Cauliflower can be a challenging vegetable to grow, so it can be devastating to see your hard work become blemished by black spots. What do these black spots mean? How can you get rid of them?
There are several reasons your cauliflower could have black spots. Many fungal diseases, such as Blackleg, Alternaria leaf spot, ring spot, downy mildew, or white mold can cause dark lesions on the leaves or heads.
Even if you already harvested your cauliflower, it can still suffer from oxidation or mold in your refrigerator.
Thankfully, you can fix this problem and the cauliflower is still edible.
Keep reading to learn how to identify which problem you are dealing with, how to save your cauliflower crop, and some tips to prevent black spots in the future.
Fungal Disease are the Major Cause of Black Spots On Cauliflower
There are several different fungi that can infect your cauliflower resulting in black spots. Soil-borne fungi are present in the soil and can splash onto the crop during a rainstorm.
Spores can also be airborne and are introduced into your garden by wind or are blown from one plant to the next.
When the weather conditions are cool and damp, fungi will cling to the plant’s foliage and infect the plant via lesions in the tissue.
Most fungi thrive in temperatures between 15°C and 21°C (59-70°F). If conditions stay moist and cool, fungi can swiftly pass from plant to plant and infect your whole crop.
Fungi can also cling to your garden tools and be carried from plant to plant, so make sure to clean your equipment, such as shears and shovels, after working with diseased plants.
There are a number of fungal diseases that can cause black spots on your growing cauliflower.
Some will damage the leaves while others infect the heads. You should distinguish each fungus by certain tell-tale signs so you know which problem you are dealing with.
The fungi that cause black spots on cauliflower are:
Let’s look at how to identify each disease, plus ways to prevent fungal diseases and keep your cauliflower safe.
Blackleg, also known as stem canker, attacks the stems and leaves of the cauliflower plant. Leaf damage is usually identified by usually dirty white areas covered with tiny black spots, and yellowing leaves.
Lesions on the stem can be various shades of brown, again accompanied by tiny black spots in the affected areas.
The black spots are very tiny and sometimes are only visible with a magnifying glass, and a pink liquid will often ooze from the spots.
Blackleg may cause the stems of young seedlings to shrivel and die. If the plant grows to maturity, cankers can form that will often severe the stem causing the cauliflower to die.
Blackleg can remain dormant in the soil from season to season and it can also become airborne traveling from plant to plant.
Poor-quality seeds can also be contaminated with blackleg so your plants are doomed before they even begin growing.
Here is a very detailed article about blackleg in canola, which has many similarities to infected cauliflower.
Blackleg is often confused with Wirestem (Rhizoctonia solani), which is very similar but lacks black spots in the lesions.
Alternaria Leaf Spot
Alternaria brassicae & Alternaria brassicicola Like most fungi, Alternaria leaf spot likes cool weather but it can tolerate warmer temperatures, too.
Its ideal range is 15.6°C to 25.6°C (59°F-78°F), and it thrives in damp environments. It creates target-shaped spots on the leaves with brown sunken centers surrounded by yellow halos.
Alternaria also infects the cauliflowers themselves causing black spots. Individual buds, or curds, will turn black, and can often spread to infect large areas of the head.
The lesions are usually superficial, and the infected area can be cut off so the head is still edible.
Mycosphaerella brassicicola, ring Spot does not affect the head itself, but its leaf damage is often confused with Alternaria leaf spot. Ring spot lesions are grey concentric rings with tiny black or white spots.
While not usually fatal, ring spots will inhibit development, and slow the growth of your plant which can be very problematic in short growing seasons.
Hyaloperonospora parasitica, Downy mildew can infect cauliflower and other brassicas at any point in their growth. Its spores spread through the air though they can survive in the soil, and it requires moisture to attach themselves to the leaves.
You can identify downy mildew by the discolored patches on the top of the leaves and a white fuzzy growth on the underside.
Seriously infected leaves can wither and fall off. The cauliflower itself can also be infected and the individual buds (or curds) can turn black, or the entire underside of the florets can have a black moldy growth.
Sclerotinia sclerotiorum & Sclerotinia minor. While this fungus does not cause black spots, it leaves black pods behind. This fungus covers lesions with a fluffy white mold.
If left to advance, the mold will produce black sclerotic (tightly packed food stores for the fungi) that are roughly the size of a grain of rice inside the white mold.
Mould can survive in the soil for several years, so crop rotation is very important.
Can You Still Eat Infected Cauliflower?
In most cases, cauliflower infected with black spots is still edible. You can discard the infected leaves, and any spotted florets can be trimmed.
Downy mildew can cause the most significant damage to the head itself, and unless the entire head had molded, the infected areas can be removed and the head can be eaten.
