Onion bacteriosis

Symptoms are often internal and may not be visible until the bulb is cut open. The tissue becomes soft, watery, and develops a yellowish or brownish discoloration.

A distinctive sign is a foul, pungent smell emanating from the decaying tissue, which is typical for bacterial soft rot outbreaks.

Affected inner scales may appear translucent or "cooked," indicating the enzymatic degradation of cell content by the bacteria.

Externally, the bulb might appear normal, but applying slight pressure to the neck often releases a cloudy or smelly liquid, a clear indicator of the presence of the pathogen.

In advanced stages, the basal plate decays, the roots detach, and the entire bulb structure collapses into a putrid mass of tissue.

Onion bacteriosis is caused by the gram-negative bacterium Lonsdalea britannica (formerly associated with Erwinia herbicola or Pantoea agglomerans). This phytopathogen primarily affects bulbous crops.

The bacterium survives in soil, crop residues, and on the surface of onion bulbs. It acts as a opportunistic pathogen that takes advantage of plant weaknesses.

Infection occurs mainly through mechanical injuries, wounds caused by insect feeding, or natural openings like the neck area if it remains moist during harvest.

Once inside the plant tissue, the bacterium produces enzymes that degrade cell walls, leading to the breakdown of internal structures and the characteristic soft rot.

Given its ability to thrive in various moisture conditions, this pathogen remains a constant threat to onion production and storage stability.

High humidity and rainfall during the late stages of crop maturation significantly increase the risk of infection by Lonsdalea britannica.

Warm temperatures (20°C to 30°C) accelerate the growth rate of the bacteria, leading to rapid disease development within the plant tissues.

Poor ventilation during storage, combined with high ambient humidity, causes condensation on the bulbs, which allows the pathogen to spread from infected to healthy onions.

Mechanical damage during harvesting, grading, or transport creates entry points, making the bulbs highly susceptible to bacterial contamination.

Harvesting bulbs with thick, poorly dried necks is a major risk factor, as moisture trapped in the neck provides a pathway for bacteria to reach the bulb interior.

The economic impact of onion bacteriosis is severe, often resulting in significant post-harvest losses during storage and transit.

Contaminated batches are unsuitable for sale or consumption, leading to reduced market value and potential loss of the entire harvest stored in bulk.

The disease spreads quickly through contact in storage environments, meaning one infected bulb can initiate a decay process in neighboring healthy ones.

Producers face additional costs for sorting, removing spoiled onions, and managing the waste produced by the high volume of decaying vegetables.

Furthermore, infection can negatively impact the germination rate and vigor of onions if used for seed or replanting purposes, affecting future yield potential.

Effective management begins with proper crop rotation and the selection of disease-resistant varieties suited to local environmental conditions.

Integrated pest management (IPM) is crucial to control insects that feed on onions, as these pests facilitate the entry of bacteria into the plant.

• Proper curing of the crop immediately after harvest to ensure a dry, tight neck.
• Maintaining optimal storage conditions (temperatures near 0°C and low humidity).
• Using high-quality, pathogen-free sets for planting.
• Ensuring adequate ventilation in storage facilities to prevent moisture buildup.
• Sanitizing storage bins, tools, and packing areas to reduce the pathogen load.

Agronomic practices, such as avoiding excessive nitrogen fertilization, help prevent the formation of succulent, late-maturing tissue that is more vulnerable to bacterial rot.

Implementing strict hygiene protocols throughout the production and post-harvest cycle is the most reliable way to mitigate the risk posed by Lonsdalea britannica.