What are those black spots on onions?

That black sooty powder commonly seen on onions is certainly not charcoal, as you may have guessed. It is usually caused by Aspergillus niger, a fungus responsible for black mould disease, and it has been the subject of debate across the internet in recent months.

Should you be worried about it? Yes and no, and you'll find out why in this article.

Aspergillus niger is a common fungal pathogen that affects several crops, including maize, grapes, and, of course, onions. It causes what farmers refer to as black mould rot. The fungus can develop both in the field and during storage, particularly when onions are exposed to excessive moisture and humid conditions. This is exactly why it is an important concern in tropical climates like Cameroon.

Aspergillus niger is widespread in the environment. It is commonly found in soil, decaying plant material, and even on the walls of poorly ventilated buildings. Most healthy people inhale its spores regularly without experiencing immediate health effects, although there are no universally accepted safe exposure limits.

However, the fungus can pose a serious health risk to certain groups of people. Individuals with respiratory conditions such as asthma or cystic fibrosis, as well as those with weakened immune systems, including people undergoing cancer treatment, are more susceptible to infections caused by Aspergillus. These infections, collectively known as aspergillosis, can affect the lungs and, in severe cases, become life-threatening. Aspergillus niger has also been associated with otomycosis, a fungal infection of the outer ear canal.

In addition to infection, some strains of Aspergillus niger are capable of producing secondary metabolites known as mycotoxins, including ochratoxin A under certain conditions. Ochratoxin A is considered a possible human carcinogen and is particularly toxic to the kidneys. Long-term dietary exposure to contaminated foods has been associated with kidney damage and may increase the risk of urinary tract cancers.

Does this mean that every onion with black soot should be thrown away? Not necessarily.

If the fungal growth is limited to the dry, papery outer skin while the inner layers remain firm, crisp, and unaffected, the onion is generally still safe to eat. Simply peel away and discard the affected outer layers, then wash the onion thoroughly under running water to remove any remaining fungal residue before use.

However, if the black streaks extend into the inner rings, or if the onion has become soft, slimy, discoloured, or gives off a musty smell, it is a sign that the fungus has penetrated the edible portion of the onion. In that case, the onion is no longer safe to eat and should be discarded immediately.