Abstract:

Sorghum is one of the important food crops in Uganda and other East African countries. Its production is curtailed largely by abiotic and biotic constraints. In Uganda and many other countries of East and Southern Africa, sorghum anthracnose is a key threat to production, yet the pathogen is highly variable, thus rapidly breaking down resistance of its host. In Uganda and East Africa in general no population studies of Colletotrichum sublineolum have been reported hence there is limited information on nature of epidemics and pathogen. In this present study, the general objective was to characterise sorghum anthracnose and its causative agent Colletotrichum sublineolum derived from various agro-ecologies of Uganda. A survey was conducted in 9 agro-ecologies of Uganda where incidence and severity was recorded and diseased leaf samples where taken for laboratory analysis. In the laboratory, DNA of isolates was amplified with three short, 17-21-mer, tandemly-repeated ISSR primers (CCA, CGA, ACA) with two (CCA) or three degenerated bases at their 5’-ends from Cape Town South Africa. The resulting DNA products were separated by agarose gel electrophoresis. The genomic data were scored for absence/presence of bands, respectively. Analysis of Variance (ANOVA) results showed that there was variation in incidence and severity sorghum anthracnose disease. J Both incidences varied significantly (P<0.05) between agro-ecologies reflecting the effect of different cropping pattern on sorghum anthracnose epiphytotics. Districts within agroecologies showed a significant variation in both incidence (P<0.05) and severity (P<0.05) of sorghum anthracnose. This might be attributed to the different crop management practices carried out by the farmers within an agro-ecology. Similar observations were made for fields within the districts with significant differences occurring at P<0.05 for both incidence and severity. The technique of Inter-Simple Sequence Repeats (ISSR) produced 37 polymorphic loci and neighbour-joining analysis of ISSR data on 124 isolates revealed two major groups. Analysis of molecular variation (AMOVA) of ISSR data on the basis agro-ecologies revealed the presence of a population structure, (OFst= 0.08 P = 0.05) leading to acceptance of the null hypothese stating a presence of population differentiation between the agro-ecologies in Uganda. Gene flow between agro-ecologies was 5.75 calculated from (bFs-r. The results of the AMOVA analysis also revealed that allelic variation (92.5%) was shared between populations. Average gene diversity over all loci ranged from 0.192 to 0.335 showing high diversity within population rather than between populations. The number of polymorphic loci were similar for the population studied. Variation in the prevalence, incidence and severity of the anthracnose epidemic in the sorghum growing agro-ecologies of Uganda suggest that sorghum anthracnose is a major threat to production in virtually all agro-ecologies in Uganda. Moreover the high severities suggest cultivation of very susceptible varieties. The cultivation of resistant varieties will support management of the disease.