Quick answer: most broccoli trouble in home gardens comes from five pests — imported cabbageworm, cabbage looper, diamondback moth larvae, aphids, and cabbage maggot flies — and three diseases — downy mildew, black rot, and clubroot. Identify first, then treat: hand-picking and Bt handle nearly all caterpillars, strong water sprays handle aphids, and only rotation and resistant varieties truly control the soil-borne diseases.
Broccoli Pests: Identification and Control
Imported cabbageworm (Pieris rapae)
The classic culprit: pale green caterpillars with a faint gold stripe, matching the leaf color, leaving wet-looking frass on the head. The white butterflies you see fluttering over the bed are the adults. Control: hand-pick, spray Bacillus thuringiensis (Bt) at first sign — it kills caterpillars within days and is harmless to bees — or exclude them entirely with floating row cover before butterflies arrive.
Cabbage looper
Green with white stripes along the sides, moves in a looper inchworm gait. Does the same damage as cabbageworms but often in larger numbers late season. Control: same as above — Bt works, and spinosad adds speed for heavy infestations.
Diamondback moth larvae
Tiny, tapered, pale green, and wriggly; they chew small window-pane holes in leaves and hang on silk threads when disturbed. Repeatedly resistant to some sprays. Control: Bt or spinosad, applied early; row cover is the best prevention.
Cabbage aphids
Gray-green clusters packed between leaf veins and under leaves, often with ants farming them. They stunt heads and spread viruses. Control: a hard water jet knocks most off; insecticidal soap or neem covers the rest; skip broad insecticides that flare mites.
Cabbage root maggot
Small white maggots tunnel into stems and roots; plants wilt on hot days and sometimes die. Most common after uncomposted manure is worked into beds. Control: ring collars or fine netting at the stem base at transplant time, rotate planting sites, and cover beds before the fly flies in spring.
Slugs and flea beetles
Slugs chew ragged holes and leave slime trails overnight — trap with boards or beer. Flea beetles shot-gun tiny holes in young leaves — row cover or diatomaceous earth at the seedling stage protects most transplants.
Broccoli Diseases: What They Look Like
Downy mildew (Hyaloperonospora parasitica)
Yellow angular patches on the leaf top with a fuzzy gray-white growth underneath, worst in cool, wet spring weather. Seedlings collapse; mature plants lose outer leaves. Management: space plants for airflow, water at the base, remove infected leaves, and choose resistant transplants. Copper sprays slow it but do not cure it.
Black rot (Xanthomonas campestris)
V-shaped yellow browning spreading inward from leaf edges with blackened veins — the veins are the giveaway. It is seed-borne and spreads by wet hands and tools. Management: there is no cure. Remove whole plants, use hot-water-treated or certified seed, rotate away from brassicas for two years, and avoid overhead watering.
Clubroot (Plasmodiophora brassicae)
Swollen, distorted roots and stunting that worsens on warm, wet, acidic soil. Once in a bed, spores last 15+ years. Management: raise soil pH to 7.2 with lime where tests show acidity, use resistant varieties (labels like “CR”), and never compost infected roots.
Alternaria leaf spot & yellow (fusarium) wilt
Alternaria makes small dark spots with yellow halos in humid weather — usually cosmetic on broccoli; treat seed with hot water and rotate. Fusarium wilt browns one side of the plant while veins darken; remove and rotate.
The 3-Step Broccoli Protection Plan
- At transplant: floating row cover + stem collars. This single step prevents cabbageworms, loopers, diamondback moths, and root maggot flies all season.
- Weekly check under leaves: crush egg clusters, spray Bt at first caterpillars, blast aphids with water.
- End of season: pull and trash (not compost) any plant with black-rot veins or swollen roots; move broccoli beds at least 3 meters each year.
FAQ
Can I still eat broccoli after spraying Bt or spinosad?
Yes — Bt has no pre-harvest interval and spinosad allows harvest 1 day after spraying. Wash heads as usual.
Why is my broccoli not heading up?
Usually heat stress, nitrogen shortage, or a too-young transplants setting button early. Keep beds evenly moist and side-dress at mid-season; for fall crops choose heat-tolerant varieties.
Is broccoli worth growing if I keep losing heads to worms?
With row cover installed at transplant, most home growers pick clean heads all season — the cover, not the spray, is what fixes chronic worm problems.
