First report of soft rot of onion bulbs in storage caused by Pseudomonas aeruginosa in Egypt

Abstract

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. Thirty-five 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 4–43°C, with optimal growth at 37°C. 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.

Keywords:

• onion soft rot
• phytopathogenic bacteria
• Pseudomonas aeruginosa
• cell wall degrading enzymes

Acknowledgements

This research was financially supported by Assuit University, Egypt.