Eight isolates of bacteria were isolated from onion bulbs exhibiting symptoms of soft rot disease. The cultural,  morphological and physiological characteristics of the selected isolates revealed that the isolated bacteria belong to Pseudomonas gladioli pv. alliicola. Pathogenicity tests indicated that inoculation of whole bulbs or their slices (of Giza-20 or Giza-6 mohasan onion cultivars) developed different degrees of soft rot, that ranging from a colourless soft rot to the dark brown discoloration. Chitosan proved the most effective compound in reducing the growth of onion soft rot pathogen, followed by sodium citrate, seaweed extract and finally salicylic acid. All tested compounds reduced the disease incidence
(DI%) and disease severity (DS%) of onion bulbs either under natural or artificial inoculation, compared with untreated control treatment. Seaweed extract (at 0.8 and 1.6%, v:v) was the most affective compound, followed by chitosan, at the same concentrations and sodium citrate (1 or 2 mg/l). Seaweed extract (at 1.6%) and chitosan (at 16 mg/l) treatments caused the highest decrease in both DI% and DS% in wounded bulbs stored 30 days after inoculation at room temperature conditions. Moreover, both seaweed extract and chitosan completely protected the sound onion bulbs against bacterial infection until 30 days of storage, either under artificial or natural inoculation. The percentages of dry matter, total sugars, colouring matter and vitamin C in bulbs of the two tested cultivars of onion were significantly decreased due to infection with Pseudomonas gladioli pv. alliicola (isolate Ps4), whereas total phenols concentration was
increased in infected bulbs compared to the healthy ones. Results also indicated that dry matter, total sugars, total phenols, colouring matter concentrations were higher in Giza-20 (red cultivar) than in Giza-6 mohasan (white cultivar), either in healthy or infected onion bulbs. Vitamin C concentration was higher in bulbs of healthy cv. Giza-6 mohasan than in cv. Giza-6, whereas bulbs infection led to decrease Vitamin C concentration in both cultivars. The percentage of reduction was more in bulbs of cv. Giza-20 mohasan.