Bacterial wilt of solanaceous crops is an important disease in warm climates, though it has been reported in Europe and in the far northern hemisphere (Thurston, 1963; Lloyd, 1975; Harris, 1976; Wilker, 1992 and Stead et al., 1996) The disease is caused by Ralstonia (Pseudornonas) solanacearum . Based on the host range and biochemical tests, five races and five biovars have been identified for the bacterium (Buddenhagen et al., 1962; Hayward, 1964;He et al., 1983 and Pegg & Moffett, 1971). The dominant strain in Egypt is race 3 (biovar II) being characterized by low virulence to tobacco and a lower optimal temperature than other biovars (Buddenhagen & Kelman, 1964 and Hayward, 1964). From the pathological point of view, the bacterium is found in nature as virulent (vi) and avirulent (av) forms . Both forms may be recovered from diseased plant tissues, though the interrelations between them is not well understood (Kelman,1954) and many questions are still unanswered. Differentiation between the (vi) and (av) forms can be easily made on media containing 2,3,5 triphenyltetrzolium chloride . Colonies of the avirulent mutants are uniformly round, butyrous and deep red in colour due to the formation of formozan on tetrazolium-containing medium, contrary to the virulent ones (Buddenhagen and Kelman, 1964 ). More recently, a Semi Selective Medium of South Africa (SMSA) has been developed for differentiation of virulent and avirulent forms (Elphinstone et al., 1996& 1998).