Pseudomonas bulb rot of ornithogalum spp. : etiology, survival and dissemination

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 xi 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