| dc.description.abstract | The bulb-rot disease of Ornithogalum in Kenya was characterized by
necrosis of the leaves which spread into the bulbs. Two common bacterial
isolates were obtained from the diseased tissues. The cultures were
designated A-II and 8-Yw. Culture A-II was found to be pathogenic to
Ornithogalum plants and was associated with the 'Pseudomonas fluorescens
complex' group. Identification of the pathogen was based on cultural,
biochemical and physiological characteristics. Characteristic symptoms of the
disease developed within 3-10 days when the leaves were inoculated by
stabbing with a needle. The yellow-pigmented culture 8-Yw which seemed to
enhance pathogenicity was identified as an Erwinia Sp.
The in vitro studies were conducted with culture A-II. The optimal
growth pH was found to be 6.5 while pH values below 4.5 and above 9 were
lethal. Some abberrant white colonies grew in nutrient agar plates with pH
values below 4.5. These colonies were not pathogenic to Ornithogalum plants.
The minimum inhibitory concentration of copper ions to the growth of culture
A-II in vitro was shown to be 150mg/1 and the value of ED50was 55.0mg/1.
The optimum temperature for growth was found to be 27°C. The maximum
growth temperature was 39°Cwhile the minimum was 4-5°C.
Stab inoculation was found to be a more suitable method on
Ornithogalum plants with 108-109cells of cultures A-II. Symptoms resulted only
if leaves had been wounded. In the absence of free moisture, lesions
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expanded very slowly. When water was supplied to the inoculation site, lesion
expansion was at a more rapid rate.
Survival of the Ornithogalum bulb-rot organism was determined on dry
leaf surfaces, in sterile soil and plant debris. Survival on leaf surfaces and in
sterile soil was determined by using an agar plate technique. There was a
rapid population decline on leaf surfaces in the first 24 hours. Once this initial
period of high death rate passed, the population declined at a low rate. The
pathogen was recovered in soil samples after 14 weeks; half-lives of the
pathogen in moist, slightly moist and dry soil samples were 1.97, 0.74 and
0.53 weeks respectively. Using a half life of 1.97, the theoretical survival time
of the bacterium would be 58 weeks. The relationships between the survival
of the pathogen in soil and control measures are discussed. The bacterium
was able to survive and remain virulent in plant debris for five months. The
highest disease incidence"was recorded from the debris that were kept in the
open and mixed with soil at sowing time. The lowest incidence was in
autoclaved soil with no debris.
It was demonstrated that bulbs harbour sufficient inoculum to cause
disease in new plants. In case of unfavourable conditions to disease
establishment, diseased plants could recover and produce marketable flowers | en |
