Black mold on a cauliflower head.

Like most crops, cauliflower is susceptible to certain diseases, which are in turn promoted by pest damage. Here are some of the diseases you should look out for:

Blackleg

Blackleg is a fungus that causes erosion of the leaves, along with blackened stems and rotting roots. It impacts plants as young as the seedling state. Prevent blackleg by using quality, disease-free seed, good drainage and air circulation, and regular crop rotation. You can also try a fungicide to kill the disease.

Cause: Fungus

Symptoms:

• leaf erosion
• black stems
• rotting roots

How it spreads:

• irrigation and wind
• crop residues
• infected seed

Treatment:

• apply fungicide

Prevention:

• apply a fungicide to soil before planting
• use disease-free seed
• provide good drainage and air circulation
• perform crop rotation

Clubroot

This is another fungal disease that likes acidic, moist soil. Roots attacked by clubroot fill with mold spores that deform the roots into useless appendages. If your plants are failing, dig down to see if the roots are healthy or if they’re smelly and deformed. If they’re damaged, discard the plant in the trash, and don’t plant brassicas in the same spot next year. You can try a fungicide too, before deciding your plant is a total loss. Well-drained, neutral soil is a good preventive.

Cause: Fungus

Symptoms:

• roots filled with mild
• deformed roots
• smelly roots

How it spreads:

• soil
• irrigation

Treatment:

• apply fungicide
• remove infected plants

Prevention:

• well-drained soil
• don’t plant in acidic soil

Damping off

Brassica seedlings may be attacked by various soil-borne fungi that feed on roots and stems, like Pythium spp., Fusarium spp., and Rhizoctonia solani. The attack appears to be sudden, with everything fine one day and seedlings dead the next.

Avoid damping off by planting in clean containers with good drainage and providing good air circulation between plants. Some seeds are even pretreated with fungicide. Other than that, there’s not much hope for plants that are victims.

Cause: Fungus

Symptoms:

• sudden dead seedlings

How it spreads:

• infected containers
• infected soil

Treatment:

• none

Prevention:

• use seeds pretreated with fungicide
• use clean containers with good drainage

Downy mildew

This is a fungus that can destroy crops. Watch for yellow patches that turn brown and fluffy white on the underside of leaves. Infected seedlings may not survive damage to their first leaves, and the disease progresses to stem and flower heads. If you spot signs of trouble, try an organic fungicide that’s safe for food plants.

Cause: Fungus

Symptoms:

• fluffy white substance on underside of leaves

How it spreads:

• irrigation and wind

Treatment:

• fungicide

Prevention:

• companion planting
• crop rotation
• pest control
• don’t compost diseased plants

Companion planting

Each crop has a garden buddy that helps out in some way: repelling pests, attracting pollinators, or contributing nutrients to the soil. Cauliflower has a number of companion plants that help it thrive, such as onions, celery, and beans (though onions and beans don’t go together, so this is not a “Three Sisters” scenario).

Cauliflower likes its garden buddies because they help fight off the pests that like to go after cauliflower crops. The most common pest of cauliflower is a tiny white butterfly called diamondback moth larvae. These little critters can strip entire plants of their leaves in no time flat, leading to a poor harvest.

Onion plants are one of the most natural and effective ways to repel diamondback moth larvae because they emit a pungent odor at the roots that is notorious for keeping pests far away. Celery makes an excellent companion plant as well, because it balances moisture in the soil, and leaves tons of good nutrients in the soil.

Beans are great companion plants for cauliflower because they help fix nitrogen in the soil, and they are a great addition to many fall gardens. By growing these plants alongside your cauliflower crops each year, you will ensure that your garden is full of healthy, vibrant cauliflower plants all season long!

Crop rotation

As tempting as it might be, it’s best not to grow the same crops in the same soil—open garden, raised bed, or container—for more than one growing season. Each plant takes certain nutrients from the soil and leaves others behind. Some crops are susceptible to soilborne diseases or particular pests. For these reasons, you want to rotate your crops from growing season to growing season. This will help ensure better soil health and healthier, more productive harvests. This practice applies primarily to annual crops; perennial crops can usually continue to grow where they’re planted.

Now, in a perfect gardening world, you would be able to implement a crop rotation plan that spans decades. There are some soilborne diseases that can live in the soil for up to 20 years!

Even so, it’s a good idea to switch things up from season to season. Below is a list of vegetable crops by category. The general rule of thumb here is to plant crops from one list in the growing space of the crops in the following list in the next growing season. So, you would go from List 1 to List 2 to List 3 to List 4 and then back to List 1.

List 1: Root, solanaceous (nightshade), and tuberous crops

• hot peppers
• carrots
• celery
• eggplant
• parsnips
• potatoes
• sweet peppers
• sweet potatoes
• taro
• tomatoes
• potatoes

List 2: Brassicas

• broccoli
• Brussels sprouts
• cauliflower
• kale
• kohlrabi
• mustard
• radishes
• rutabaga
• turnips

List 3: Legumes and pod crops

• broad beans
• lima beans
• okra
• peas
• runner beans
• snap beans

List 4: Alliums

• bulb onions
• garlic
• leeks
• oriental bunching onions
• scallions
• shallots
• Welsh onions

Here’s a short example of a common crop rotation plan:
potatoes > cauliflower > beans > onions > tomatoes > broccoli > peas > garlic > peppers

If you can’t do a complete crop rotation, consider alternating what you grow from one season to the next—a virtual crop rotation of sorts. With containers and raised beds, you have the option of changing out the soil, depending on what you want to grow there. Some container soil, especially, will be severely depleted of nutrients at the end of the growing season and should just be replaced.

Other best practices include:

• Buy healthy, disease-free seeds from reputable sources.
• Plant in sites with good drainage; if planting in open ground, choose a higher spot for better drainage.
• Check plants regularly for signs of disease.

How do you prevent diseases on your cauliflower plants? Please share your techniques with us in the comments.