What causes Onion Rot?

What causes Onion Rot?

Onion bulb rot may be caused by bacterial or fungal pathogens.

Accurate diagnosis of bulb rot (bacterial, fungal, or other) is important to select an effective management strategy.

For example, fungicides are not effective in controlling bulb rot caused by bacteria.


Bulb rot caused by bacteria

Botrytis neck rot

Bulb rot caused by fungus (Botrytis neck rot)

Bacterial bulb rot may be caused by dozens of different types of bacteria.

It is generally not possible to determine the type of bacteria that caused the rot based on bulb rot symptoms. Samples would have to be sent to a diagnostic lab.

To learn how to identify bulb rot caused by bacterial pathogens, and to distinguish it from bulb rot caused by other factors including fungal pathogens and abiotic factors (such as bruising), check out this video:

English video (4 min)

Spanish video (4 min)

In the field, bacterial diseases cause chlorosis, water-soaking, and collapse of the inner leaves of the plant.


In severe cases, all of the leaves collapse and die.


Not all plants with foliar symptoms of bacterial disease result in bulb rot.

Sometimes bulb rot results from onion plants without foliar symptoms of bacterial disease.