| dc.description.abstract | The damage caused by cassava diseases has been on the rise in Africa in recent years. Most
farmers obtain planting materials from their own fields or neighbours consequently enhancing
the spread of the disease. The most feasible option of managing diseases is to improve
existing cultivars through resistance breeding. This study was therefore conducted to
evaluate the performance of cassava half-sib progenies arising from hybridization between
diverse parental cassava germplasm deemed to be tolerant or resistant to that disease with
the aim of determining their genetic inheritance. Five parental genotypes each with at least
twelve progenies generated through polycross mating design were evaluated for agronomic
and disease resistance. The half-sib families were developed from five elite parents selected
based on their performance, disease resistance and farmers preferred traits. The experiments
were established at the Kenya Agricultural and Livestock Research Organization
(KALRO) at Kakamega and Alupe Research Stations from June 2016 to June 2017. Randomized
completely block design was used. Data on emergence, plant height, height to the
first branching, number of roots per plant, root yield, harvest index, dry matter, starch and
cyanide content were recorded. Monthly assessment was done for cassava mosaic disease,
cassava brown streak disease, cassava green mite and whiteflies infestation. Inheritance of
agronomic traits and disease resistance was determined by calculating the general combining
ability and estimating the heterosis between progenies and their best parents. There
were high significant (P<0.01) differences in the reaction of genotypes to cassava mosaic
disease and cassava green mite damages. Twenty three genotypes had a mean score of 1.0
to cassava mosaic disease and three to cassava green mite. Alupe site was observed to have
high number of genotypes showing susceptibility compared to Kakamega, indicating the
effect of the environment on the evaluated genotypes. Genotype, P4G1 followed by genotype
P2G3 gave the highest fresh storage root yield across the study sites, while P3G6 and P5G9 recorded the lowest yield of 8.5t/ha. Significant correlation was observed among the
agronomic traits, levels of cassava mosaic disease, green mite damage, fresh storage root
yield, starch and cyanide content. Most parental cultivar expressed varying general combining
ability effects in the two sites for most of the evaluated traits. Parental cultivars
MM96/4271, MM96/0686, MM97/0293 and Kaleso had good general combining ability
for cassava mosaic disease resistance. MM96/4271 was the most resistant parent among
the five for cassava green mite with a negative general combining ability effect in both
sites. Parental cultivar Kaleso had negative general combining ability effects for cassava
mosaic disease and for the progression of the disease in both two locations. The progenies
from Kaleso and MM96/4271 had high positive heterosis for fresh storage root yield, harvest
index and storage root number, and the most negative better parent heterosis for cassava
green mite and cassava mosaic disease incidence. Though there were significant differences
between parents and their respective progenies in the reaction to cassava mosaic
disease severity, there were a varying number of symptomless clones generated from different
cassava families involved. This suggests that these genotypes may be suitable as genetic
stock that could combine cassava mosaic disease and cassava green mite damage resistance
in one background. Evaluation of new cassava varieties under local disease conditions
would likely improve the productivity of cassava through selection of resistant
clones. The parental cultivars and progenies identified here are potential candidates for
producing a new generation of segregating progenies that could in future be released to
farmers to increase the productivity of cassava in a number East African country. | en_US |
