Aspergillus rot of onions

Pathogen and disease type. The pathogen Aspergillus welwitschiae is a fungus belonging to the black Aspergilli group. It is the primary cause of black mold rot in onions and garlic. This disease is highly infectious, spreading through airborne spores that can colonize hosts in both the field during wet harvest conditions and within storage facilities where ventilation is poor.

Affected crops. The disease primarily impacts Allium species, including onions, garlic, and shallots. The pathogen typically gains entry into the bulb through neck tissues that have not fully dried after harvest, or through physical abrasions, cuts, and cracks on the outer scales resulting from mechanical harvesting or transport handling.

Symptoms and signs. The characteristic symptom is the appearance of a dark, sooty-black powder on the surface of the bulbs, particularly around the neck. This black dust is composed of millions of fungal spores. As the infection progresses, internal scales become soft, water-soaked, and eventually shrink as the fungus consumes the plant tissue, leaving the bulb shriveled and hollow.

Development conditions and impact. The fungus thrives in warm, humid environments, with temperatures between 25°C and 30°C being optimal for rapid growth. The economic impact is severe, as it is one of the most destructive storage diseases. If not controlled, it can lead to total loss of stored batches. Furthermore, infected bulbs often become sites for secondary infections by soft-rot bacteria, worsening the damage.

Protection and prevention. Controlling the disease requires a rigorous post-harvest strategy.

• Ensure proper drying and curing of bulbs immediately after harvest.
• Minimize mechanical damage during handling.
• Maintain storage conditions with temperatures near 0°C and relative humidity below 70%.
• Use proper ventilation to prevent moisture accumulation on the bulb surfaces.
• Practice crop rotation and field sanitation to reduce initial inoculum levels in the soil.