This study evaluated common bean populations for resistance to common bacterial blight (CBB) through phenotypic and genotypic screening. Phenotypic screening identified 43 lines as resistant to CBB. Genotypic screening found that 28 resistant lines possessed the SU91 marker, while 2 lacked both markers tested and 13 had a combination of SU91 and SAP6. The findings provide a basis for advancing lines with improved CBB resistance and reducing the number needing field verification. Further crosses incorporating additional resistance markers could enhance resistance.

1. PHENOTYPIC AND GENOTYPIC EVALUATIONS FOR CBB
RESISTANCE IN COMMON BEAN POPULATIONS
L. Kachulu, M.S. Mwala, R. Chirwa and L. Madubanya
10th African Crop Science Society Conference 10-13 Oct, 2011, Maputo,
Mozambique

2. Bean Production areas in
Africa
Expanding to
tropical
lowland areas
of western
Africa
Source: Bean
Atlas, 1998

3. Major production biotic constraints of
beans in Africa (Wortmann, et al. 1998)
Constraint Yield loss ton (p.a.)
Angular leaf spot 384,200
Anthracnose 328,000
Bean stem maggot 297,100
Root rots 221,100
CBB 220,400

4. Common bacterial blight (CBB)
 CBB –a worldwide seed-
borne disease of common
bean- yield loss of up to
40%.
 Contaminated seed (internal
or external) acts as primary
source of inoculum can
survive for extended period
10 years
 Quantitative trait controlled
by more than one gene of
QTLs

5. Breeding for CBB resistance
• CBB resistance breeding -constrained by the
instability of resistance because of its
quantitative nature
• Despite the instability-resistance QTLs have
been introgressed into breeding lines
• Markers (SCARs) for these QTLs are
available and employed in MAS as a way to
improve the selection of cultivars with CBB
resistance

6. Breeding for CBB resistance
 SCAR markers BC420, SU91, and SAP6 are tightly-
linked with three major QTLs on chromosomes, B6, B8,
and B10 respectively.
 Different chromosomal positions of these SCAR
markers makes them attractive sources for introgressing
independent QTLs conditioning resistance to CBB into
susceptible bean lines

7. Objective of the study
 The study was carried out to evaluate the
reaction of F4.6 lines to CBB and validate the
presence of SCAR markers SU91700,
SAP6820 and BC420900

8. Materials and Methods
 The crosses were made from CBB resistant
sources of Meso (VAX3, VAX6 ) and Andean
(RMX2, RMX19 & RMX20), and susceptible
recipient parents of Andean origin
 Exp1: Phenotypic screening for CBB
resistance- Conducted in the G/house using
isolates Xf260 and Xf410. Fully expanded
trifoliate leaves were inoculated using multiple
needle method and scored at 1-9 CIAT scale after

9. Materials and Methods
 Exp2: Genotypic screening for CBB
resistance-DNA was extracted from the
greenhouse plants, template DNA was used in
PCR reaction to amplify SCAR markers SU91,
SAP6 and BC420.
 Presence or absence of the resulting fragments
for each marker was determined using agarose
gel electrophoresis

10. Results :
EXP1:Phenotypic screening-
Level of resistance for CBB in the populations were intermediate.
Average population scores of 5 and 6- due to segregation

11. Results :
CBB scores for the advanced lines within populations
revealed presence of some resistant (1-3) genotypes,
as well as intermediate and susceptible
Population Number of lines and reaction to CBB
Resistant (1-3) Intermediate (4-6) Susceptible (7-9)
BRB211/VAX3 3 19 4
BRB214/VAX3 10 17 9
BRB215/VAX6 0 7 3
BRB265/VAX6 6 8 6
CMB107/RMX2 0 6 10
RMA70/RMX20 0 13 6
RMA72/VAX6 21 45 22
BRB264/VAX3 3 5 10
RMA72/RMX19 0 9 18

12. Results from Exp. 2
Parent SU91 SAP6 BC420
VAX 3 + + _
VAX6 + + _
• SU91 marker was
present in the VAX
RMX2 _ + _
parents,
RMX19 _ + _
• Andean parents
BRB211 _ + _ possessed SAP6.
BRB214 _ _ _ • SAP6 was also
BRB215 _ _ present in some
BRB265 _ + _ susceptible
cultivars.
CMB107 _ _ _
• None of the
RMA70 _ _ _ parents carried
RMA72 _ + _ the BC420 marker
BRB264 _ _ _

13. The SCAR markers were easily scored as present (+)
or absent (-) of a single band on agarose gel.
Population -SAP6/- SAP6 SU91 +SAP6/
SU91 +SU91
Some resistant genotypes
*BRB211/VAX3
1 2
lacked both SCAR markers
BRB214/VAX3 10
SU91 marker was associated
BRB264/VAX3 1 2 with genotypes that showed
RMA72/VAX6 8 13 higher levels of phenotypic
resistance to CBB.
BRB265/VAX6 6
*BRB264/VAX3 2 7 1 SAP6 was found in
RMA70/RMX20 3 3 susceptible genotypes
CMB107/RMX2 3 7
*Resistant genotypes
RMA72/RMX19 9 9
*Susceptible genotypes
RMA72/VAX6 8 7

14. Conclusion...
 The findings of this study show that, 43 F4.6 lines
were resistant to CBB. These will further be
used as donor parental lines in the breeding
programmes or evaluated for yield perfomance
 28 of the resistant lines possessed SU91 , 2
lacked both markers and 13 had a combination
of the two markers i.e. SU91 and SAP6.

15. Conclusion...
 The findings of the study provide a basis for
advancing lines with improved levels of
resistance to CBB and reducing the number of
lines that need to be verified in the field
 Given the absence of BC420 marker in the
study populations, resistance could be
enhanced further by crossing with resistance
sources carrying the BC420marker
 Need to find other markers for resistance to
CBB other than SU91, SAP6 or BC420

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