| dc.description.abstract | The susceptibility of cassava cultivars to disease is one
of the most important factors that limit cassava production.
Cassava bacterial blight caused by Xanthomonas campestris Pv
manihotis often results in considerable, sometimes total
losses in tuber yield, leaves and planting material. Therefore
this study was designed to survey the distribution of cassava
bacterial blight and its causal agents in different
agro-ecological zones of Kenya, interactions between the two
strains of the pathogen, K. campestris QY manihotis and K.
campestris QY cassavae and to screen cassava cultivars for
resistance against the above disease.
A survey was undertaken to map the distribution of the
disease and its causal agents in different agro-ecological
zones of Kenya. The survey revealed the disease and its causal
agents as not being spread allover cassava growing areas but
being confined to Western province and lake region areas of
Kenya. Disease severity and incidence differed with locality
depending on the amount of rainfall. Areas with higher
rainfall had higher disease severity and incidence ratings.
Disease incidence was also influenced by the method of farming
being practiced. Monocrop showed more severe symptom
development as compared to mixed cropping where cassava was
intercropped with maize and bananas.
Physiological and biochemical tests indicated that the
two organisms are similar in most characteristics except in
the utilization of sucrose and production of hydrogen sulphide
from peptone. Xanthomonas campestris QY cassavae produced acid
from sucrose and hydrogen sulphide from peptone, which was not
the case with Xanthomonas campestris QY manihotis.
In-vitro and in-vivo studies of the two strains of the
pathogen and epiphytic micro flora found on cassava leaves were
carried out in the laboratory and in the greenhouse
respectively. Results showed that Xanthomonas campestris QY
cassavae partially inhibits Xanthomonas campestris QY
manihotis. Epiphytic microflora had no effect on the progress
of Xanthomonas campestris QY manihotis wi thin leaves and
stems. For cassava plants separately inoculated with the two
pathogens, Xanthomonas campestris QY manihotis was more
virulent and invaded stems faster as compared to QY cassavae.
It also grew faster on nutrient agar.
Twelve selected cassava cultivars were screened against
cassava bacterial blight disease in the green house using
three isolates identified from the survey. Leaf clipping and
stem puncture methods of inoculation developed by CIAT were
used . Leaf clipping method was rapid in operation and showed
higher susceptibility of varieties than the stem puncture
method. Isolates were significantly different in their
reaction to the cultivars. Of the twelve cultivars tested,
5543/156, Mwakazanga, Kasimbiji green were resistant and
46106/27, 4670/37, 4763, 47234/114, 5543/19, 5043/11,
Kibandameno and Guzo were susceptible to cassava bacterial
blight. A local variety Kasimbiji green showed consistent
resistance against the disease in all the methods and isolates
used in these study. | en |
