Black mold: Fungal spores on infected onion bulb

Pathogen: Aspergillus niger

Symptoms

Post-harvest black discoloration at neck; lesions on outer scales; black streaks under outer dry scales; entire surface of bulb turning black and shriveling.

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.