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Bacteria of the genera Pseudomonas. Erwinia. Klebsiella, Enterobacter and Escherichia were isolated from internal swollen leaf bases of naturally infected onion bulbs and were pathogenic to onion tissue on reinoculation. In pathogenicity tests, inoculation of slices from symptomless bulbs with one bacterium or with a sterile needle often resulted in isolation of P. aeruginosa, Serratia marcesvens and Bacillus cereus. It is postulated that the bacteria isolated form part of the endophytic microflora of an onion bulb, and that they may become opportunistic pathogens when changes in host physiology are triggered by high temperatures at bulb maturity.