What Causes Black Spots on Broccoli?

Black spots on broccoli almost always come from a plant disease. Either a mold-like fungus, a related organism called downy mildew, or bacteria that rot the tiny flower buds got into the plant. Occasionally the cause is a shortage of the nutrient boron while the plant was growing. Much of the damage starts in the field and only shows up later, sometimes inside the stalk where you can’t see it until you cut.

How to Tell What Kind of Black Spot You Have

Smell and texture are the quickest clues. A bacterial infection is more likely than a fungus to produce an offensive odor.1Eat Or Toss. Black patch on top of your broccoli Bacterial rot also turns the tissue soft and mushy as it spreads.2Eat Or Toss. Black patch on top of your broccoli Fungal spots, by contrast, tend to stay dry and firm, with sharp edges.

Where the spots sit gives another clue:

In the kitchen, smell the spot up close, since a sour or rotten odor points toward bacteria. Press it to see whether it feels firm or gives way. Cut the stalk lengthwise to look for dark streaks or a hollow center.

Not every dark area is disease. Cut ends and bruised florets darken in air, much as an apple slice does, and that browning stays on the cut or bruised surface instead of spreading in patches. Yellowing florets are simply a sign of age.

No single clue settles the cause, so weigh them together. Fungal and bacterial problems can also share a head, with dry spots in one place and soft rot in another. If any part is soft or smelly, treat the whole head by the rot rules. Early on, bacterial rot and fungal spotting both look like small dark patches, so look again a day later: rot spreads and softens, while dry spots mostly stay put.

Gardeners can also check the leaves, where round dark spots point to black spot fungus. If several plants in a row show the same spots, an infection may be spreading between them.

Fungal Black Spot and Downy Mildew

Black spot fungus causes dark dead spots on broccoli leaves and heavy crop losses.7Applied Sciences. Identification of Black Spot Resistance in Broccoli (Brassica oleracea L. var. italica) Germplasm Resources On young plants, spots can appear about 3 days after the spores land.8PubMed Central. Genomic and Functional Characterization of an Alternaria brassicicola Isolate Causing Black Spot Disease on Broccoli Leaves By day 5 the spots grow and merge together, and by day 7 the leaves can wilt badly.9PubMed Central. Genomic and Functional Characterization of an Alternaria brassicicola Isolate Causing Black Spot Disease on Broccoli Leaves

The spots are typically round and dark, and as they enlarge they often show rings like a target. Spores landing on a head from infected leaves can leave small dark spots on the florets, which is one way spotting reaches the broccoli you buy.

Black spot fungus is a true fungus, a living organism that feeds on plant tissue and spreads by tiny spores that drift on air, water splashes, and tools. Downy mildew is not a true fungus but a similar-looking organism that behaves much the same way in the garden. Both thrive when plants stay damp, which is why wet, humid weather brings outbreaks.

The practical difference for a cook is that downy mildew can run through the whole plant, especially when seedlings are infected.10Pacific Northwest Pest Management Handbooks. Broccoli (Brassica oleracea)-Downy Mildew (Staghead) It grows through the tubes that carry water and food, darkening them up into the stalks and heads.11Vegetables Canada. Managing Downy Mildew In Broccoli The black streaking inside stems and heads often doesn’t show until close to harvest.12Pacific Northwest Pest Management Handbooks. Broccoli (Brassica oleracea)-Downy Mildew (Staghead) So the outside of a broccoli head may show only spots while the inside is dark brown or black.13Hortsense. Broccoli, Cole crops: Downy mildew

That’s how store broccoli can look fine and be dark inside: infection can start in the seedling and grow out of sight for weeks. Cutting the stalk is how you see the damage.

Cut stalks affected by downy mildew can show brown to purple streaks in a net-like pattern.14Pacific Northwest Pest Management Handbooks. Broccoli (Brassica oleracea)-Downy Mildew (Staghead) A clean, pale green stalk with only a few surface spots on the florets suggests the problem stayed shallow. Dark streaks running well into the head mean the discoloration is internal, so trimming the surface won’t remove it.

Bacterial Head Rot

Several kinds of bacteria can cause head rot, and some break down plant tissue very strongly.15Vegetables Canada. Managing Head Rot In Broccoli One type identified in rotting broccoli heads was especially good at dissolving the material that holds plant cells together.16Australian Journal of Agricultural Research. The etiology of head rot disease of broccoli The rot starts as water-soaked or greasy patches on small groups of unopened buds.17Eat Or Toss. Black patch on top of your broccoli Those buds turn brown to black and the rot spreads.18Vegetables Canada. Managing Head Rot In Broccoli The tissue turns soft and gives off a very bad smell.19Eat Or Toss. Black patch on top of your broccoli

Bacteria usually enter through small wounds, such as insect damage, hail or wind injury, or a cut, and then move into the tight clusters of buds. A broccoli head’s dense dome of tiny buds traps water, giving bacteria the moisture they need to multiply. Rain or sprinkler water splashing from the soil onto heads can carry bacteria upward.

The same moisture explains why wet broccoli rots faster in your fridge. Bacteria spread easily in water sitting between tight buds, and washing before storing leaves moisture in the head. Wash broccoli just before cooking instead.

Bacterial rot should go in the trash, since it spreads through the tissue and cutting off visible patches won’t remove it. Move nearby produce away and wipe the drawer it sat in.

A head that is limp and rubbery but dry and odorless is only losing water, and it often perks up in cold water before cooking. Rot is different: the tissue becomes wet, slimy, and discolored in patches rather than just soft overall.

