Common Bok Choy pests

Flea beetles are tiny black or bronze jumping beetles that riddle young leaves with a shotgun pattern of small round holes, and they hit seedlings hardest. Cabbage worms (the velvety green larvae of the cabbage white butterfly), cabbage loopers (inchworm crawlers), and diamondback moth larvae all chew ragged holes and leave dark green or brown droppings in the leaf crowns. Cabbage aphids form gray green waxy clusters on undersides and in the heart, causing curled and stunted growth, while flea beetle and slug feeding, plus root maggots that tunnel into the base and stunt the plant, round out the main threats along with occasional deer, rabbit, and groundhog browsing.

Common diseases

Downy mildew shows as pale yellow angular patches on the upper leaf surface with a fuzzy gray to purple growth on the underside, favored by cool wet weather. Damping off rots seedlings at the soil line and topples them, and Alternaria and other leaf spots appear as dark spots with concentric rings, sometimes yellow halos. Black rot is a bacterial disease that causes yellow V shaped lesions spreading in from the leaf margins with blackened veins, while clubroot deforms and swells the roots and causes wilting and stunting above ground, and white rust can produce chalky white pustules on undersides.

Prevention

Rotate brassicas so bok choy does not return to the same bed for three to four years, and clear and compost or remove crop debris after harvest to deny pests and pathogens a place to overwinter. Space plants for good airflow, water at the base in the morning rather than overhead to keep foliage dry, and avoid working among wet plants to limit disease spread. Start with clean seed and disease free transplants, keep clubroot in check by liming acidic soil toward a more neutral pH, and time plantings for the cool ends of the season to sidestep peak flea beetle and worm pressure.

Organic control

Cover beds with floating row covers from transplanting or emergence to block flea beetles, cabbage butterflies, moths, and root maggot flies, since bok choy does not need pollinators the covers can stay on until harvest. Handpick caterpillars and crush aphid colonies or knock them off with a strong spray of water, and set out yellow sticky cards to monitor and reduce flying pests. Bacillus thuringiensis subspecies kurstaki (Bt) applied to leaf tops and bottoms controls young caterpillars, insecticidal soap or neem oil helps with aphids and soft bodied pests, and copper based products can suppress downy mildew and bacterial spread. Always read and follow the product label for what each product treats, timing, and how to apply, and favor evening applications to protect bees and beneficial insects like lady beetles, lacewings, and parasitic wasps.

Scouting & monitoring

Scout at least twice a week, checking leaf undersides and the plant heart where aphids and caterpillars hide, and inspect seedlings daily for the flea beetle shot holes and toppled damping off that appear soonest. Watch for cabbage white butterflies fluttering over the bed as a cue to check for freshly laid eggs and start row covers or Bt. After cool damp spells look for the yellow patches and fuzzy underside growth of downy mildew and the V shaped margin lesions of black rot, and pull and discard any plant showing swollen clubroot galls at the roots.

Tips

Row covers from day one are the single most effective home garden defense for bok choy because they exclude nearly every major pest before damage starts. Grow it fast in cool weather with steady moisture and clean rotation, and remove stressed or infected plants promptly so small problems do not spread through the bed.

General home-garden IPM guidance, focused on prevention and least-toxic control. Always read and follow the product label for any pesticide, including organic products. Page updated Aug 4, 2026.