Rotten to the Core: The Center Rot Disease of Onion

What is center rot?

Center rot is a bacterial disease of onion caused by Pantoea ananatis, P. agglomerans, P. alli, and P. stewartii subsp. indologenes. This disease causes significant economic loss to the onion bulb industry in the U.S. by causing damage to both the leaves and onion bulbs. Efficient and effective management of center rot remains a significant challenge in onion-producing regions, including Pennsylvania, since damage can occur both pre- and post-harvest.

What does center rot look like?

Newly infected leaves develop small, water-soaked lesions. These lesions expand rapidly into bleached white areas of necrosis. Disease symptoms eventually progress, and the entire leaf becomes water-soaked and soft. As the disease severity increases, multiple leaves become infected and wilt (Figure 2).

Over time, the leaf infection progresses down into the onion neck and corresponding bulb scale(s). Infected bulb scales show discoloration, primarily light to dark brown or yellow, water-soaked lesions. Similarly, discolored scales are rotted and soft. Center rot is often associated with interior bulb scales, leaving the outer bulb scales intact, making it difficult to detect at harvest (Figure 1), and symptoms are typically limited to one to a few scales and not a complete rot of the bulb interior. Hot and humid conditions favor disease development in the field.

Where does center rot come from?

Infected seeds, infected asymptomatic transplants, and weeds, as well as thrips, including onion and tobacco thrips, are the primary sources of Pantoea spp. inoculum in onion production (Figure 3). Natural openings or wounds in the leaves or bulbs provide entry for the bacterium. Thrips can acquire the bacteria when feeding on contaminated plants and transfer it to new healthy plants during feeding (Dutta et al., 2014). Similarly, rain and overhead irrigation water also aid in the spread of the bacteria through splash dispersal within and between plants in the field.

How to manage center rot?

Cultural Practices

Purchase high-quality seeds and transplants from reputable sources. Choose a less susceptible onion cultivar. In general, pungent cooking onions are less susceptible than Spanish sweet onions. Crop rotation for two or more years, along with the elimination of volunteer onions and weeds is recommended. Avoid overhead irrigation, especially late in the season, since this can spread the pathogen (Belo et al., 2023). Using reflective or biodegradable plastic mulch can help reduce soil temperatures, leading to lower disease incidence. Reflective mulches can also aid in thrips management by disrupting their vision and ability to locate onion plants. Maintain optimum plant fertility according to soil and foliar nutrient analyses to avoid nutrient imbalances and plant stress.

Harvest onions when approximately 50% of the plants have lodged or dried down in the field. However, if a greater incidence of foliar symptoms is observed in the field, one strategy to reduce loss would be to harvest early, sacrificing the bulb size for the healthy bulb. Additionally, a study in Georgia has shown that cutting the neck short at harvest causes greater loss than cutting the neck long (Dutta et al., 2022). Keep a long neck length (approx. 3 inches). The neck will dry faster and restrict bacteria from moving to the bulb. Handle harvested bulbs gently and remove diseased or damaged bulbs prior to storage. Cure onion bulbs under dry conditions using forced air. Store onion bulbs at 32°F to 37°F (0°C to 2°C) and 65–70% relative humidity.

Chemical Management

Copper-based fungicides can be used preventatively to help manage this disease, although the results have been variable. These products are protectants used to prevent bacterial infection and reduce secondary spread of bacterial pathogens. Tank mixing of basic copper sulfate (BCS) with ethylene bisdithiocarbamate (EBDC) fungicides like mancozeb has shown improved efficacy, increasing the availability of copper ions (Conover and Gerhold, 1981). Insecticides can be used to reduce thrips population to slow the spread of Pantoea spp. among plants and fields, as well as minimize wounding of the leaf surface.

Integrated pest management (IPM) is the best approach to combating the center rot disease of onions. While there may not be a "silver bullet" solution, integrating cultural, biological, and chemical strategies can help manage the disease sustainably.

References

Belo, T., du Toit, L. J., & LaHue, G. T. (2023). Reducing the risk of onion bacterial diseases: A review of cultural management strategies. Agronomy Journal, 115(2), 459–473.

Conover, R.A., Gerhold, N.R. 1981. Mixtures of copper and maneb or mancozeb for control of bacterial spot of tomato and their compatibility for control of fungus diseases. Proceedings of the Florida State Horticultural Society 94: 154-156.

Dutta, B., Barman, A. K., Srinivasan, R., Avci, U., Ullman, D., Langston, D. B., and Gitaitis, R. 2014. Transmission of Pantoea ananatis and Pantoea agglomerans, causal agents of center rot of onion (Allium cepa L.) by onion thrips (Thrips tabaci Lindeman). Phytopathology; doi: 10.1094/PHYTO-07-13-0199-R.

Dutta, B., Greene, R. and Tyson, C. 2023. Evaluation of neck-clipping length on post-harvest incidence of external and internal bacterial bulb rot in onion, Georgia, 2022. PDMR 17: V008