Collection of soft rotted diseased samples of onion Diseased plant samples of onion were selected based on visible symptoms of soft rot and characteristic odor described by Rich (1983) and Shing (2001). Symptoms were observed at various levels of post harvested onion bulbs during survey. Soft rotted samples of onion were collected from farmers’ homes and markets of different locations of Bangladesh. The onion varieties were Kalashnagari, BARI-1, Taherpuri, Faridpuri, Indian varieties and locations were Santhia (Pabna), Sujanagar (Pabna), Gazipur, Rangpur, Karwan Bazar (Dhaka) and Faridpur. After collection of rotted samples brought to the microbiology laboratory of Bangabandhu Sheikh Mujibur Rahman Agricultural University (BSMRAU), Gazipur and the soft rotted bacteria were isolated. Isolation of soft rot bacteria Soft rot bacterial isolates were isolated from different sample of onion by “Streak plate” technique as described by Mortensen (1997) and Kim et al. (2002). A common bacteriological medium, yeast peptone dextrose agar (YPDA) was used for isolation of soft rot bacteria. YPDA was prepared by dissolving yeast 3.0 g, peptone 0.6 g, dextrose 3.0 g, and agar 15 g in 1000 ml of distilled water. The pH of the medium was adjusted to 7.0 using 0. 1 N KOH and cooked on hot plate. After cooking the medium was autoclaved for 20 min at 121o C under 1.1 kg/cm2 pressures. The medium was poured into petridishes at the rate of 20 ml/ plate and were cooled in a clean bench. To isolate the causal bacteria, small part from the margin of rotted tissues of the infected bulbs of onion samples were separated with a scalpel and were surface disinfected with 1% sodium hypochlorite (NaOCl) for 2-3 min. Sterilized samples were washed several times in sterilized water to remove the residual hypochlorite. The samples were placed in petridishes containing sterilized water and were crushed with a sterile scalpel. After crushing, the petridishes were kept undisturbed for 10-15 min to release the bacteria associated with rotted tissues. One loop full of resulting suspension (water containing bacteria) was streaked on the solidified YPDA medium in each plate. The plates were incubated at 300 C for 48 hr. Characteristic individual bacterial colonies that appeared on YPDA medium were picked up using a bacterial loop and transferred to another plate. Purification of bacterial colony was done by re-streaking of single colony on another fresh plate. Potato and onion soft rot test All of the bacterial isolates originated from single colonies were tested for their ability to cause soft rot on potato tubers and onion bulbs following standard procedure (Lelliot et al., 1966). Potato tubers and onion bulbs were sterilized with 70% ethyl alcohol, rinsed in sterile distilled water and aseptically cut into slices (ca. 1 cm). The slices were put in petridishes containing sterilized filter paper impregnated with ca. 2 ml of sterilized distilled water. The soft rot tests were repeated at least twice for fulfilling the Koch's postulates. The slices were inoculated with needle pricking method. The inoculated slices were maintained in moistened petridishes (Togashi, 1988; Nabhan et al., 2006) and incubated at 300 C for 2-3 days. The bacterial cultures produced characteristic symptoms of soft rot on potato and onion slices were selected and preserved more virulence isolates for further studies. Preservation of pathogenic bacterial strains Pure single colonies of each of pathogenic soft rot bacterial isolates of onion were preserved in test tubes containing sterilized distilled water and in test tube with half slant culture containing YPDA overlapped with sterilized liquid paraffin. The tubes containing culture were preserved in a cool room (140 C). Characterization of the pathogenic bacterial strains A series of physiological and biochemical tests were performed for characterization of the isolated more pathogenic isolates. The physiological and biochemical tests were a. Potato soft rot test b. Fermentation of glucose (OF test) (Hugh and Leifson, 1953), c. Gram reaction (Suslow et al., 1982), d. Oxidase reaction (Kovacs, 1956), e. Catalase production (Hayward, 1992), f. Gelatin Liquefaction test (Schaad, 1988), g. Urease production (Schaad, 1988), h. Nitrate reduction test (Lelliott and Dickey, 1984), i. Indol test (Lelliott and Dickey, 1984), j. Lecithinase test (Clung and Tobae, 1947), k. Acetoin production (Dye, 1969), l. Methyl red test, m. Arginine dihydrolase (Thornley, 1960), n. Gas formation (Hugh and Leifson, 1953), o. Hypersensitive reaction (Klement and Goodman, 1967) and p. Utilization of carbon (Ayers et al., 1919). Three soft rotting bacterial strains Erwinia carotovora subsp. carotovora ATCC-15713, E. chrysanthemi Ura-2 and Burkholderia cepacia ATCC 25416 were used as reference strains in this experiment.