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Scientific Name: Pseudomonas and Erwiniaspp.

Several bacteria (Pseudomonas and Erwinia spp.) cause a range of diseases known as slippery skin, sour skin and soft rot.  Although caused by different bacteria, they share a common biology.  Bacteria that cause these diseases live in the soil and volunteer onions left in the field; they all require rain splashing on to the plants; and they all tend to infect wounded tissue in the field. They often start in the field and often are not detected until the bulbs have been in storage for some time.

Identification

• All are characterized by a watery rot and a strong smell
• Slippery skin onions may appear sound on the surface but the inner rotted portions will slide out through the neck when squeezed
• Sour skin appears as tan or brown rotted leaves followed by a soft rot near the neck; it’s not as watery as slippery skin and diseased scales separate from healthy ones
• Soft rot develops near the neck and these leaves can be easily pulled off the onion; symptoms can range from a spongy, water-soaked scales to complete bulb breakdown

Often Confused With
Neck Rot

Biology
Fields that are frequently irrigated, particularly during periods when temperatures are hot (>30˚C), are more likely to become diseased if the causal bacteria are present in the soil. Heavy rains, or overhead irrigation splash the bacteria contaminated soil onto leaves. The bacteria then infect through natural openings or wounds made by thrips, diseases, damaging winds, pounding rain or hail.  Once the bacteria infect the leaves, they multiply inside the tissue and move downward into the bulb killing the tissue as they advance.

Very warm temperatures favour the rapid multiplication of the bacteria once inside the tissue resulting in the rapid onset of symptoms. Even at cool to moderate temperatures, the bacteria are still capable of multiplying but symptoms often develop much slower and go unnoticed during harvesting.  Once in storage, the disease slowly invades the bulb tissue resulting in severe losses in quality.

Period of Activity
In general, these bacterial diseases occur during wet periods and are favored by warm temperatures and damaged tissues. Initial foliar symptoms are characterized by severe breakdown of one or more leaves.  Tissues damaged by onion maggot or other diseases are most prone to infection. 

Scouting Notes
Regular field scouting is required to identify diseased plants in the field. 

Thresholds
None established

Management Practices

• Minimize injury (sunscald, bruising and mechanical damage) and provide good storage conditions.
• Since the bacteria are soil-borne pathogens, crop rotation with non-host crops such as cereals for at least 3 years will significantly reduce soil population levels.
• Avoid planting near cull piles or in fields with a history of these diseases for at least 3 years, particularly if the disease was slippery skin.
• Always provide adequate plant and row spacing so that air movement can dry the crop quickly and reduce conditions favourable for infection after a severe storm or heavy irrigation.
• Use a moderate fertilizer program and do not apply nitrogen after bulb initiation since this can exacerbate disease development and losses.
• Do not put diseased or damaged onions into storage, be sure that the bulbs are cured and the necks are closed before placing them in a cool dry storage facility.
• Avoid using irrigation when the crop is approaching maturity, especially with Spanish onion types. 
• Harvest only when onions are fully mature.