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
ix
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.