FIRST REPORT OF PSEUDOMONAS AERUGINOSA CAUSING INTERNAL BROWN ROT OF STORED ONION BULBS IN TAIWAN
C.J. Huang, C.H. Lin
Onion (Allium cepa L.) may suffer from various bulb rot diseases during postharvest storage, including internal brown rot caused by Pseudomonas aeruginosa (Cother et al., 1976). Onion bulbs, stored at room temperature, were sampled in Chiayi city, Taiwan in July 2017. The bulbs were externally asymptomatic but symptoms of internal brown rot occurred in 6% of bulbs. Repeatedly a bacterium was isolated on nutrient agar from surface-sterilized tissue of diseased onion bulb and fluoresced under UV light while cultured on King’s B plates. Several isolates were purified and maintained on King’s B plates, and produced fluorescence. Two isolates, OP01 and OP03, were stored in Luria-Bertani broth with 20% glycerol at -80°C. They were identified as Pseudomonas aeruginosa by API 20 NE system (Biomerieux) and phenotypically similar to a reference strain of P. aeruginosa (ATCC 27853). Furthermore, the 16S rDNA sequence of the strain OP01 was amplified using the primers fD1/rP1 (Weisburg et al., 1991) and sequenced (GenBank accession number MF946565). Blastn analysis of 16S rDNA indicated that the onion-pathogenic strain was P. aeruginosa (100% identity to P. aeruginosa ATCC 27853, AB594760). For pathogenicity assay, surface-sterilized onion bulbs were injected with a bacterial suspension (Schwartz and Otto, 2000). P. aeruginosa ATCC 27853 was included as a control strain. After incubation at 28°C for 14 days in closed plastic bags in the dark, the artificially inoculated bulbs exhibited symptoms indistinguishable from those observed in natural infections, but not the control bulbs. The same fluorescent bacterium could be consistently re-isolated from the inoculated bulbs, fulfilling Koch’s postulates. To our knowledge, this is the first report of P. aeruginosa causing internal brown rot of stored onion in Taiwan.