Black mold: Fungal sporangia on infected onion bulb
Pathogen: Aspergillus niger
Symptoms
The region below the neck blackens and clusters of black fungal spores from along veins and on or between the outer papery scales of the bulbs. Infected tissues are water-soaked and dry and shrivel over time. In advanced stages of the disease, the entire surface of the bulb turns black and all the scales are infected. Discoloration extends from neck to the central fleshy scales and the disease is usually associated with soft rot bacteria.
Disease development
Spores of this fungus are very common in the air and soil. Black mold is most common when temperatures are higher than 30°C in the field or 24°C in the storage. Free moisture for 6 hours or longer on the onion surface is necessary for infection to occur.
Disease management
Treat seeds with appropriate fungicide prior to planting to reduce rot in mature bulbs; avoid bruising bulbs during and after harvest; storing at temeperatures below 15°C (59°F) prevents mold from spreading but it will resume once temperature increases.
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