Location: Vegetable Crops ResearchTitle: Factors contributing to bacterial bulb rots of onion Author
Submitted to: Onion World
Publication Type: Trade Journal
Publication Acceptance Date: 1/15/2016
Publication Date: 2/11/2016
Citation: Havey, M.J. 2016. Factors contributing to bacterial bulb rots of onion. Onion World. 32(2):17. Interpretive Summary:
Technical Abstract: The incidence of bacterial rots of onion bulbs is increasing and has become a serious problem for growers. This increase is likely due to a combination of factors, such as high bacterial populations in soils and irrigation water, heavy rains flooding production fields, higher temperatures, etc. It may be that recent increases in bacterial bulb rots are due in part to the drive towards larger bulb sizes. Sizes of onion bulbs are controlled by both genetic and environmental factors. Environmental factors include direct seeding versus transplanting, planting density and stand establishment, and heat units accumulated throughout the summer. A major genetic factor is maturity; later cultivars have more time to produce larger bulbs. Historically, breeders of long-day storage onions selected for tight necks that went “tops down” prior to harvest in the fall. These onions tended to store better because the collapsed necks provided a barrier to bacterial or fungi entering the bulb, although bulbs may be smaller because “tops down” indicates that plants reached full maturity in the field. However if growers plant relatively late cultivars hoping to enhance bulb sizes, these onions may retain larger necks later into the growing season and may not go “tops down”. The grower may undercut the onions and roll the tops to hasten maturity. The result is that green foliage may lie on the soil and provide the opportunity for bacterial and/or fungal infection, especially if rains occur shortly after undercutting bulbs or rolling the tops. These plants will then be picked up, topped, and placed in storage, where rotting of the specific leaves (rings) infested by pathogens can occur. If one cannot avoid higher inoculum loads and excessive moisture, growers may wish to consider earlier maturing cultivars that have a greater chance to go naturally “tops down” with tighter necks.