Ten isolates of Ralstonia solanacearum bacterium were isolated from potato tubers and soil samples which were collected from Qalubiya and Beheira governorates. The obtained isolates were identified initially according to their morphological, physiological and biochemical characteristics then, immunofluorescent antibody stains (IFAS) and polymerase chain reaction (PCR) techniques were used to confirm the identification of R. solanacearum bacterium. All tested isolates caused symptoms of bacterial wilt disease on potato plants compared with the un-inoculated control with superiority of R. solanacearum (R6) which recorded the highest percentage of infection and disease severity of brown rot disease at 35 day post inoculation the sterilized soil with brown rot pathogens. All tested potato cultivars were susceptible to infect with R. solanacearum when cultivated in sterilized soil and un-sterilized soil. In the sterilized soil, isolate R6 recorded the highest percentage of infection and disease severity on cvs. Gelabica and Cara whereas, the isolates R2 and R1 were the most virulent on cvs. Barren and Spunta, respectively. However, isolate R6 on cvs. Gelabica and isolate R4 on Cara, and R3 on Barren and R4 on Spunta recorded the highest disease severity in the un-sterilized soil.