Genetic characterization, priming and screening for Cercospora leaf spot resistance of different Bambara groundnut (Vigna subterrannea [L.] Verdc.) landraces.
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Date
2017
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Abstract
The Bambara groundnut (Vigna subterranea [L.] Verdc.) is an African plant species which
has been cultivated for many years even longer than the groundnut (Arachis hypogaea L.).
Bambara groundnut is mostly grown by farmers as a subsistence crop. This legume has an
admirable balance of carbohydrates (63%), protein (19%) and fats (6.5%) which is beneficial
in improving protein deficiencies in cereals. Bambara groundnut has great potential to
alleviate food insecurity, especially as the world moves towards a drier future. It may provide
a sustainable food source since it is able to adapt well to dry conditions. However, Bambara
groundnut is not afforded a lot of attention in scientific research.
The objectives of this study were: 1) to identify the suitable duration of priming Bambara
groundnut seeds in order to enhance germination and seedling establishment 2) to assess
Bambara groundnut landraces for their reaction to Cercospora leaf spot (CLS) with a view to
identifying landraces which may be used in breeding programs to develop CLS resistant lines
and 3) to genotype 22 Bambara groundnut landraces obtained from various geographical
regions in Southern Africa using 20 simple sequence repeat (SSR) primers developed
specifically for Bambara groundnut.
Five landraces (Keledi, Mokgalo, SCAM, LMS and MMB) were obtained from the National
Genetic Resources Centre (NGRC) in Botswana. Seeds were primed using distilled water
(hydropriming) at 24 and 36 hours; and controls (0 hours) were not primed. After priming,
seeds were air-dried and planted. The number of days to 50% seedling emergence and the
number of seedlings established at 20 days after planting were the two major parameters
assessed in this study. Results of this study revealed that priming time had a significant
(p = 0.043) effect on days to 50% seedling emergence where seeds primed for 36 hours took
15 days to reach the aforementioned level; the least amount of time relative to the other
treatments. It was also observed that there was a significant difference (p < 0.0001) on
seedling establishment as affected by the interaction of landrace and priming time. Landraces
SCAM and LMS were the best performers in the group of landraces used.
Nineteen Bambara groundnut landraces were planted at the Ukulinga Research farm on a plot
with previous history of Cercospora leaf spot infection. Disease was evaluated using a
disease rating scale (0 – 4) in order to calculate the disease index; the disease indices were
used to calculate area under the disease progress curve (AUDPC) and finally, the apparent
rates of infection were also calculated using Vanderplank’s Logistic equation. None of the
landraces were resistant to infection by the pathogen. Statistically, there were no significant
differences (p > 0.05) observed among landraces in final foliar disease percentage and
AUDPC. The Bambara groundnut landraces were categorised into reaction groups
(resistance) based on the calculated AUDPC values. Eight landraces were considered to be
moderately resistant (501 – 1000 units) and 11 were considered to be susceptible (1001 –
1500 units). Landraces KB05 and STN 05 were categorized as moderately susceptible to CLS
and a recommendation was made that they could be used in Bambara groundnut breeding
programs.
DNA was extracted from 22 Bambara groundnut landraces and was then used in touchdown
polymerase chain reaction (PCR) using 20 Bambara groundnut specific SSR primers. The
PCR products were subjected to capillary electrophoresis using a genetic analyzer. A
phylogenetic tree was constructed using DARwin 5.0 software. After analysis, 110 alleles
were detected, with a mean of 5.50 alleles per locus. The mean polymorphic information
content (PIC) was 0.62 and the mean expected heterozygosity (He) was 0.64; the latter
indicating a high gene diversity among the genotypes. The neighbour-joining analysis
generated three major genetic groups, where the genotypes were clustered irrespective of
their geographic origin. Landraces SCAM and LMS which had improved seedling emergence
and seedling establishment did not appear in the same cluster. Similarly, landraces KB05 and
STN 05 which were moderately susceptible to Cercospora leafspot did not appear in the
same cluster.
Poor emergence, disease infection and a lack of improved varieties are three major
production constraints of Bambara groundnut that are identified in this study. Concerted
efforts in scientific research are necessary in the amelioration of these problems and in the
improvement of production of Bambara groundnut.
Description
Master of Science in Plant Pathology. University of KwaZulu-Natal, Pietermaritzburg, 2017.