Boron Shortage and Weather Damage

Broccoli short of boron develops watery tissue inside the stem.20Yara United Kingdom. Boron deficiency | Broccoli | Yara UK That tissue turns brown or sometimes black, then dries out, leaving hollow spaces.21Yara United Kingdom. Boron deficiency | Broccoli | Yara UK Light brown spots on the head are another sign.22Michigan State University Extension Service. Disorders of Cole Crops These spots come from a nutrient problem, not from an infection.

The damage happens while the plant grows, so the hollow stem is already there when the broccoli is picked and won’t get worse in your fridge. Cooks mostly meet it as a hollow, brownish stalk when cutting a head open. Boron spots are light brown, don’t smell, and don’t spread in storage the way rot does.

Weather can also mark the plant. Several rainy or dark days followed by strong sun can burn broccoli leaves.23PlantVillage. Is this downy mildew on my broccoli? Leaves that were shaded and then suddenly exposed can burn the same way.24PlantVillage. Is this downy mildew on my broccoli? The flatter part of the leaf, where it starts to curve down, is the most likely to burn.25PlantVillage. Is this downy mildew on my broccoli?

Sunburn shows up all at once on the most exposed leaves after a change in weather. Fungal spots appear over time and keep growing and merging. Once the weather settles, sunburn doesn’t spread to new leaves.

Is Broccoli With Black Spots Safe to Eat

  • Firm head with a few small dry spots: cut the spots away with a margin of healthy tissue and use it.
  • Light brown spots with a hollow stem: trim and cook as usual.
  • Soft, slimy, or foul-smelling patches: discard the head.26Eat Or Toss. Black patch on top of your broccoli
  • Fuzzy mold or spots across much of the head: discard it.

The germs that cause these plant diseases are not known to make people sick. The bigger risk is spoilage and other microbes growing on rotting tissue. When unsure, throw it out. Cooking won’t rescue rot either, since a bad smell will usually carry into the cooked dish.

To trim a spotted head, cut well past the visible spot, then look at the cut face to check that it is clean. If it still shows brown or dark streaks, cut further or give up on that section.

Separate the floret clusters before judging them. A cluster that is bright green, firm, and odorless can be used even if its neighbor is spotted. Rinse and cook trimmed florets soon, since a head with any damage deteriorates faster than a sound one.

How to Store Broccoli So It Stays Green

Broccoli heads age quickly once picked.27HortScience. Hydrogen Sulfide Alleviates Dark-promoted Senescence in Postharvest Broccoli Even at 10°C, about the temperature of a cool cellar, plain broccoli stayed sellable for about 7 days but not 14.28Elsevier. Unique molecular mechanisms revealed for the effects of temperature, CA, ethylene exposure, and 1-MCP on postharvest senescence of broccoli Broccoli kept in low-oxygen, high-carbon-dioxide storage still looked good after 14 days.29Elsevier. Unique molecular mechanisms revealed for the effects of temperature, CA, ethylene exposure, and 1-MCP on postharvest senescence of broccoli Sealed packaging kept 67 to 83% of broccoli acceptable after 8 days, compared with about 37% unpackaged.30MDPI Foods / PubMed Central. Modified Atmosphere Packaging Delays Senescence and Chlorophyll Degradation by Enhancing Antioxidant Capacity in Postharvest Broccoli Those figures come from commercial tests, not home kitchens, so treat them as showing which conditions help rather than as exact shelf lives.

Ethylene, a gas given off by ripening fruit, made broccoli unsellable within 7 days.31Elsevier. Unique molecular mechanisms revealed for the effects of temperature, CA, ethylene exposure, and 1-MCP on postharvest senescence of broccoli Store broccoli away from apples, bananas, and tomatoes.

The crisper drawer, near 0 to 4°C, is colder than the storage temperatures above. Put the head in a loose or perforated plastic bag, or wrap it loosely in a slightly damp paper towel, so humidity stays up without water pooling on the buds. Plan to eat broccoli within about 3 to 5 days. When you buy, check the cut end and buds and skip heads that are already dark or soft.

Preventing Black Spots in Your Garden

Broccoli varieties differ a lot in resistance to black spot fungus.32Applied Sciences. Identification of Black Spot Resistance in Broccoli (Brassica oleracea L. var. italica) Germplasm Resources Many hybrid varieties sold today carry less resistance than they could.33Applied Sciences. Identification of Black Spot Resistance in Broccoli (Brassica oleracea L. var. italica) Germplasm Resources Look for varieties labeled as resistant or tolerant. Resistance lowers the risk but doesn’t remove it. Buy seed or seedlings from reputable sources, since infected seedlings can carry downy mildew into your bed from the start.

To keep fungus and bacteria in check, water at the soil level in the morning so the leaves dry. Space plants so air can move between them, and remove and bin infected leaves and plants rather than composting them. Rotate cabbage-family crops to a different bed each year.

For boron, a soil test shows whether it is low. Add it only in small amounts, because too much harms plants. Sandy soils that drain fast and heavily limed soils are more often short of boron.

Against sunburn, watch for strong sun after rainy spells and harden off seedlings gradually before moving them outdoors. Sunburned leaves recover once new growth comes in, so the plant usually survives.

At the end of the season, clear away old broccoli plants, because disease organisms can survive on leftover debris. Clean tools and stakes that touched diseased plants so you don’t carry spores to healthy ones.