What you see: A black area inside your orange (which we’re guessing is a navel orange).
What it is: Most likely a confrontation between a fungus and the orange.
Eat or toss?: Toss!
Black spot inside navel orange
This navel orange looked fine on the outside.
But on the inside?
It appears a stealthy fungus is at work. One that may have slipped in through the crevices on the navel end, traveled down the center of the orange, known as the columella, and then settled in and started popping open the little sacs of juice. Its signature? An inky black stain in the middle of the fruit.
While it’s difficult to diagnose a microbial infection based on an image, Megan Dewdney, a plant pathologist at the University of Florida, said the photos in this post look similar to oranges she’s studied that were afflicted with an Alternaria alternata fungus that has a thing for oranges.
Unlike fuzzy molds you might find on the outside of an orange, this fungus has structures too small to detect with the human eye; you won’t see soft filaments. Instead, you’ll just see its dark color staining your orange.
“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.
- Megan Dewdney. Associate professor and plant pathology extension specialist. University of Florida/Institute of Food and Agricultural Sciences Citrus Research Center. Email correspondence August 2022.
- J. David Miller. Distinguished Research Professor. Carleton University. Helped draft guidelines for mold and dampness published by Health Canada. Co-editor of World Health Organization report, “Mycotoxin Control in Low- and Middle-Income Countries.” Interview and email correspondence Sept. 2022.
- Mycotoxins. World Health Organization. May 2018.
- Dietary exposure assessment to Alternaria toxins in the European population. European Food Safety Authority. 23 December 2016
- Questions and Answers to Citrus Management. Third Edition. Peggy A. Mauk, Ph.D. and Tom Shea. University of California Cooperative Extension. 1994.
- Diseases of Citrus in Arizona. Mary Olsen, Mike Matheron, Mike McClure and Zhongguo Xiong. Arizona Cooperative Extension. College of Agriculture and Life Sciences – University of Arizona.
- Matheron, M. & Maurer, Michael & Bacon, Dean & Truman, James & Lopez, Al. (1996). Development of Control Measures for Alternaria Fruit Rot on Roanges in Arizona.
- Ghanei Ghooshkhaneh, N., Golzarian, M. R., & Mamarabadi, M. (2022). Spectral pattern study of citrus black rot caused by Alternaria alternata and selecting optimal wavelengths for decay detection. Food Science & Nutrition, 10, 1694– 1706.
- Phylogenetic Analysis of Alternaria species Associated with Citrus Black Rot in Iran. Shideh Mojerlou and Naser Safaie (Department of Plant Pathology, College of Agriculture, Tarbiat Modares University, Tehran, Iran). Journal of J Plant Pathology & Microbiology. 2012.
- Postharvest Technology of Horticultural Crops. Technical Editor: Adel A. Kader. University of California – Agriculture and Natural Resources. Third edition. 2011.
- Black rot of oranges. Botanical Gazette. 1902. NEWTON B. PIERCE, U. S. Dept. Agric., Bureau of Plant Industry. Pacific Coast Laboratory, Santa Aloa, California.