What to Do About Black Spots on Your Onion

Finding black, sooty residue on your onions can be an unwelcome discovery. This guide will clarify the cause of this common fungus, address safety concerns, and provide methods for preventing this issue from developing on your stored onions or in your garden.

Identifying the Cause of Black Spots

The black, powdery substance on your onions is a fungus known as Aspergillus niger. It is naturally present in soil and air and appears as a dry, sooty powder between the papery outer skin and the first fleshy layer of the onion, often near the neck.

Aspergillus niger flourishes in warm, humid conditions. It infects the onion through small wounds, bruises, or through the neck as the plant’s tops dry during maturation. The visible black spots are masses of fungal spores, which can form in streaks that follow the veins of the onion.

Are Onions with Black Spots Safe to Eat?

In most situations, an onion with black mold is safe to consume after taking a few precautions. If the mold is confined to the outer dry, papery layers, these can simply be peeled away. You can rinse the onion under cool running water to wash off any residual spores before proceeding to use the unaffected flesh.

If the mold has penetrated the first fleshy layer of the onion, you can still use the bulb. Simply cut away the affected portion, making sure to leave about a one-inch margin around the moldy area. After trimming, wash the remaining part of the onion thoroughly before it is prepared and eaten.

An exception applies to certain individuals. People with known allergies to mold or those with compromised immune systems should not consume any part of an onion that has shown signs of mold. If an onion feels soft or mushy, it should also be thrown away, as this can indicate a more advanced decay.

Preventing Black Mold on Stored Onions

Proper handling after harvest is a primary defense against black mold. A process called curing prepares onions for long-term keeping by letting them dry for several weeks in a warm, well-ventilated area. This dries the necks completely and makes the outer skins papery, creating a protective barrier that seals the neck from fungal spores.

Once cured, the ideal storage environment is a cool, dark, and dry location with good air circulation. To inhibit mold growth:

• Store them in a temperature range between 34 to 59 degrees Fahrenheit.
• Use mesh bags, wire baskets, or nylon stockings to allow air to move freely around the bulbs.
• Avoid storing onions in plastic bags, as these trap moisture.
• Do not store them in a refrigerator due to its high humidity.

Preventing Black Mold in the Garden

Prevention can begin in the garden before harvest. Ensuring your onion patch has good soil drainage is important, as waterlogged soil can promote fungal growth. When planting, give each onion enough space for adequate air circulation between the plants as they mature.

Careful handling during the growing season and at harvest is also needed. Physical damage, such as nicks from a hoe or bruising when onions are pulled from the ground, creates an opening for the fungus to enter. When the onion tops naturally begin to yellow and fall over, handle them gently to reduce the chances of infection.