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

Source: Bean

Atlas, 1998

Expanding to

tropical

lowland areas

of western

Africa

Bean Production areas in

Africa

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

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

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

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

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

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

14 days

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

Materials and Methods

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

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

Results :

Results from Exp. 2

Parent SU91 SAP6 BC420

VAX 3 + + _

VAX6 + + _

RMX2 _ + _

RMX19 _ + _

BRB211 _ + _

BRB214 _ _ _

BRB215 _ _

BRB265 _ + _

CMB107 _ _ _

RMA70 _ _ _

RMA72 _ + _

BRB264 _ _ _

• SU91 marker was

present in the VAX

parents,

• Andean parents

possessed SAP6.

• SAP6 was also

present in some

susceptible

cultivars.

• None of the

parents carried

the BC420 marker

The SCAR markers were easily scored as present (+) or absent (-) of a single band on agarose gel.

Population -SAP6/-

SU91

SAP6 SU91 +SAP6/

+SU91

*BRB211/VAX3 1 2

BRB214/VAX3 10

BRB264/VAX3 1 2

RMA72/VAX6 8 13

BRB265/VAX6 6

*BRB264/VAX3 2 7 1

RMA70/RMX20 3 3

CMB107/RMX2 3 7

RMA72/RMX19 9 9

RMA72/VAX6 8 7

Some resistant genotypes

lacked both SCAR markers

SU91 marker was associated

with genotypes that showed

higher levels of phenotypic

resistance to CBB.

SAP6 was found in

susceptible genotypes

*Resistant genotypes

*Susceptible genotypes

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.

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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