Common Kohlrabi pests

The most common insect pests are the brassica caterpillars: imported cabbageworm (velvety green worms from small white butterflies), cabbage loopers (green inchworms that arch as they crawl), and diamondback moth larvae, all of which chew ragged holes in leaves and leave dark droppings. Flea beetles are tiny black beetles that pepper young leaves with pinhole shot holes, and they hit seedlings hardest. Cabbage aphids form gray-green waxy clusters on the undersides of leaves and in the growing tips, causing curling and stunting, while cabbage root maggots tunnel into roots and cause sudden wilting of transplants. Watch also for harlequin bugs (shield-shaped black-and-orange stink bugs that cause white or yellow stippling), cutworms that sever seedlings at the soil line, and slugs; larger animal pests include rabbits, groundhogs, and deer that graze the tender leaves.

Common diseases

Black rot is the most serious disease, a bacterial infection that shows up as yellow, V-shaped lesions along leaf margins with blackened veins. Clubroot is a soilborne disease that causes swollen, distorted roots along with wilting, yellowing, and stunted growth even when the soil is moist. Downy mildew produces pale yellow patches on the upper leaf surface with a grayish, fuzzy growth underneath, and it favors cool, wet weather. Alternaria leaf spot appears as dark brown spots with concentric target-like rings, and bacterial soft rot can turn the bulb mushy and foul-smelling in wet conditions.

Prevention

Rotate brassicas so kohlrabi does not follow cabbage-family crops in the same bed for at least three to four years, which is the single best defense against black rot and clubroot. Space plants well and keep beds weeded so leaves dry quickly, and water at the base in the morning rather than wetting the foliage. Start with disease-free transplants or certified seed, choose resistant varieties when available, and keep clubroot in check by liming to raise soil pH where that disease is a known problem. Clean up crop debris at the end of the season and do not compost diseased plants, since many of these pathogens overwinter on old residue.

Organic control

Floating row covers placed over the bed right at planting are the most effective organic tool, blocking cabbageworms, flea beetles, root maggot flies, and harlequin bugs; just remove or vent them during hot spells and rotate the bed so pests are not sealed in. Hand-pick caterpillars and harlequin bugs and knock aphid colonies off with a strong spray of water, and use Bt (Bacillus thuringiensis) or spinosad for heavy caterpillar pressure. Insecticidal soap or neem oil helps with aphids and young flea beetles, and beneficial insects such as lady beetles, lacewings, and parasitic wasps will keep aphids in check if you avoid broad-spectrum sprays. For downy mildew or Alternaria in wet seasons, copper-based fungicides can slow spread; always read and follow the product label for any spray, and apply in the evening to protect pollinators.

Scouting & monitoring

Scout at least once or twice a week, checking the undersides of leaves and the growing tip where aphids and caterpillars hide, and look for fresh chewing holes, dark droppings, or shot-hole damage. Watch seedlings and new transplants most closely, since flea beetles, cutworms, and root maggots do the greatest harm in the first few weeks. In cool, damp weather, inspect for the yellow patches of downy mildew and the V-shaped margin lesions of black rot, and pull any suddenly wilting plant to check the roots for the swollen galls of clubroot. Set out yellow sticky cards or check under boards for slugs to catch buildups before they explode.

Tips

Cover the bed with row cover from day one and you will avoid the majority of kohlrabi pest problems without any spraying. Harvest bulbs while they are young and tender, about tennis-ball size, because stressed or overmature plants are far more prone to pests and disease.

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.