Why Are My Swede Leaves Curling?

Cabbage aphid

Cabbage aphid (Brevicoryne brassicae) is one of the most damaging pests of swede in the UK. The grey-green, waxy aphids form dense, compact colonies on the underside of leaves and at the growing tips, causing the leaves to curl inward around the feeding sites. The colonies are distinctive in appearance: tight, powdery-grey clusters with a waxy, dusty texture quite different from the shiny green colonies of other aphid species. Heavily infested growing tips become completely distorted, and severe infestations significantly reduce plant vigour and root development.

What to do

• Squash small colonies by hand immediately. The waxy coating reduces spray penetration so early intervention before colonies are established is the most effective approach.
• Apply insecticidal soap or pyrethrin-based spray, ensuring thorough coverage of the underside of leaves. Repeat every 5 to 7 days.
• Cover young plants with fine insect mesh from sowing to exclude the winged aphids that colonise in late spring and early summer.

Downy mildew

Downy mildew (Peronospora parasitica) on swede produces pale, yellowish, angular patches on the upper surface of the leaves, with a downy, grey-purple fungal coating visible on the underside in humid conditions. Leaves curl and yellow around the infection sites, and the disease progresses rapidly in cool, wet weather with poor airflow. Downy mildew is most common on closely spaced plants or in seasons with persistently wet conditions from June onward.

What to do

• Remove and destroy affected leaves promptly to reduce spore spread. Do not compost infected material.
• Thin swede seedlings to the correct spacing (23 to 30 centimetres between plants) as soon as they are large enough to handle. Crowded plants are far more susceptible to downy mildew than well-spaced ones.
• Rotate all brassica crops each year: downy mildew oospores and other brassica pathogens persist in soil and plant debris.

Flea beetle

Flea beetle (Phyllotreta species) attacks swede seedlings in exactly the same way as other brassica seedlings, chewing numerous small, round holes in the leaves of young plants. Severely attacked seedlings curl their leaves as a stress response and may fail to establish. Attacks are most damaging in the first 4 to 6 weeks after germination; once the plants are well established they are able to grow away from flea beetle damage without serious harm to the final crop.

What to do

• Cover the seedbed with fine insect mesh immediately after sowing and keep it in place until the plants are large enough (10 centimetres or more) to outgrow beetle attacks.
• Water the seedbed regularly during dry periods: flea beetles are far less active when the soil surface is moist.

Clubroot

Clubroot (Plasmodiophora brassicae) does not directly cause leaf curl but causes progressive wilting and yellowing of swede leaves that can be confused with drought or aphid attack. Infected plants pull from the ground easily and reveal massively distorted, club-like root structures in place of the smooth, round swede root. The disease persists in the soil for 20 years or more and makes affected beds unsuitable for any brassica family crop for the very long term.

What to do

• Check any wilting or yellowing plants by lifting them gently and examining the roots. Club-shaped root distortions are diagnostic of clubroot and are not caused by any other disease.
• Rotate brassica crops each year and lime the soil to raise the pH to 7.0 to 7.5: clubroot spores are less viable at high pH.
• Avoid importing soil or brassica transplants from unknown sources, as this is the primary route by which clubroot enters new gardens.

Drought

Swede is more drought-tolerant than some other vegetables because of its deep taproot, but extended drought during the growing season produces smaller, tougher roots and can cause the roots to develop internal brown areas (a condition called brown heart, related to boron deficiency exacerbated by drought). The foliage of drought-stressed swedes wilts and curls inward during hot periods.

What to do

• Water swedes during extended dry spells in summer and early autumn, particularly during the period of rapid root development from August onward.
• Apply a mulch of compost between the rows to conserve soil moisture and reduce the watering frequency needed in dry summers.

Frequently asked questions

Why are my swede leaves curling?

Swede leaves curl most often from cabbage aphid infestation, downy mildew, or flea beetle damage in the seedling stage. Cabbage aphid (Brevicoryne brassicae) forms dense, grey-waxy colonies on the underside of swede leaves and at the growing tips, causing the leaves to curl inward around the feeding colonies. Downy mildew (Peronospora parasitica) produces pale, angular patches on the upper leaf surface with a grey-purple fungal coating below, causing leaf curling and yellowing. Flea beetle attacks young swede seedlings, creating shot-holes in the leaves that cause stressed seedlings to curl.

How do I prevent clubroot on swedes?

Clubroot (Plasmodiophora brassicae) is the most damaging disease of swedes and other brassicas in the UK. It causes the roots to become massively distorted into large, irregular clubs, and infected plants wilt and yellow as the root system fails. Prevention is far more effective than attempted control: rotate all brassica family crops to a different bed each year; lime the soil to raise the pH to 7.0 to 7.5 (clubroot is suppressed at high pH); avoid bringing soil into the garden from unknown sources; grow transplants in pots of fresh compost rather than sowing direct into infected soil. There is no chemical control for clubroot available to UK home gardeners.

When should I sow swedes in the UK?

Swedes are sown direct in the UK from late May to mid-June. Sowing earlier risks the plants bolting (running to seed) before producing a usable root; sowing later reduces the growing season and may result in small roots. Swedes need a long growing season (approximately 20 to 26 weeks) to develop large, sweet roots. Sow seed thinly in shallow drills and thin seedlings progressively to a final spacing of 23 to 30 centimetres. Swedes are cold-hardy and improve in flavour after frost; they can be left in the ground and harvested as needed from October through to the following spring.

What is the difference between a swede and a turnip?

Swede (Brassica napus subsp. napobrassica) and turnip (Brassica rapa subsp. rapa) are related but distinct vegetables. Swede is larger, slower-growing, and produces roots with yellow-orange flesh and a sweet, earthy flavour. The skin is usually purple-yellow and the root is solid and dense. Turnip is smaller, faster-growing, and produces white-fleshed roots with a sharper, more peppery flavour. Swede leaves are smooth and grey-green, similar to other Brassica napus crops; turnip leaves are hairy and rougher in texture. Both improve in flavour after frost, but swede is typically available from October onward while turnips sown in July to August are available from autumn.