Kohlrabi is a cool-season brassica grown for its swollen, bulb-like stem that forms just above the soil line. The texture is crisp and the flavor mild, similar to a broccoli stem. It matures quickly, making it useful for both spring and fall plantings in upstate New York.
Grow it fast and steady. Kohlrabi needs consistent moisture and cool temperatures to size up before turning woody or bolting. Harvest the bulbs when they reach 2–3 inches across; larger bulbs become tough and fibrous. Heat and drought stress cause cracking and poor texture, so the fall crop often performs best in Zone 6b.
Clubroot
Swollen, distorted roots; wilting in midday heat; stunted growth and yellowing foliage. Plants recover poorly even with watering.
Remove and destroy infected plants with roots. Raise soil pH toward 7.2 with lime. Improve drainage and rotate out of brassicas for several years.
Black Rot
Yellow, V-shaped lesions spreading inward from leaf margins, with blackened veins. Severe cases cause wilting and leaf drop.
Remove and destroy infected plants. Use disease-free seed, rotate brassicas on a 3-year cycle, avoid overhead watering, and keep tools and beds clean.
Cabbage Worm
Ragged holes in leaves and green velvety caterpillars on undersides; dark green frass collects in leaf crevices and head.
Handpick caterpillars and eggs. Apply Bacillus thuringiensis (Bt) weekly during egg-laying. Use floating row cover to exclude white cabbage moths.
Flea Beetle
Small round shot-holes scattered across leaves; tiny dark beetles that jump when disturbed. Heavy feeding stunts young plants.
Use floating row covers on seedlings. Remove crop debris and weeds that harbor beetles. Apply kaolin clay or spinosad if damage is severe.
Downy mildew
Yellowing on top of leaves with grey-purple fuzz underneath. Spreads rapidly in humid conditions.
Remove affected leaves. Improve air circulation. Avoid overhead watering. Copper spray as preventive.
Aphids
Clusters of small soft insects on new growth and flower buds.
Knock off with a strong jet of water. Ladybirds and lacewings are natural predators. Insecticidal soap as last resort.