MECHANISMS OF SEED TRANSMISSION OF BACTERIAL BLIGHT OF COWPEA (Xanthomonas axonopodis pv. vignicola) (BURKHOLDER) DYE
MECHANISMS OF SEED TRANSMISSION OF BACTERIAL BLIGHT OF COWPEA (Xanthomonas axonopodis pv. vignicola) (BURKHOLDER) DYE
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Date
2013-02
Authors
AMODU, Umoru Simeon
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Abstract
Understanding transmission mechanisms of cowpea bacterial blight Xanthomonas
axonopodis pv vignicola (Xav) from seed to seedling will form the bases for appropriate
control measure to be initiated. In order to study the parts of seed where the pathogen
usually colonize, three methods were used (seed washing and seed plating) were used to
detect the external borne pathogens while the third (separation of seed into components
and maceration) were used to detect the internally borne pathogen. The results showed
that hilum was the most colonized part of the seeds. The embryo colonization was low
and there was no Xav detected on cotyledons. In order to study the populations build up
in vivo and symptom manifestation, seed samples were collected from various sources to
reflect the various ways farmers obtain seeds (local markets, seed companies, research
institute and treated seeds with fungicides). Fungicides had no significant effect on the
pathogen survival and pathogenicity. There was significant difference between the
treatments in terms of disease incidence and severity on the vegetative parts. There was
steady population build up in all the treatments both in/on the plants. There was no
significant difference between all the treatments in terms of Xav population build up on
the floral parts. Mechanism of transmission of Xav was studied by soaking seeds of
sorghum, maize, millet and cowpea in bacterial suspension adjusted to 4.7x107cfu/ml for
two and four hours respectively. After 2 h of soaking seeds in bacterial suspension,
cowpea seeds had higher population of Xav attached to it while after 4 h of soaking; all
the seeds had Xav attached to it. The study of biofilm formation were carried out by
growing bacterial in maize, millet, sorghum, cowpea extracts and extracts of cowpea +
0.5 g glucose and nutrient glucose agar (NGA) in a 96 microlitre wells. There was
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statistical difference between the biofilm formation induced by the different extracts and
NGA. Studies on the route of transmission of Xav to seeds indicate that the bacterium
was able to transmit systemically and epiphytically from inoculated seeds to harvested
seeds and also from inoculated vegetative parts into the harvested seeds. Since adhesion
of bacteria to seeds was just on commensal basis, studies were conducted to evaluate the
possibility of non-host seeds of cereal crops to transmit the pathogen. Maize, millet and
sorghum seeds were collected from cowpea producing states for the isolation of the
pathogen. The pathogen (Xav) was consistently isolated from millet samples collected
from all the states except Kebbi state. In other to determine whether the Xav can directly
transmit systemically from seeds to harvested seeds or indirect transmission through the
floral parts to the non-host seeds (maize, millet and sorghum), seeds were inoculated
before sowing and also, the floral parts were inoculated at the flowering stage. The
pathogen (Xav) was able to systemically transmit from seeds to harvested seeds in millet.
Maize and sorghum did not transmit CoBB pathogen systemically. However, millet and
sorghum were florally transmitting Xav to seeds but maize did not.
Description
A DISSERTATION SUBMITTED TO THE SCHOOL OF
POST GRADUATE STUDIES AHMADU BELLO
UNIVERSITY, ZARIA
NIGERIA
IN PARTIAL FUFILMENT OF THE REQUIREMENTS
FOR THE AWARD OF DOCTOR OF
PHILOSOPHY IN CROP PROTECTION
DEPARTMENT OF CROP PROTECTION
FACULTY OF AGRICULTURE
AHMADU BELLO UNIVERSITY
ZARIA
FEBRUARY, 2013
Keywords
MECHANISMS, SEED TRANSMISSION, BACTERIAL BLIGHT, COWPEA