In April 2006, sweet onions (Allium cepa) that were grown in Wayne County, GA displayed symptoms typical of either center rot caused by Pantoea ananatis or a foliar blight caused by Iris yellow spot virus (IYSV). After samples tested negative for IYSV by enzyme-linked immunosorbent assay and polymerase chain reaction, isolations were made from basal areas of leaves of infected plants where healthy and diseased tissues converged. All samples yielded yellow colonies on trypticase soy broth agar (TSBA) that were nonfluorescent when transferred to King's medium B. Four strains were characterized and tentatively identified as a Pantoea sp. by yellow pigmentation of colonies, oxidative and fermentative use of glucose, and lack of oxidase. However, the inability to produce indole from tryptophan, negative ice-nucleation activity, ability to reduce nitrate to nitrite, and the presence of phenylalanine deaminase were characteristics more typical of P. agglomerans than P. ananatis. Furthermore, all test strains utilized cellobiose, raffinose, lactose, gelatin, melibiose, and malonate. The identity of the bacterium was confirmed as P. agglomerans by BIOLOG (Hayward, CA). In addition, the 16S gene was amplified using universal primers (forward 5′-AGTTTGATCCTGGCTCAG-3′ and reverse 5′-TACCTTGTTACGACTTCGTCCCA-3′ (1) and sequenced. A BLAST search of the sequence against the NIH GenBank nucleotide library also confirmed the identity of the onion pathogen as P. agglomerans (97% identity) by having 8 of the top 10 bacteria providing significant alignments identified as P. agglomerans. The remaining two matches were uncultured bacteria from environmental samples. To confirm pathogenicity, two onion plants for each of the four test strains were inoculated with a turbid, aqueous bacterial suspension (~1 × 108 CFU ml-1) or sterile water in the lab (n = 8) and the field (n = 8). In addition, two plants each were inoculated with P. ananatis as a positive control and with a water blank and a nonpathogenic strain of P. agglomerans from peach (Png 86-2) as negative controls. All test strains of P. agglomerans produced severe blighting and withering of onion leaves in 4 days, while the water control and Png 86-2 were negative. Results were the same for both lab and field trials. Bacteria recovered from the plants infected with the test strains demonstrated the same characteristics of P. agglomerans as described above. Although P. agglomerans was originally reported as a pathogen of onion in South Africa (2), to the best of our knowledge, this is the first report of P. agglomerans causing a disease of onions in the United States. The long-term impact on the onion industry at this time is unknown. However, considering the close relationship of this organism with P. ananatis and the similarity of disease symptoms with those caused by center rot, there is potential that this bacterium could become established in the onion-growing area of Georgia and become part of a center rot ‘complex’.
References: (1) T. De Baere et al. J. Clin. Microbiol. 42:4393, 2004. (2) M. J. Hattingh and D. F. Walters. Plant Dis. 65:615, 1981.
