Black patch on top of your broccoli

What you see: Black on top of your broccoli head.
What it is: Rot from microbial growth.
Eat or toss: Cut off the affected area, with generous margins. Inspect the rest of the broccoli. If it looks good (no brown, black, squishy or wet tissue or fuzz), go ahead and prepare it. 

Is black on a broccoli head mold?

Between the farm, processing, shipping, the store and our homes, raw produce like broccoli has plenty of opportunities to pick up stray microbes. This is one of the reasons why we wash it, of course, and also why many types of produce are cleaned before they’re shipped off to consumers. 

But washing is imperfect; a review in Frontiers in Microbiology found that after typical minimal processing, like washing and packaging, fresh produce can have between 1,000 and a million units of bacteria per gram. That stat doesn’t even account for fungal spores, which are there too. In the case of this broccoli, it’s likely that it picked up a fungus somewhere between the field and my kitchen. When the broccoli was first harvested, it was probably too robust for the fungus to penetrate. But, along the way, the broccoli became more vulnerable and the fungus was able to develop into the black mass at the top of this head. It’s easier for fungi and other microorganisms to grow on produce when it’s injured (a scrape or even a bruise can break open cells; those busted cells leak nutrients and moisture that fungi love). As produce ages, it also becomes weaker and less able to fend off microbial threats. If produce is wet during storage, microbes also have an easier time diving into the dirty work of decomposition. 

“So if you’re leaving it at room temperature or not in cold conditions or high humidities or high oxygen conditions, those fungi will show up because conditions become more favorable for them to grow,” said Tie Liu, an assistant professor in horticulture sciences at the University of Florida. 

The fungus in question here is likely from the genus Alternaria, which is often implicated when black patches form on fruits and vegetables (like this orange). But an image alone is rarely enough to identify a fungus, so we can’t say for sure.  

You don’t want to eat an unidentified fungus, so it’s certainly best to cut off the black area, with wide margins. USDA recommends cutting an inch around the fungus on firm fruits and vegetables (broccoli qualifies as firm as far as I can tell), while advising consumers to pitch any soft fruits or vegetables, like tomatoes, that go moldy. The guidance differs because mold can more easily penetrate softer foods, even without obvious evidence of its presence. Several molds produce toxins that are harmful to eat. People with allergies to airborne mold spores will want to avoid smelling moldy food or handling it roughly–this can knock invisible-to-the-naked-eye spores into the air. 

Bacteria can also cause black areas on broccoli

Several kinds of bacteria can cause a similar kind of rot on broccoli heads–you might get a clue whether fungi or bacteria are at work because a bacterial infection is more likely to produce an offensive odor.  When bacteria are at work you’ll initially see a “water-soaked or greasy discoloration on the surfaces of small groups of the unopened flowers,” plant pathologist Steven Koike wrote in a University of California blog post.  Then the affected areas will turn brown to black, the infection will spread, the tissue will become soft and give off a very bad odor. 

Whether the initial infection is bacterial or fungal, once the tissue is broken and diseased, secondary molds and bacteria can settle in and further decay the broccoli. 

Conditions leading to head rot

Cool, wet conditions, like long periods of rain, dew and fog, make fungal growth more likely on broccoli. The shape and topography of the head are important too. Domed heads, which water more easily runs off, are less prone to microbial growth than flatter heads, or heads where the buds are looser and there are more places for water to collect. Generally, rots, whether from bacteria or fungi, are most likely in areas of the head that collect water and stay wet the longest. Spores of Alternaria are spread by wind and splashing water. Wet weather also prompts fungal spores to germinate.