|Du toit, L.|
Submitted to: Plant Pathology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/1/2012
Publication Date: N/A
Citation: Interpretive Summary: Storage onion bulbs are produced on more than 43,000 acres annually in the United States, with a farm value of $622 million. About 66% of this production is in the western United States. Losses to onion bulb rots in storage can have significant negative financial impact on producers. Bulb rot symptoms are caused by many different types of bacteria. Field curing and/or postharvest curing are used widely to prepare onion bulbs for long-term storage. We found that bacterial storage rots of onion bulbs produced in the Columbia Basin of Washington and Oregon can be caused by multiple bacterial pathogens, present alone or in combinations. Although E. cloacae has historically been considered a pathogen of minor importance, it was the predominant bacterium associated with bulbs showing bacterial rot symptoms. E. cloacae is associated with onion bulbs exhibiting bacterial rots in the Pacific Northwest, and appears to have an asymptomatic, foliar phase that can spread into the bulb. Environmental conditions in onion fields prior to harvest, postharvest curing practices, and storage conditions can influence development of bacterial storage rots. Surveys of onion bulbs are needed to assess the diversity of onion bacterial bulb rot pathogens and assist producers, storage managers and scientists in developing management strategies to reduce onion bulb rot.
Technical Abstract: Approximately 1.6 million metric tons of onion bulbs are produced annually in the Pacific Northwest USA. Bulb decay can be a major problem and is caused by a variety of plant pathogens. Onion bulbs exhibiting symptoms of bacterial rot were sampled to determine the causal agents. Enterobacter cloacae, alone was isolated from 43.3% of the bulbs, and was isolated in combination with other bacterial plant pathogens from an additional 20.7%. Burkholderia gladioli pv. alliicola was isolated from 31.1%, B. cepacia from 14.2%, Pectobacterium carotovorum subsp. carotovorum from 13.2% and Pantoea ananatis from 4.7% of the bulbs. E. cloacae was the most prevalent bulb rot pathogen and investigations were completed to study the E. cloacae-onion interactions leading to bulb decay. E. cloacae inoculated into onion leaf tissue at log10 5.7 CFU plant-1 was detected acropetally up to 26 cm and 20 cm basipetally from the inoculation site and attained a final density of log10 7.7 CFU plant-1 by 6 weeks post-inoculation. Inoculated leaves remained asymptomatic. When bulb scales were inoculated with log10 3.4 CFU scale-1, an average of log10 8.0 CFU scale-1 was recovered after 48 h and necrosis was observed after 4 days. This study demonstrates that E. cloacae is associated with onion bulbs exhibiting bacterial rots in the Pacific Northwest, and appears to have an asymptomatic, foliar phase that can spread into the bulb.