How To Prevent Fungal Diseases In Cauliflower
The best way to avoid black spots on your cauliflower is to keep fungi from ever setting foot in your garden or field. Here are some tips to prevent fungus from infecting your garden:
1: Grow Disease Resistant Varieties
If you know your garden is susceptible to certain fungi, grow varieties of cauliflower that are resistant to the diseases. Most reputable seed companies will list which varieties are disease resistant.
2: Don’t Water With Sprinklers
Most fungus attaches to leaves when they are wet, so water the soil instead of the plant. Overhead waterers, like sprinklers, soak the leaves and can splash contaminated soil onto the foliage.
A much better option is soaker hoses which seep water onto the soil. They also provide water directly to the roots and help water conservation by reducing evaporation.
3: Space Out Plants
Fungi like cool moist conditions. Closely spacing plants creates an environment that excludes air and sunlight and traps moisture, so space out your plants to allow sunlight and air to penetrate beneath the foliage to warm and dry the area.
Space your cauliflower at least 45cm to 60cm (18-24 inches) apart.
4: Crop Rotation
Most fungi can survive in the soil for several months, sometimes for several years. Do not plant cauliflowers in the same plot for 3 to 4 years to make sure the pathogens die.
Remember, cauliflowers are part of the brassica family, so you also want to avoid planting cabbages, mustards, broccoli, and other brassicas in that area.
5: Cull Infected Plants
If your cauliflower does become diseased, remove any infected leaves immediately. If the disease has really taken hold, remove the entire plant.
Do not dispose of infected foliage in the compost, as the heat of the composting process might not be sufficient enough to kill the pathogens.
Instead, remove the diseased plants from your garden completely, or burn them.
5: Companion Planting
Companion planting is the practice of growing two crops together so they mutually benefit each other.
For example, try growing garlic or other alliums with your cauliflower as it is high in sulfur which is a natural fungicide.
Another benefit of companion planting is that it “confuses” pests and diseases. By growing several crops together, a fungus that likes cauliflower is less likely to take hold if there are beets or beans (to name a few) growing amongst them.
Good companion plants or cauliflower include dill, mint, chamomile, rosemary, sage, alliums, beans, beets, cucumbers, radishes, carrots, celery, lettuce, spinach
Avoid planting cauliflower near nightshades (tomatoes, potatoes, eggplants, peppers) as they make the soil too acidic for cauliflower, winter squash, and strawberries.
Treating Fungus In Cauliflower
But what can you do when a fungus has already taken hold of your cauliflower crop? Before you reach for a bottle of dangerous chemical fungicides, try one of these natural, homemade fungus treatments:
Dark Spots In Storage
Maybe your cauliflower crop has made it through the growing season with a blemish. But now that it is in your fridge, it has started to form small black spots! What is happening? What to do?
Your cauliflower is most likely suffering from oxidation or it is starting to mold.
These are both most common in cauliflower that has been stored for a long time so is more likely with cauliflower from the store but it can happen to your own products as well.
Oxidation is the result of exposure to light and air, similar to how a slice of an avocado or piece of an apple turns brown when cut and left on the cupboard.
Moreover, condensation often causes the moisture to settle on the head, so try and keep excess moisture off the head in your fridge.
Oxidation will cause small brown or black spots on the buds, or curds (which are the small little individual balls that bounce all over your kitchen when you cut it).
They often start as individual buds going light brown, but they can turn black and even spread to entire florets.
Can You Eat Oxidized Cauliflower?
Yes! Oxidized cauliflower is perfectly edible though the brown or black spots might not have the greatest flavor, especially when they are large.
Carefully scrape off the dark spots with a sharp knife, or cut away any large areas.
Oxidation is, however, the start of decay. If the areas start to get mushy with a foul odor, then they have started to rot.
You can still probably remove the rotten pieces, but if the affected part covers most of it, it would be safer to discard the cauliflower instead.
Sometimes, the dark spots on your cauliflower are actually black mold and not oxidation. You can easily identify black mold by its slightly fuzzy appearance.
Can You Eat Cauliflower That Has Moulded?
If the moldy spots are still very small, just removed them like oxidized cauliflower, and then wash thoroughly.
If, however, black mold has infected large areas of the head, it is far safer to throw it away. Black mold can be extremely dangerous if ingested in large quantities.
Whether it is white, yellow, or purple, a healthy cauliflower head emerging from the dense foliage of leaves is a sight to behold.
But after weeks of patient cultivation, it is a shame when the perfect head got blemished by unsightly dark spots.
With good garden management and a natural fungicide at the ready, you can combat these diseases and keep your cauliflower growing healthy and strong.
Amber Noyes was born and raised in a suburban California town, San Mateo. She holds a master’s degree in horticulture from the University of California as well as a BS in Biology from the University of San Francisco. With experience working on an organic farm, water conservation research, farmers’ markets, and plant nursery, she understands what makes plants thrive and how we can better understand the connection between microclimate and plant health. When she’s not on the land, Amber loves informing people of new ideas/things related to gardening, especially organic gardening, houseplants, and growing plants in a small space.