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SummaryThe brown rot disease of potato is important plant disease because it leads to great decrease in potato yield in Egypt and different locations in the world. Therefore, this study aimed at isolate the causal agent of this disease (Ralstonia solanacearum) fDifferent isolates of Ralstonia solancearum were been obtained from infected potato tubers collected from different locations in El-Dakhlia (6 locations) and El-Sharkia (2 locations) Governorates. Most of these isolates (13 isolates) contained both viruleFrom only the clay soil of El-Dakhlia Govenorate, the isolated phages (active against R. solanacearum) were obtained. On the other hand, no-phage was obtained from the sandy soil of El-Dakhlia Govenorate and from the soil of El-Sharkia Govenorate.The isolated phages could be distinguished into three types according the plaque morphology and electron microscopy that were designated by RSP1, RSP2 and RSP3. Furthermore, all the isolated phages were active only against the avirulent strains.The plaques of isolated phages could be distinguished into clear plaques (plaques of phages RSP1& RSP3) and turbid plaques (plaques of phage RSP2) Also, plaques of phage RSP1 were the largest ones.It was observed that there were no lysogenic bacteria found for the three isolated phages after 7 days of incubations.Plaque diameters of the isolated phages were increased with further incubation (growing plaques) and this increase continued for 6 days. After this period, the plaque diameters of phage RSP1 were the largest (20 mm), while those of phage RSP2 were the smaAccording to the electron microscopy, these phages belong to group B (phages RSP1 & RSP3) and group C (phage RSP2) of classification of Bradley (1967) and Family Styloviridae (phages RSP1 & RSP3) and family Podoviridae (phage RSP2) of classification of MaStudying the rate of adsorption experiment clarified that phage RSP3 had the highest value of adsorption rate constant (9.75X10-4 PFU/ CFU /ml) and the lowest time of highest adsorption (6 minutes). While, phages RSP1 and RSP2 had the same time of maximumStudying the One-step growth curve experiment revealed that the isolated phages had the same latent period (2 minutes) and the same rise period (6 minutes). However, the burst size values were different for the three phages, where phage RSP1 had the higheMolecular studies of R. solanacearum strains and their phages revealed that the chromosomal DNA of the two strains (virulent and avirulent strains) of the plant pathogen had a similarity of 92.7% and there was a positive relationship between the two straiThe three isolated phages were resistant to high temperatures, where the complete inhibition of the three isolated phages was observed at 100 ºC after 15 minutes (phage RSP1) or 20 minutes (phages RSP2 and RSP3).Furthermore, these phages were resistant to exposure UV-irradiation, where complete inhibition was observed after exposure to this irradiation at a distance 30 cm for 65 minutes (phage RSP3) or 70 minutes (phages RSP1 and RSP2).Studying the effect of pH on the infectivity of the tested phages indicated that the optimum pH value for the growth of these phages was almost pH 8.0 Furthermore, these three phages were resistant to pH values. Where the complete inactivation was occurreStudy of the effect of some organic solvents (Petroleum ether, acetone, ethyl alcohol and chloroform) on the infectivity of phages showed that the infectivity of these phages decreased with further incubation with these solvents. However, these phages werStudy of the effect of fumigation by some oils on the infectivity of tested phages indicated that the infectivity approximately did not affected by fumigation of these oils.Study of the effect of some diluents on the infectivity of isolated phages revealed that some of them were stimulant and others were inhibitors. Phage RSP1 was stimulated by the highest number of used diluents, where it was stimulated by distilled water, Storage of the tested phages in different solutions SPB, distilled water, saline solution and soil extract solution) at different temperatures (4ºC, room temperature 30ºC) for 6 weeks revealed that all phages were maintained viability in all cases. Furthe
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