Mold On Onions: Black Mold & Powdery Mildew Still OK To Eat?

Nobody wants to come home after a trip to the grocery store to find little black powdery spots all over their freshly bought onions.

So, what are the black spots, is it mold, is it dangerous, can they still be eaten and is there anything you can do to stop it from happening again?

In this article, we will be answering all these questions and more, so, for everything you could ever need to know about mold growth on onions, keep reading.

black mold on onion

What causes mold on onions?

There are two primary causes of mold growth occurring on onion bulbs and plants, improper storage conditions and downy mildew occurring during cultivation.


Onions must be stored in dark dry conditions after harvesting, as moisture levels greater than 70% will quickly lead to mold and mildew growth. Likewise, the temperature needs to be carefully controlled to prevent not only mold but bacteria growth. Temperatures of 32-40 degrees Fahrenheit (0-5 Celsius), should be maintained to keep onions at their most fresh.

Under perfect storage conditions, onions can be kept in storage for up to one year.

It can be really tough to keep check of the humidity level in your home, so I’ve added a few links below to the Amazon store where you can pick up a hygrometer. These small alarm-clock-sized units will tell you the average humidity levels of whichever room you place them in, so if storing onions or other vegetables at room temperature in your kitchen, you will be able to clearly see if the humidity level is likely to make them grow mold more quickly.

You can also place several of them throughout your property to ensure you don’t get patches of mold growing in your bathroom or kitchen. Clicking the links below will take you directly to the Amazon store so you can pick one up easily. They are highly rated and very affordable.

DOQAUS Digital HygrometerOpens in a new tab. (US)

ThermoPro TP49-2 Digital Room HygrometerOpens in a new tab. (UK)

Powdery mildew

Onions, beans, artichokes, parsnips, and many other varieties of vegetables are affected by powdery mildews, a type of fungus, most often the Podosphaera xanthii strain from the Erysiphaceae family.

Comfortable temperature ranges for powdery mildew are between 68-81 degrees Fahrenheit, with humid environments speeding up the growth of the fungus, which makes warm wet summers typically to blame for powdery mildew growth. However, improper storage conditions that create high humidity can also promote growth or exacerbate existing growth.

What type of mold grows on onions?

As we have previously stated, powdery mildew growth on onions is a highly common occurrence, however, what you may be more likely to associate with the black “soot-like” substance found on purchased onions, which is caused by one pathogen a fungus named aspergillus niger, which is often found within soil.

This strain is often mistaken for black mold due to its dark hue, however, this is rarely the case, and Aspergillus niger is far more likely to be the cause. This strain is often seen with dark colorations such as very dark brown, dark grays and of course, black. It’s usually described as having a velvety or cotton-like texture, although it can also become powdery over time.

Is the mold dangerous?

Inhalation of aspergillus niger spores by healthy individuals is usually harmless, for people with weakened immune systems, caution should be taken, as inhalation, ingestion, or coming into direct contact with the mold or its spores can lead to symptoms such as:

  • Asthma Attacks
  • Gastrointestinal discomfort
  • Nausea
  • Rash

It is recommended to have as little contact as possible with molds and their spores. This is because it can be extremely difficult to ascertain the strain of mold by vision alone, and so despite the high likelihood of aspergillus niger being the culprit for onion mold growth, you will not be able to tell for certain, and you may in fact be dealing with a strain of higher toxicity.

What happens if you eat moldy onions?

Mold is more commonly found growing on the outer paper-like layers of onions. This means in the majority of cases, the fleshy part of the vegetable is still safe to eat once the skin has been removed.

There are several factors to be cautious of, however, one of which is that if once the onion has been cut open, you spot the insides have begun to turn soft, you should discard it, as the exterior mold may have entered the center, as well as other bacterias.

Another factor is the appearance of any other types of mold formation, such as strains with bluey-green, white, orange, or brown coloration. If these types of mold are present, it is likely that they will have developed within the flesh of the onion also, and the onion should be discarded.

In the majority of cases, accidentally investing a small amount of mold is unlikely to cause major harm if you have a fully functional immune system. For weakened immune systems, symptoms of nausea, gastrointestinal discomfort, headaches, and nausea could result from eating moldy onions.

Can you still use onions with mold?

Whether or not you can still use an onion that has mold growing on it depends on whether or not the person intending to consume it has any allergies.

A person who has been diagnosed with allergies to Aspergillus, or any other strain of mold should not use onions with mold and should discard any purchased that show signs or symptoms of mold or mildew growth.

For individuals without mold-specific allergies, washing the outer layers and removing any parts of the onion with obvious signs of growth will make the onion safe to eat, however, if mold growth is affecting large parts of the onion, it is best discarded entirely.

How to store onions to prevent mold growth

Onions are stored for many months before they reach the grocery stores where the public can purchase them, mold growth can occur during this time, so a very simple method to ensure that the rest of your onions do not become moldy, is to be careful whilst you are purchasing them.

Be on the lookout for small patches of what looks like black powder on the external skin of the onion, as well as any soft patches. Mold can quickly spread from one onion to another, so it only takes a single moldy onion to affect a whole string.

Washing onions that show any signs of exterior mold growth can be helpful in preventing them from spreading to other onions or being stored loose. However, be very careful to make sure they are completely dry before storing them.

You should also aim to keep your onions as dry as possible, this means not storing them in the refrigerator, as they are high-humidity areas, instead, keep them in an open basket, and If at all possible, keep them in a dark, well-ventilated area, such as a pantry with air vents. This will allow them to last longer whilst maintaining their characteristics.

When should you throw away moldy onions?

Small amounts of black powdery mold on the surface of an onion do not always require throwing the item away as long as the person planning to consume or cook with it has no allergies. In this case, the affected area can be washed or cut away.

When blue-green, white, or any other color of mold is seen growing on the outer skin or within the core of an onion, it should certainly be discarded. Eating an onion that has this level of mold can lead to unpleasant symptoms of mold exposure that can be otherwise easily avoided.


Do onions go moldy in the fridge?

Onions will become moldy faster in a refrigerator than they would if they were to be kept on a kitchen counter. This is because refrigerators are high-humidity environments and unpeeled onions can quickly absorb the moisture that mold needs. A lack of ventilation further exacerbates this issue by trapping moisture.

What is the grey mold on your onions?

Gray mold formation on onions is the result of Peronospora destructor, a strain of powdery mildew. Onions and shallots are most commonly affected by the pathogen, with chives and leeks less often. Warm and moist conditions make powdery mildew growth more likely. Any items found with powdery mildew growth should be discarded.

How long does it take for an onion to mold?

Mold can form rapidly on an onion when the correct conditions are met. With adequate moisture levels and temperature ranges, mold and mildew can begin to grow within 6-12 hours, with visible mold growth becoming present within 1-2 days.

What does mold on an onion look like?

Onion mold can be white, black, or green, with black mold often appearing in small clusters of circular dots with a powdery texture. White mold is frequently a symptom of a serious condition called white rot, which can be seen as a white fluffy growth originating from the base of the bulb.


Mold growing on onions is a common occurrence that often begins during the cultivation of the onion plant, or whilst storing. Aspergillus niger is often mistaken for black mold and is usually harmless to people with healthy immune systems.

Chris Walker

Chris Walker has struggled for several years with mold after buying his own property. After finding the solutions to several issues around his home, he decided to create this site in order to answer as many questions about mold and mildew as possible to help others dealing with the same problems.

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