Kohlrabi Pest Control: Common Pests and Diseases

Kohlrabi is one of the easier brassicas to grow. It matures fast — usually 45 to 60 days — which means it spends less time in the ground than cabbage or broccoli, giving pests and diseases a shorter window to cause problems.

But “easier” doesn’t mean “immune.” Kohlrabi belongs to the brassica family (along with cabbage, broccoli, kale, and cauliflower), and that family has a well-known list of enemies. If you’ve grown any brassica before, many of these pests will be familiar. If kohlrabi is your first, here’s everything you need to watch for.

For general growing guidance, start with how to grow kohlrabi.

Insect Pests

Cabbage Worms and Cabbage Loopers

What they are: The larvae of white butterflies (imported cabbageworm) and brown moths (cabbage looper). You’ll see small white or off-white butterflies fluttering around your plants — they’re laying eggs.

Damage: Irregular holes chewed through leaves. Heavy infestations can skeletonize leaves entirely, leaving only the veins. The caterpillars are green and well-camouflaged, so you often see the damage before you see the pest.

On kohlrabi specifically: The bulb itself usually isn’t targeted — these caterpillars prefer leaves. But severe leaf damage reduces photosynthesis, which stunts bulb development.

Control:

• Row covers are the most effective prevention. Cover plants immediately after transplanting with lightweight floating row cover fabric. This physically blocks the butterflies from landing and laying eggs. Keep covers on until harvest.
• Bt (Bacillus thuringiensis): An organic bacterial spray that kills caterpillars when they eat treated leaves. Apply every 7-10 days and after rain. Completely safe for humans, pets, and beneficial insects.
• Hand-picking: Check the undersides of leaves every few days. Pick off any caterpillars and crush the small, oval, yellowish eggs.

Flea Beetles

What they are: Tiny (1-2mm) black or bronze beetles that jump when disturbed.

Damage: Distinctive “shothole” damage — dozens of tiny round holes in the leaves. Individual holes are small but heavy infestations make leaves look like they’ve been hit with birdshot. Young seedlings can be killed; established plants tolerate moderate damage.

On kohlrabi specifically: Flea beetles are most dangerous to young kohlrabi transplants and direct-seeded plants. Once the bulb starts forming and the plant is established, flea beetles become more of a cosmetic problem than a growth threat.

Control:

• Row covers again — the single best defense. Put them on at planting and leave them.
• Diatomaceous earth: Dust around the base of plants. Effective when dry, useless when wet. Reapply after rain or watering.
• Trap crops: Plant radishes nearby as a sacrificial crop. Flea beetles often prefer radishes and will concentrate there instead of your kohlrabi.
• Strong transplants: Set out robust transplants rather than direct-seeding. Larger plants withstand flea beetle damage much better than tiny seedlings.

Aphids

What they are: Small, soft-bodied insects that cluster on the undersides of leaves and on new growth. Usually gray-green on brassicas, but can be other colors.

Damage: Curled, yellowing leaves. Sticky residue (honeydew) on leaf surfaces, which can lead to sooty mold. Stunted growth in severe infestations.

On kohlrabi specifically: Aphids often colonize the crevices where leaves meet the bulb. Check these joints regularly — a colony can establish itself quickly in a protected spot.

Control:

• Strong water spray. A hard blast from a garden hose knocks aphids off plants and many can’t make it back. Do this every 2-3 days at the first sign of aphids.
• Encourage beneficial insects. Ladybugs, lacewings, and parasitic wasps eat enormous numbers of aphids. Plant flowers nearby — alyssum, dill, yarrow — to attract these predators.
• Insecticidal soap. Mix 2 tablespoons of pure castile soap per gallon of water and spray directly on aphid colonies. Must make contact to work. Reapply every few days.
• Neem oil. Works as both a contact killer and a repellent. Spray in the evening to avoid burning leaves in direct sun.

Cabbage Root Maggot

What they are: Larvae of a small fly (Delia radicum) that lays eggs at the base of brassica plants. The white maggots burrow into roots and stems below the soil line.

Damage: Wilting plants that don’t recover with watering. Yellowing leaves. If you pull up the plant, you’ll see tunnels and rot in the lower stem and roots.

On kohlrabi specifically: This is one of the more serious kohlrabi pests because the swollen stem sits right at or near the soil line — exactly where root maggots operate. Damage to the base of the bulb can ruin the harvest.

Control:

• Cabbage collars. Cut 4-6 inch circles from cardboard, landscape fabric, or carpet padding. Slit to the center and fit snugly around the base of each transplant at planting time. This prevents the fly from laying eggs at the stem base.
• Row covers. Prevents the adult flies from reaching the plants entirely.
• Beneficial nematodes. Apply Steinernema feltiae nematodes to the soil around plants. These microscopic organisms parasitize root maggot larvae. Water them in well.
• Crop rotation. Don’t plant brassicas in the same spot two years in a row. Root maggot pupae overwinter in the soil.

Slugs and Snails

Damage: Large, irregular holes in leaves, especially on young plants. Silvery slime trails on and around plants. Worst in wet weather and in gardens with heavy mulch or lots of ground cover.

