1. T. L. Munga, R. Melis, P. Shanahan and M. Laing
Screening germplasm for resistance to cassava brown streak disease in coastal Kenya By
T. L. Munga, R. Melis, P. Shanahan and M. Laing

2. Introduction: Cassava in Coastal Kenya:
Constitutes 30% of the country total production
2nd most important food crop after maize
Important food security crop
A main source of income for many communities
Production low 5-9 t ha partly due to CBSD
Breeding for CBSD resistance is hampered by lack of resistant parental lines

3. Objective: To screen germplams for resistance to CBSD

4. Materials and methods
64 cassava genotypes screened in the field after mechanical inoculation with CBSV by grafting infected scions at 2 sites in 8 * 8 simple lattice design with 2 replications
Incidence and severity of above ground symptoms was rated at 6
Incidence and severity of root necrosis rated at 12 MAP
Data analyzed as row x column design to adjust for the heterogeneity of the plots across the experimental site

5. Five genotypes with lowest incidence of CBSD across sites (S) & rating time (RP):
6 MAP
12 MAP
Mean
Overall
Rank
5632/8
64.3
42.2 30 31
42.0 1
Gushe
80.0
54.7
15.7
21.9
43.1 2
Plot19
34.3
51.6
42.9
53.2
45.5 3
5043/11
60.0
50.0
32.9
40.7
45.9 4
Kwl199
57.8
40.0
43.8
51.5 5
Overall Mean
80.8
69.9
56.9
56.7 66
LSD0.05 G
8.29
LSD0.05 RP
1.47
LSD0.05 S
LSD0.05 G x RP
11.72
LSD0.05 RP x S
2.07
LSD0.05 G x RP x S
16.58
CV (%)
0.40

6. Five genotypes (G) with lowest severity of CBSD across sites (S) & rating time (RP):
6 MAP
12 MAP
Mean
overall
Rank overall
5312/22
3.00
2.50
2.75 1
Kwl155 2
Mwakazanga
3.25
3.13
3.06 4
Kwl200
3.50 5
3.63
3.55
3.59
3.61
LSD0.05 G
0.38
LSD0.05 RP
0.07
LSD0.05 Gx RP
0.54
CV (%)
0.29

7. Five genotypes (G) with lowest incidence of root necrosis across sites (S) & rating time (RP):
6 MAP
12 MAP
mean 6 12
rank
5312/22
0.0
1.6
0.8
0.4 1
Mld119
1.3
0.6
0.9
0.5 2
Pyt336
1.0
Trn43
2.1 4
Msa140
4.0
2.0 5
Mean
13.4
8.7
11.0
12.1
11.4
11.8
LSD0.05 G
14.94
LSD0.05 RP
2.64
LSD0.05 S
LSD 0.05 RP x S
3.74
LSD0.05 G x S
21.13
LSD0.05 G x RP
LSD0.05 Gs x RP x S
29.88
CV (%)
0.60

8. Five genotypes (G) with lowest incidence of root necrosis across sites (S) & rating time (RP):
6 MAP
12 MAP
mean 6 12
rank
5318/3
1.0
2.0
1.5
1.3 1
Kwl160
Msa140
12701
3.0 5
50298/21
2.5
1.8
Mean
1.73
2.83
2.28
1.63
3.62
2.44
2.36
LSD0.05 G
0.38
LSD0.05 RP
0.07
LSD0.05 G* RP
0.54
CV (%)
0.80

9. ICBSD SCBSD IRN -0.01 -0.21 0.22 SRN 0.07 0.25* 0.67**
Correlations between symptoms of cassava brown streak disease
ICBSD
SCBSD
IRN
-0.01
-0.21
0.22
SRN
0.07
0.25*
0.67**

10. Conclusion
Over the sites and rating periods none of the genotypes had resistance to root necrosis. However, three genotypes, 5318/3, Kwl160, and Msa140 had high field tolerance (lowest mean severity scores) to root necrosis and may be used as new sources of CBSD root necrosis resistance
High severity of root necrosis would be associated with high incidence of root necrosis but not always with high severity of above ground CBSD

11. Acknowledgements
KARI
Rockefeller Foundation
HarvestPlus