الفهرس 
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Abstract
Bacterial wilt caused by Ralstonia solanacearum (Yabuuchi et al.,1995) phylotype II sequevar I (race 3 biovar 2), is considered one of the most destructive bacterial diseases of potato crop. The present work was concerned with surveying and diagnosing the disease. Determination of phylotype and sequevar identity of the pathogen was carried out. Large number of samples was collected from different habitats, at governorates (El-Behera, El-Minufiya, El-Gharbia and El-Isamailia)levels in Egypt, over three seasons (2010 to 2013). Potato tubers, irrigation water and weeds were collected from different potato districts at the governorates (El-Behera, El-Minufiya, El-Gharbia and El-Isamailia). Based on pathogenicity as well as growth characteristics on semi selective medium of South Africa (SMSA) and immunofluorescence antibody staining (IFAS) tests, variable percentages of positive reactions were recognized. In this regard, 162 isolates were selected from different habitats. The pathogenic variation between isolates was assessed on the basis of the degree of virulence on inoculated tomato seedlings under greenhouse conditions. The highest pathogen potential was detected by 12 isolates, as shown by the degree of wilting expressed, out of 162. Identification and race determination of the aforementioned twelve isolates were done based on the physiological and bacteriological characteristics. The conclusion of results revealed the identity with characteristics described for Race 3, biovar II of R. solanacearum. No variation was observed between these isolates in morphological, cultural, physiological and biochemical characteristics. All tested isolates produced a single band at the size of 288 bp using PCR technique. Also, phylotype specific multiplex (Pmx) - PCR revealed that, all the twelve isolates of R. solanacearum
belonged to phylotype II as 372- bp amplicon was observed with all tested isolates.
Resistance of five commercial potato cultivars to R. solanacearum was studied under the greenhouse conditions. Spunta, Hirmas, Nicola, Draga and Cara cultivars showed different degrees of susceptibility to the pathogen in concern. In this respect, Spunta was considered as the most sensitive to infection, Hirmas as quite sensitive one, and Nicola appeared to be moderate in this regard. Cara and Draga showed reasonable tolerance to R. solanacearum infection. These results may be due to the genetic variability existed in genes controlling brown rot resistance among the tested potato cultivars.
Results indicated that crude plant extracts from Corchorus olitorius, Solanum nigrum, Portulica oleracea, Ricinus communis appeared more effective against the R. solanacearum. Many chemical compounds were found to have distinct antimicrobial potential as concluded by HPLC results. Keywords: Ralstonia solanacearum, different habitats, weeds, , potato bacterial wilt, potato varieties, plant extracts, PCR, Real-Time PCR, phylotype analysis and HPLC technique.