In May 1997, sweet onions (Allium cepa L.) grown in Toombs County, GA, displayed symptoms of blighted leaves, bleached and rotted seed stalks, and rotted bulbs. Gram-negative bacteria were isolated from infected tissues on nutrient agar and shown to be from the genus Pantoea on the basis of cell morphology (rod-shaped), yellow pigmentation, utilization of glucose in an oxidative and fermentative manner, presence of catalase, and absence of oxidase. These characteristics are typical of bacterial strains belonging to the Enterobacteriaceae (facultative anaerobes). Initially, these bacteria were thought to be P. agglomerans, a common saprophyte associated with plant material. However, fatty acid analysis, using bacterial identification software (MIDI, Dewark, DE), identified (second choice) some strains as possibly being P. ananas.. Further testing indicated that all strains utilized cellobiose, melibiose, inositol, glycerol, and sucrose, but not pectin, starch, or gelatin. However, those strains identified by fatty acid analysis as P. ananas were differentiated from P. agglomerans on the basis of indole production, lack of phenylalanine deaminase, and lack of nitrate reductase. To confirm pathogenicity, three strains of each species (total of six strains) were grown overnight in nutrient broth shake cultures. Bacterial cells were harvested by centrifugation and suspended in 0.01 M phosphate-buffered saline (0.85%). Inoculum was adjusted to approximately 5 × 108 CFU/ml with a spectrophotometer and misted with a chromatography sprayer onto onion leaves of approximately 10-week-old onion plants in the greenhouse. Onions were predisposed by placing them under plastic bags for 18 h prior to inoculation. Inoculated plants were left covered with plastic bags for an additional 24 h after inoculation. There were two plants per pot, each test had three pots, and the test was conducted twice. The three strains of P. agglomerans and buffer control resulted in no symptoms. The three strains of P. ananas produced severe blighting, rapid collapse of tissues, and rapid drying so that leaves were light tan and dry within 3 days. Disease on plants infected with P. ananas continued to develop until death of all foliage and bulbs shriveled and collapsed. Results were consistent for all replications and both trials. Bacteria recovered from diseased tissues were gram-negative, yellow, and facultative anaerobic, and produced indole but not phenylalanine deaminase or nitrate reductase; i.e., the bacteria demonstrated the same characteristics as P. ananas. Although P. agglomerans has been reported to produce similar symptoms in South Africa (1), our P. agglomerans strains were nonpathogenic. To our knowledge this is the first report of P. ananas causing a disease of onion.
Reference: (1) M. J. Hattingh and D. F. Walters Plant Dis. 65:615, 1981.
