الفهرس 
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Abstract
1.Potato (Solarium tuberosum L) is grown in Egypt from September to February in three distinct growing seasons. Brown rot caused by Ralstonia solanacearum was studied in samples collected from traditional potato districts in Egypt. More than five thousands samples collected over four-years period, (1996-1999), were examined. Sixty-four samples showed symptoms, and eighty-five showed latent infections. Potato cultivars, Nicola and Spunta showed the highest frequency of the brown rot infection.
2.Isolation of the pathogen was made on KB, CPGA, and SMSA media. Pathogenicity of isolates was tested in 3-week-old tomato seedlings, under greenhouse condition. Stem inoculation was made Abith - a water- suspension’ kl x1108 S efu ’ he most Viftilent (pathogenic on tomato plants) isolates were tested for pathogenicity on the target host (potato) of this study using the above-mentioned concentration under the green house condition.
3.Identification of the pathogen was made based on traditional and recently developed techniques employed routinely worldwide. Morphological, physiological and biochemical characteristics were studied. Serological (1FAS), Fatty acids profiling (FA), protein electrophoresis (PE) and nucleic acid-PCR based techniques were used. The employed techniques revealed that race 3 biovar II was the only detected pathovar affecting potatoes in Egypt. The Egyptian isolates exhibited very close similarity with the European isolates of R. solanacearum tested in this study.
4.Isolation of R. solanacearum from potato fields either planted with potatoes and/or with previous potato history was tried. Samples
collected from sixteen traditional potato districts with different soil types were tested. Successful isolation was only achieved from two districts in Nubaria using the usual plating technique on SMSA medium. The recorded densities were limited and ranged from 100-300 cfu /g dry soil. Centrifugation of soil suspension and plating on SMSA as another isolation trail, improved the results, and gave 2400 and 1600 cfu 1/g dry soil. Isolation from clay soil with previous history of infestation with brown rot was quite difficult as compared to sandy soil. In addition, the cover crop also had an indirect effect on the results of isolation.
5.Isolation of the brown rot bacterium from watercourses was carried out. More than 200 water samples collected from water canals in Kalyubia, Gharbia, Ismailia and Sharkia were tested. Of the Kalyubia samples, six samples showed contamination. Drainage water on the other hand showed five positive samples out of sixty samples collected from Kalyubia and Gharbia. The densities of the pathogen in all of the tested samples were low, they were did not exceed 1000 cfu /L in drainage water as compared to 1333 cfu /L in irrigation water.
6.Densities of bacterial flora in the rhizosphere of three different potato cultivars (Diamant, Cara and Alpha) were determined using the usual plating count technique on soil extract agar medium. The cv. Diamant showed the highest microbial densities in the rhizosphere compared to the other two cultivars used. However, a peak rise increase in densities was observed 75 days after planting.
7.The antagonistic bacteria in Diamant potato rhizosphere were investigated. Colonies randomly selected from the countable plates, were tested for their antagonistic potentials towards
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R. solanacearum. The most predominant antagonist in the rhizosphere was Acinetobacter baumanii. In soil apart, however, the latter along with P. chlororaphis (P. putida) were detected. The other cultivars, namely Cara and Alpha, did not show any antagonistic bacteria in the rhizosphere at the used dilutions, indicating very low densities of these organisms in the rhizosphere.
8. Bacterial flora in the rhizosphere of eggplant (Solarium melongena L. cv. ’Black Beauty’) and sweet pepper (Capsicum annuum) were determined at the late maturity stage along. The percentage of antagonistic bacteria that showed in vitro antagonistic potential against R. solanacearum was determined. The eggplant rhizosphere was characterized by high bacterial density as compared to sweet pepper. The rhizosphere of eggplant supported greater proportion of antagonists. The rhizosphere of sweet pepper at the used dilutions, however, did not show any antagonistic
bacteria.
9.Identification of antagonists in the rhizosphere of eggplant by fatty acid (FA) profiling revealed Stenotrophomonas maltophilia currently referred to as Pseudomonas maltophilia or Xanthomonas maltophilia.
10.Satellite (associated) bacteria, derived from naturally infected potato tubers were also tested for their antagonistic potential to the target pathogen. The FA profiling revealed that the isolated antagonists were P. chlororaphis and P. putida. By using the biochemical tests, both isolates belong instead to the species putida and fluorescens respectively. Also, two isolates were identified as
Bacillus cereus.
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11.The effect of trace elements on the potential of the antagonistic bacteria was studied on King’s medium B. The growth of P. chlororaphis (P. putida) and P. fluorescens was favoured by iron and zinc. Manganese had no effect on the antagonistic potential. Moreover, this element promoted the growth of Stenotrophomonas maltophilia. Poor growth on King’s medium B either containing or devoid of trace elements was reported for Acinetobacter baumannii. Fe inhibited the antagoniostic potential of A. baumannii and P. fluorescens. While antagonism with S. maltophilia was not affected by any of these trace elements tested.
12.The In vivo trails involved the use of two physiologically by different cultivars. (cvs. Diamant and Nicola, early and late maturing, respectively). The selected biocontrol agents used were P. putida, A. baumannii, P. fluorescens and S. maltophilia. In this study, A. baumannii and S. maltophilia were used for the first time in biological control against bacterial pathogens. The application of the antagonists was made separately in soil and
through bacterization the potato eyepieces. P. putida, A.
baumannii, P. fluorescens and S. maltophilia proved to be
effective as biocontrol agents, as used in eyepiece bacterization. P. putida failed to control the disease as soil treatment, S. maltophilia on the other hand gave the best control results followed by P. fluorescens, while the other two antagonists gave poor results.
In another experimental trial the bacterized eyepieces with A. baumannii and S. maltophilia were used in sterile and non-sterile soil. It is worth to note that S. maltophilia was also efficient in controlling the Ralstonia brown rot. The disease control by A. baumannii was very poor
in this regard.
In a third trial, A. baumannii was used on three week-old tomato and was not effective in disease control.