Characterisation of cowpea germplasm in Uganda.

Abstract

Characterisation of cowpea (Vigna unguiculata (L.) Walp accessions is an important starting point in crop improvement programs worldwide. Analysis of genetic variability among and within crop species is a prerequisite to any crop improvement program. The main objective of this study was to determine the variation among and within cowpea population collected from major cowpea growing districts of Uganda. Their reaction towards aphids and false rust was lso evaluated. Twenty four cowpeas accessions (Vigna unguiculata (L.) Walp from Uganda were characterized using morphological traits and Simple Sequence Repeats (SSRs). Their response in terms of resistance to false rust and cowpea aphids was also determined. The accessions were scored for qualitative morphological traits of growth pattern, twinning tendency, plant pigmentation, terminal leaf shape, plant hairiness, raceme position, immature pod pigmentation, seed shape, growth habit, green colour of the leaf and seed colour. Analysis of the cowpea accessions showed clear variability amongst them and when the data was subjected to cluster analysis using the NTSYC Pc system, the dendogram generated showed greater variation by the clustering patterns into different groups. The ANOVA analysis of the quantitative morphological characters showed significant differences at 95% level of significance. Using SSRs, variation among the accessions was observed. The nine SSR primer pairs used yielded a total of 57 bands of which 41 (71.9%) were polymorphic. Genetic similarity among the genotypes ranged from 59% to 93% clearly showing existence of variabilty in the accessions studied. Investigations for aphid and false rust resistance were carried out at MUARIK over two seasons. The cowpea aphid incidence and severity of false rust was recorded across two seasons. The population densities were estimated using visual rating of 1-6 and basing on this the incidence of cowpea aphids varied across the accessions under study. Nine of the accessions were found to be susceptible to aphid attack, five accessions had moderate resistance and ten accessions were considered to be resistant because of the negligible number of aphids that were found on these accessions. Severity of false rust was also recorded across the 24 accessions over two seasons. The area under disease progress curve (AUDPC) was calculated and these values were plotted against the accessions and the resultant graph divided the 24 accessions into three distinct groups. Ten accessions were grouped together in those that were susceptible, six accessions were categorized as those with moderate resistance to false rust and eight were found to be conferring resistance to false rust. No clear relationship between morphological and SSR analysis with resistance to cowpea aphids and false rust was found to exist. The existence of variability among and within collected cowpea germplasm can be exploited in crop improvement programs in the breeding activities of the national program. There is need to carry out multi-locational trials to validate the results obtained on-station for purposes clearly separating accessions with desirable traits. Key words: cowpea, characterisation, Simple Sequence Repeats (SSR’s), morphological, molecular