What Causes an Onion to Mold?

By Kiva Bottero

Onions are a staple in many different cuisines around the world thanks to their versatility, pungent flavor and wonderful aroma. However, like any vegetable, this underground bulb is susceptible to mold, which usually forms as little black spots known as Aspergillus niger. This fungus can infect onions while in the field or during storage. Care needs to be taken when choosing and storing onions to prevent them from becoming moldy.

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Onion Identification

Onions are generally categorized as either fresh or storage. Fresh onions are known for their sweetness. They generally come out in early April and are named after the region they are grown in, such as Vidalia or Maui, for example. They are called fresh because they are meant to be eaten soon after purchase, not stored.

Storage onions are picked in the bountiful summer harvest when onions are in ample supply. They are dry-cured to help slow the decomposition process and kept for long-term storage. They are more firm than fresh onions due to their lower water content and are more pungent, but less sweet than fresh onions. Yellow, red, white and Spanish onions are some examples of storage onions.

Growing Conditions that Cause Mold

The black mold Aspergillus niger is a common fungus found in soil and on dead plant matter. Onions usually get infected by this fungus through wounds on their body or roots. Wounds are sometimes the result of pests eating away at the onions or farmers cutting them with tools when harvesting.

The fungus thrives in warm, humid climates such as Florida, but also grows in the southern desert states.

Improper farming practices contribute to bulb diseases. Some examples include not rotating crops, not using clean seeds and not having good soil drainage.

Handling Conditions that Cause Mold

When handling onions during harvesting, transportation and storage, bruising can often occur, causing fungus infection. Bruised parts of onions are where mold is most likely to grow. Onions should have no smell whatsoever. Smell indicates that they are bruised under their skin and are going bad.

Storage Conditions that Cause Mold

Improper storage of onions can cause mold. Warm, moist environments contribute to mold growth. Therefore, storage onions are best kept in a single layer in a dry, dark, cool and well-ventilated place such as a cellar. However, it is essential to protect stored onions from freezing temperatures. Storing onions in close proximity to potatoes, which give off moisture, can also cause onions to rot.

Fresh onions need to be refrigerated. They generally last up to one week, but their shelf life can be extended slightly by storing them so that they are not touching.