Genetic characterization, priming and screening for Cercospora leaf spot resistance of different Bambara groundnut (Vigna subterrannea [L.] Verdc.) landraces.
Gama, Nompumelelo Phumelele.
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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.