Control:

• Iron phosphate bait (Sluggo or similar). Organic, safe around pets and wildlife. Scatter around plants.
• Beer traps. Bury a cup so the rim is at soil level, fill halfway with cheap beer. Slugs crawl in and drown.
• Reduce hiding places. Remove debris, boards, and dense mulch from around kohlrabi beds during peak slug season.
• Water in the morning. Evening watering leaves the soil surface moist all night — prime time for slugs. Morning watering lets the surface dry by nightfall.

Diseases

Clubroot

What it is: A soil-borne disease caused by Plasmodiophora brassicae. The most feared brassica disease for good reason — it can persist in soil for 15-20 years.

Symptoms: Wilting during the day, partial recovery at night. Stunted growth. Yellowing leaves. If you pull up an affected plant, the roots will be swollen, distorted, and club-shaped — hence the name.

Prevention and control:

• Soil pH. Clubroot thrives in acidic soil. Liming your beds to raise the pH to 7.0-7.2 significantly reduces infection risk. Test your soil before the growing season.
• Crop rotation. Avoid planting any brassicas (kohlrabi, cabbage, broccoli, kale, Brussels sprouts, cauliflower, turnips, radishes) in the same bed for at least 4 years.
• Drainage. Clubroot spreads fastest in wet, poorly drained soil. Raised beds can help.
• Resistant varieties. Some kohlrabi varieties show tolerance to clubroot, though true resistance is limited in kohlrabi compared to cabbage.
• No cure for infected plants. Remove and destroy them. Do not compost.

Black Rot

What it is: A bacterial disease (Xanthomonas campestris) that enters through leaf pores or wounds.

Symptoms: V-shaped yellow lesions starting at leaf edges, with darkened veins. Leaves yellow and drop. In severe cases, the bacteria reaches the bulb, causing internal dark discoloration and soft rot.

Prevention and control:

• Start with clean seed or disease-free transplants. Hot water treatment of seed (122°F/50°C for 25 minutes) kills the bacteria.
• Avoid overhead watering. Water at the base of plants to keep foliage dry.
• Crop rotation. 2-3 year rotation away from all brassicas.
• Remove infected plants immediately. Black rot spreads rapidly in warm, wet conditions.

Downy Mildew

What it is: A fungal-like pathogen (Hyaloperonospora parasitica) that thrives in cool, humid conditions — exactly the weather kohlrabi grows best in.

Symptoms: Yellow patches on upper leaf surfaces with a fuzzy gray-purple mold on the undersides. Older leaves are affected first.

Prevention and control:

• Air circulation. Space plants properly. For kohlrabi, that typically means 6-8 inches between plants. See growing kohlrabi in containers for spacing in small spaces.
• Morning watering so leaves dry during the day.
• Copper-based fungicides can slow the spread but won’t cure infected tissue. Remove badly affected leaves.

Alternaria Leaf Spot

What it is: A fungal disease (Alternaria brassicicola) that causes round, dark spots with concentric rings (target-like patterns) on leaves.

Symptoms: Dark, circular spots that start small and expand. Heavily spotted leaves yellow and drop. Most common in warm, humid weather.

Prevention and control:

• Remove crop debris after harvest. The fungus overwinters on old plant material.
• Crop rotation. 2-3 years away from brassicas.
• Avoid wetting foliage. Drip irrigation is ideal.
• Neem oil or copper sprays can provide some protection when applied preventively.

Integrated Pest Management Strategy

If you want to minimize pest problems with the least effort, here’s the priority list:

The Non-Negotiables

1. Use row covers. This single action prevents 80% of insect problems. Lightweight fabric lets light, water, and air through while keeping butterflies, flea beetles, and root maggot flies out. It’s the closest thing to a magic solution in organic gardening.

2. Rotate crops. Never plant kohlrabi (or any brassica) where brassicas grew the previous year. A 3-4 year rotation breaks disease cycles and reduces soil-dwelling pest populations.

3. Start with healthy transplants. Buy from reputable sources or grow your own from treated seed. Most bacterial diseases arrive in the garden via infected seedlings.

The Helpful Habits

4. Companion planting. Certain plants deter kohlrabi pests or attract their predators. Aromatic herbs (thyme, sage, rosemary) confuse pest insects searching by smell. Flowers (alyssum, marigold) attract beneficial predators. Read more in kohlrabi companion planting.

5. Monitor weekly. A 5-minute walk through the garden, checking leaf undersides, looking for holes, noting any wilting. Early detection makes everything easier.

6. Clean up after harvest. Remove old kohlrabi plants and debris promptly. Don’t leave brassica stalks sitting in the garden over winter — they harbor pests and diseases.

When Not to Worry

Some damage is normal and doesn’t significantly affect your harvest:

• A few small holes from flea beetles on established plants. Cosmetic damage only.
• Outer leaves yellowing as the bulb matures. The plant redirects energy to the bulb — this is normal, not disease.
• Minor aphid colonies that beneficial insects are already visiting. Nature often handles small outbreaks on its own.

Kohlrabi’s short growing season is your biggest ally. Most pests and diseases don’t have time to build to devastating levels before the bulb is ready to harvest. If you start with a fast-maturing variety, use row covers, and practice basic crop rotation, you’ll dodge the majority of problems without reaching for a single spray bottle.