Black Mold

Black mold on onionBlack mold on onion
Onion with brown discolorationOnion with brown discoloration
Onion with symptoms of black moldOnion with symptoms of black mold
Black mold visible under onion skinBlack mold visible under onion skin


  • Onion


Aspergillus niger is a commonly found fungus that causes mold on many vegetables including onion. Without a microscope the spores can be mistaken for smut another fungus that can affect onions. However, in Utah smut is rare.


Aspergillus spores are airborne, soilborne and seedborne. The fungus can survive on plant debris. Infection usually occurs wounds in the bulb.

The disease is worse under warm condition especially during storage. Optimum growing conditions are 82-93°F. Harvested onions should be stored at cool temperatures as soon as possible. If long-distance transport occurs under warm conditions the fungus will resume growth even if onions had been stored in cool conditions.


  • Black discoloration in the neck area
  • Dark lesions with spores developing under outer scales
  • Bulb rot


  • Prevent wounding or bruising of bulbs
  • Store and transport bulbs below 59°F








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