Rotten onion bulbs were collected from different markets in Assuit governorate and screened for the presence of
phytopathogenic bacteria using yeast extract-dextrose-calcium carbonate medium and King’s medium B. Thirtyfive
bacterial isolates were recovered from 15 onion bulb samples. Twenty-five bacterial isolates were classified as
putative Pseudomonas aeruginosa. Ten other unidentified bacteria were isolated at the same time and designated
as Gram negative bacteria and nonflourescents. None of these bacteria have been reported as being pathogenic
and it is likely that these other bacteria were present as concomitant/commensal bacteria. The causal agent of
onion bulb rot was identified as Pseudomonas based on phenotypic characteristics. One representative isolate of
these has been further identified as a Pseudomonas aeruginosa by sequencing of the 16S rRNA gene. The bacteria
have the ability to grow in a temperature range of 4
438C, with optimal growth at 378C. Virulence of the
Pseudomonas aeruginosa was also confirmed by the production and secretion of a large variety of enzymes
capable of degrading the complex polysaccharides of the plant cell wall and membrane constituents. Scanning
electron micrographs indicated that the Pseudomonas aeruginosa isolates were able to abundantly colonize the
internal tissue of fleshy scales of onion bulbs. The virulence of the pathogen may be attributed to biofilm
formation on the plant cell wall via fibrous materials as revealed by scanning electron microscopy as well as high
activity of proteases, lipase, pectinases, and alkaline phosphates. This is the first evidence showing that a
Pseudomonas aeruginosa strain E can cause soft rot of onion bulbs.