Abstract:

Bacterial blight of cassava, Xanthomonas campestris
pv. manihotis (Arthaud-Berthet, Starr) Dye, is one of the
most important diseases of cassava in Uganda. The
infection of cassava by the disease may result in
defoliation, tip dieback, and reduction in tuber yield
of up to 90-100% in susceptible varieties.
This thesis reports results of three field,
laboratory and green house studies conducted during 1985-
87 on the role of insects in the epidemiology of cassava
bacterial blight. The first experiment was carried out
to identify the most suitable cassava varieties and the
insects associated with them. Four cassava varieties
(Bukalasa 11, Bukalasa 8, Bintiminti and Ebwanateraka)
were used. The insects associated with healthy and CBB
infected leaves were monitored monthly. The monthly
records were taken at early morning, mid-day and evening.
The second experiment was carried out to prove that the
insects associate with CBB infected leaves and to
the role they Play in CBB epidemiology.
yeasts were used to mimic insects
establish
Mollasses and brewers'
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from CBB infected to CBB free plants while dimethoate and
unsprayed control were used to exclude insects from
visiting healthy cassava and to represent natural
conditions respectively. The last experiment was carried
out to ensure the potency of bacterial isolates from
insects. Bacterial isolates from body surfaces and faeces
of the insects collected from the field were cultured in
the laboratory. Their population in culture and
infectivity in cassava seedlings were determined.
A total of twenty insect species from seven orders
were associated with bacterial blight infected cassava
plants. The insects preferred bacterial blight infected
than healthy cassava leaves. Neither the cassava
varieties used nor the time of the year affected the
insect preference.
Nevertheless, time of the day significantly affected
the number and abundance of each insect species on the
infected cassava leaves. All the insect species studied
were most abundant on the infected leaves during the
warmer part of the day (12.30 1.30 p.m.) than either
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earlier in the morning (6.30 7.30 a.m.) or late in the
afternoon (5.30 6.30 p.m.).
The population of insects on the infected cassava
leaves reached two peaks. Variations in peak populations
the number of
bacterial blight lesions on the crop and the weather
factors (particularly rainfall).
All the insect species studied had movements
directed towards locating bacterial blight lesions and
were probably mediated by some kariomones from blighted
lesions. Once a lesion was located, the insect either
sucks the exudates from the lesion or chews parts of the
lesion. The feeding behaviour and sites of chewing
species were of two types. For instance Catantops
momboensis centralis and Lagria villosa preferred feeding
on edges of the leaf blight lesions on which they create
irregular feeding sites. Svstates castaneipennis prefers
to feed on the middle of blight lesions and creates
smooth round "holes".
were probably due to the age of the crop,
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yeast sprayed on to healthy
leaves, significantly attracted higher number of insects
to the crop. Insect feeding sites and bacterial blight
lesions were also significantly higher in plots treated
with the 'attractants 1 . The number of lesions were
highly correlated with the number of insects: implying
that the insects were responsible for the observed
increases.
The application of Dimethoate to suppress insects
resulted in a significant reduction in the
number of bacterial blight lesions on cassava and in the
insect feeding sites. In general. it was estimated that
under natural conditions, the insects contributed up to
35.43% of bacterial blight epidemiology. When moll^sses
used, the insects contributed
41.19%-56.64% of bacterial blight increase in the field.
Generally the population of the pathogen isolated
from the exoskeleton and feaces of the insects were lower
than those from bacterial blight infected lesions.
Cassava leaves inoculated with isolates produced typical
bacterial blight lesions.
Mollasses and brewers'
on cassava,
and brewers yeasts were
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The senario of CBB epidemiology seems intrieably
linked up ecology many insect species
attracted to bacterial blight infected cassava leaves by
kariomones from disintegrating cassava leaf tissues.
They acquire the pathogen while feeding on bacterial
blight lesions and disseminate them to healthy cassava
The
insects Play an important role in bacterial blight
epidemiology and measures aimed at controlling the
disease should integrate the use of insecticides in
reducing the number of insects.