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Discovery of
Bruchid Resistance Trait in a Malawian Landrace of Dry Beans
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Plate 1. Part of
the collected germplasm |
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Plate 2.
Acanthoscelides obtectus(Common bean weevil) |
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Plate 3.
Zabrotes subfasciatus (Mexican bean weevil) |
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Plate 4.
Laboratory screening of bean accessions under controlled conditions |
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Plate 5. Two
potential landraces showing resistance to the two bruchid species.
Accession KK35 consistently showed no damage symptoms |
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Plate 6. KK25
damage by Acanthoscelides obtectus |
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Plate 7. The
resistant landrace KK35 in the field at Chitedze Research Station |
Geoffrey Kananji
ACCI PhD
Student/Chief Agricultural Scientist
Ministry of
Agriculture, Chitedze Research Station
P.O. Box 158,
Lilongwe, Malawi
Phone: +265 1
707087
Email:
gkananji@yahoo.com
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Background
information.
Beans are an
important legume in Malawi. They contribute significantly to household
nutrition and economic activities in the economy. An annual bean production
of up to 100,000 MT has been recorded in favourable cropping seasons. The
National Bean Improvement Programme (NBIP) in partnership with the
International Centre for Tropical Agriculture (CIAT) and the Bean/Cowpea
Collaborative Research Support Program (CRSP) have released more than 15
improved bean varieties to farmers in Malawi in the recent past. However,
almost all these varieties seriously succumb to bruchid infestation during
storage. The extensive damage caused by bruchid to stored beans has forced
farmers to utilize short term indigenous control methods while other farmers
use hazardous insecticides such as DDT to reduce losses. A long term
solution to the problem of bruchid damage in Malawi is therefore required.
Germplasm
collection.
Malawi is
considered a secondary center of bean genetic diversity in Africa, and this
gave the impetus to exploit the existing genetic variability for sources of
resistance to bruchids. Over the years, farmers might have done their own
selection of preferred cultivars and in the process have, unknowingly,
selected genes for resistance to bruchid strains found in Malawi. Bean
germplasm collection was conducted in 2003 which targeted bean growing areas
of the country. A total of 99 different bean landraces, with varied seed
colours and sizes, were collected (Plate 1). The collected landraces were
planted at Chitedze Agricultural Research Station in Lilongwe, Malawi to
study their phenological and qualitative traits and also generate sufficient
seed for the laboratory experiment.
Laboratory
testing.
After obtaining
sufficient seed quantities from the research plots, a laboratory experiment
was conducted for 150 bean landraces of different backgrounds. The landraces
included lines obtained from CIAT-Malawi, the National Bean Improvement
Programme, Bunda College of Agriculture Bean Research Programme and Oregon
State University who provided 3 SMARC lines. The SMARC lines contain arcelin,
a protein that is said to confer resistance to beans against Zabrotes
subfasciatus. All these lines were subjected to attack by the two
bruchid species, common bean weevil (Zabrotes subfasciatus) and
Mexican bean weevil (Zabrotes subfasciatus) under controlled
laboratory conditions and a no choice test method was used (Plate 4).
Landrace resistance indicators such as duration of weevil development (DWD),
number of progeny emerged (NPE) were measured and susceptibility index (I)
for each accession were calculated after 60 days of laboratory testing.
Results.
Two dry bean
landraces KK25 and KK35 (Plate 5) were not damaged by any of the two bruchid
species i.e. Zabrotes subfasciatus and Acanthoscelides obtectus.
KK35 in particular, had the lowest susceptibility index and no F1 adults
emerged. Verification experiments carried out to confirm the observed
resistance did not show any damage symptoms for KK35. The other landrace,
KK25, however, which initially showed some level of resistance eventually
succumbed to attack by A. obtectus (Plate 6). These results will be
used to strengthen the bean improvement programme through multiplication and
delivery of KK35 landrace seeds to farmers (Plate 7). Farm level awareness
events will also be organized to educate farmers on the planting of
resistant and other improved bean varieties.
Table 1:
Varietal characteristics of KK35 bean landrace (D50F: days to 50% flowering)
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Landrace |
Varietal
traits |
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Seed size |
Seed colour |
Growth habit |
Flower color |
Average pod
length (mm) |
D50F |
Seed yield |
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KK35 |
Medium |
Red speckled |
Determinate
bush |
Pink |
14 |
35 |
average |
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KK25 |
Medium |
purple |
Indeterminate
bush |
Pink |
12 |
36 |
average |
Additional reading
about bruchids
Description, biology and damage by
bruchids
C.S.A. Dayler eta al. 2005.
Identification of a novel bean α-amylase
inhibitor with chitinolytic activity. FEBS Letters
Volume 579, Issue 25 , Pages
5616-5620
S. Nchimbi-Msolla and R.N. Misangu.
Seasonal Distribution of Common Bean (Phaseolus
Vulgaris L.) Bruchid Species
in Selected Areas in Tanzania. Proceedings: Bean Seed Workshop,
Arusha, Tanzania January 12-14
Protective mechanism of the Mexican bean weevil against high levels of
alpha-amylase inhibitor in the common bean. M Ishimoto and M J Chrispeels,
Plant Physiology
volume111(2).
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