To characterize the genetic variation between virulent and avirulent isolates of Ralstonia solanacearum race 3 (biovar II), the causal agent of potato brown rot. Nine isolates of R. solanacearum recovered from the natural habitats (potato tubers, weeds, soil and water) were used. Six virulent and three avirulent isolates were characterized in terms of pathogenicity and molecular level using PCR technique with specific primers targeting three-virulence related genes located on the megaplasmid which was involved directly or indirectly in the disease process. Both virulent and avirulent forms of R. solanacearum were pathogenic to potato plants causing different symptoms. The two forms of the pathogen were containing the megaplasmid which carry three genes hrpB, egl and fliC. Detection anddigestion of the amplification PCR of the three genes by two restriction endonuclease enzymes EcoRI and DraI showed differences between the two forms of R. solanacearum, whereas digested egI gene by the DraI enzyme gave two bands with all virulent isolates, while it gave only one band with the avirulent isolates. On the other hand, there were no differences in bands with the two genes hrpB and fliC.
Key words: Ralstonia solanacearum, virulent, avirulent, pathogenicity, potato, PCR, megaplasmid, restriction enzymes.
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