Abstract:

Cassava (Manihot esculenta Crantz), is the world’s most widely grown starch storage root
crop. It is a principal food staple in sub-Saharan Africa where it accounts for approximately
one-third of the total production of staple food crops. It plays a key role as a food security and
an income-generating crop for millions of smallholder farmers. In Uganda, cassava ranks
second to bananas (Musa spp.) in terms of area occupied, total production and per capita
consumption; however, nearly 5% of the total population experiences hunger with the
prevalence of food energy deficiency at the country level standing at 48%. Cassava is a crop
with high potential to alleviate food shortages and energy deficiencies, owing to its unique
advantages of producing acceptable yields and starch on infertile soils amidst erratic rainfall,
when most other crops would fail. Hoewever, its yield potential has not been fully realised
since most of the cassava cultivars grown are susceptible to pests and diseases, low yielding
and late bulking. The main objective of the research was to develop high yielding, early
bulking cassava genotypes that combine resistance to cassava brown streak disease (CBSD)
and cassava mosaic disease (CMD) with farmer preferred traits for cultivation in Uganda. The
specific objectives were to: (i) evaluate farmers’ attitudes to and/or perceptions of cassava
early bulking, production constraints and cultivar preferences; (ii) determine the extent of
genetic variability in storage root bulking and other important traits of selected cassava
genotypes; (iii) assess the effects of genotype x environment interaction on early bulking and
related traits of selected cassava genotypes; (iv) develop and evaluate cassava F1 families
for early bulking in terms of the attainment of early, high fresh storage root yield (FSRY) and
resistance to CBSD and CMD; and (v) determine the combining ability and gene action
controlling early bulking and yield-related traits, as well as resistance to CBSD and CMD.
Through the farmer participatory survey, a number of cassava production constraints were
identified, key of which were: diseases, especially CBSD and CMD; lack of early bulking
cultivars; rodents and insect pests. Farmers rated early bulking as the second most important
preferred trait after FSRY, but suggested that early bulking should be complemented with
high dry mass content (DMC), sweetness, high FSRY and resistance to pests and diseases.
The analysis of variance of 12 cassava genotypes selected for evaluation in three diverse
locations and at five different harvest times indicated significant variation among genotypes,
harvest times, locations and their interactions for FSRY and most of the other traits
evaluated. Fresh storage root yield and the other traits evaluated were predominantly under
the control of genetic variation, indicating that genetic advance would be achieved through
iii
hybridisation of the test genotypes. Additive main effects and multiplicative interaction (AMMI)
analysis of the data collected at nine months after planting (MAP) indicated a non-significant
GEI for early FSRY, but significant GEI for other traits assessed. Eight of the 12 genotypes
analysed had relatively low interaction with locations for early FSRY, signifying that these
genotypes were relatively stable for early FSRY. Thirty-six Ft families were generated from a
9x9 diallel and exhibited a high degree of variation between and within families for all the
traits assessed at the seedling evaluation stage. Diallel analysis at the seedling evaluation
stage at 10 MAP indicated that additive gene effects were predominant in the expression of
early FSRY and most of the other traits analysed. At the clonal evaluation stage, the 36
families were assessed for early FSRY at 8 MAP and this trait together with most of the other
traits assessed were found to be predominantly under the control of non-additive gene
effects. High mid- and better-parent heterosis for early FSRY was recorded in most families
at the clonal evaluation stage with NASE3 x Nyara, Nyara x B11 and NASE3 x B11 recording
the highest. Selection from the 36 families at the clonal evaluation stage based on farmers’
top two preferred traits, viz. early bulking for FSRY and DMC, plus resistance to CBSD and
CMD identified 50 genotypes that had early FSRY of £25 t ha’1 at 8 MAP compared to the
best parent, CT1 that had 15.9 t ha’1 at 8 MAP. The selected genotypes also had high DMC
and dual resistance to CMD and CBSD. Advancement of the selected genotypes should go a
long way towards increasing cassava yield per unit time, reducing food shortages and
increasing the income of smallholder farmers in Uganda.