Pathogenic potential of Ralstonla solanadarum isolated from different sources was determined. Mixed inoculum of potato tuber isolates caused the highest wilt severity, on inoculation in tomato seedlings, compared to those from the above ground potato stems. Isolates recovered from Rumex dentatus and Solanum nigrum induced severe wilt similar to those recovered from potato tubers. Isolates from soil, irrigation water, and potato foliage showed limited pathogenic potential. of Plating isolates on SMSA medium revealed phenotypic change from virulent (vi) to the known avirulent (av) form. The latter form, however, showed pathogenic potential on tomato seedlings, thus referred to as atypical (at) form. Isolates stored in sterilized tap water amended with easily assimilated organic material survived for only 110 days, indicating a detrimental effect of the treatment. The pathogen persisted for 6 months in either sandy or day soil maintained at 75% water holding capacity (WHC) and temperature ranging from 28.8°C to 15.2°C (July to December, respectively). Under the same temperature conditions and in dry soil, the pathogen survived in sandy soil for 6 months as well, with very high densities in December. On the other hand, densities in dry clay soil were extremely low after 5 months A e., in November, with higher densities of the pathogen in sandy soil compared to those recorded in day soil. The biofertilization with a biosystem microorganisms product (EM) showed seasonal fluctuation in densities of R. solanacearum. In January and February, however, R. solanacearum could not be detected in the assayed samples. Key words : Ralstonia solanacearum, atypical (at), virulent (vi), avirulent (av), survival in water, survival in soil, preferential host organ effect, phenotypic change in colony morphology , effect of organic material on persistence.