Why is this orange black inside?

“The fungus is heavily melanized (containing the pigment melanin) so has a dark appearance at high concentrations,” Dewdney wrote in an email.

Is it safe to eat an orange with black on the inside?

You definitely won’t want to eat the black areas – they’ll taste terrible. As for the rest of the orange, even areas that look fine could contain icky-tasting enzymes, biochemicals or other substances the fungus has deployed or generated as it breaks down the orange.

“Generally these fruit are considered to have an off-taste and are unpalatable,” Dewdney wrote. “I would not eat a fruit infected with this disorder.”

In terms of food safety, first, consider that it’s not possible to diagnose a fungus from sight alone, so you can’t be 100 percent certain that your orange and the orange we’re discussing here are afflicted with the same issue; that alone is a reason to be cautious.

Some fungi, including Alternaria alternata, can produce chemicals of varying toxicity. While the toxins associated with this fungus aren’t notorious the way some fungal toxins are, caution is still warranted. There’s a lot we don’t know about them and the impacts of eating them. 

Interestingly, the Alternaria alternata species includes a number of strains tailored to different host plants. Dewdney said that the strain that’s likely depicted in this post actually doesn’t generate any toxins to break down the orange. But she couldn’t speak to any human health impacts of anything else it might produce.

This fungus is probably a respiratory allergen

Alternaria alternata fungi are respiratory allergens. David Miller, a research professor at Carleton University who studies human health impacts of fungi, told me that they’re part of a group of fungi that causes a reaction that many people call “hay fever.” 

“It’s a ubiquitous fungus on the surfaces of leaves. It’s very common in outdoor areas,” he said. Miller noted that it would be harder for a spore within the wet tissue of an orange to be knocked airborne. Handling an orange like this would probably only marginally increase your regular exposure to this fungus’s spores in the air and probably won’t impact most people. But those who are very sensitive to allergens should be careful. 

“If it is A. alternata, it is important to remember a fair percentage of the American population is allergic to it,” Miller wrote in an email. “While most people would not be bothered, some would.”

Miller, who has advised governments and agencies around the world on risks and best practices for managing fungi, said he wouldn’t eat any part of an orange that looked like this. Between not knowing for sure what the fungus was, and the unknowns of whatever it might have left behind, even in the tissue that still looks normal, he said he’d drop it into the compost.

Oranges with inner fungal infections like this can have more intensely colored peels

While there often aren’t obvious clues of this type of Alternaria infection on the outside of oranges, the fungus can produce an interesting effect: the fruit might be a deeper shade of orange. This is because, as the orange attempts to fight off the infection, it produces more ethylene than usual. Ethylene, a natural plant hormone, speeds ripening and can thus cause the oranges’ color to intensify more than their normally ripening peers. If you wait a really long time before attempting to eat an affected orange you might notice that the peel becomes thin and dark. 

You might also be more likely to find this type of black decay in an orange that has a “poorly formed navel.” That would make it easier for the fungus to get into the fruit. The fungus might also get in through a wound or even during flowering. Stressed fruit are more susceptible to infection. If the black rot advances before harvest, the fruit can fall from the tree prematurely.  

Dewdney said that fruit packers and processors usually reject loads of oranges they suspect have this issue. Affected oranges can ruin large batches of juice. But just because one of your oranges has a blackened center, doesn’t mean the entire bag is a lost cause. 

“Hopefully there are only one or two fruits like this in a bag,” she wrote.