 MR

Namikonga 34 82 3 3 MR

130040 101 78.2 4 4 MS

I00142 111 77.5 5 5 MS

TZ/130 80 61.3 6 6 MS

Kigoma 16 56.3 7 7 MS

182/00661 9 33.3 8 8 MS

*Wilcoxon ranking and RC- Resistance category as obtained from table 2, *No – number of S1 that had roots to evaluate

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 S1 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 S1 and S0 in a Clonal evaluation trial is ongoing to evaluate the impact of inbreeding on CBSD resistance