Causal Agent: Burkholderia gladioli pv. allicola (Sny. Pseudomonas gladioli pv. allicola)

Distribution: Worldwide

Symptoms: Field symptoms often appear as one or two wilted leaves in the center of the leaf cluster. These leaves eventually turn pale-yellow, and die back from the tip, while older and younger leaves maintain a healthy green appearance.

During the early stages of this disease, the bulbs may appear healthy except for a softening of the neck tissue. In longitudinal sections, one or more inner scales will look watery or cooked. The disease progresses from the top of the infected scale to the base, where it can then spread to other scales, rather than spreading crosswise from scale to scale. Eventually, all the internal tissue will rot.

Finally, the internal scales dry out, and the bulb shrivels. Squeezing the base of an infected plant causes the rotten inner portion of the bulb to slide out through the neck, hence the name 'slippery skin'.

Conditions for Disease Development: The bacterium requires moisture for infection, and grows in the temperature range of 5 to 41 degrees C (41 to 105 degrees F). Severe disease can occur during periods of high rainfall combined with strong winds or hail. Heavy irrigation and persistent dews are also conducive to this disease.

This bacterium is soilborne, and can be readily water-splashed to the foliage and necks, where it can enter through wounds. As the plant matures, it increases in susceptibility, with the mature plant being highly susceptible. In warm weather (30 degrees C / 86 degrees F), infected bulbs can decay within 10 days. However, in storage, the decay moves slowly, often requiring 1 to 3 months for total decay.

Control: Onions should be harvested when the bulbs have reached full maturity, and should not be stored until they have been properly dried. Minimizing stem and bulb injury and avoiding overhead irrigation when the crop is approaching maturity can reduce losses from this disease.