Why Are My Sweet Potato Leaves Curling?

Sweet potato virus disease (SPVD)

Sweet potato virus disease (SPVD) is the most destructive disease of sweet potato worldwide and results from a synergistic co-infection of two viruses: sweet potato feathery mottle virus (SPFMV, a potyvirus spread by aphids) and sweet potato chlorotic stunt virus (SPCSV, a crinivirus spread by whitefly). Together, these two viruses cause far more severe symptoms than either alone. Infected plants show severe leaf distortion, puckering, and curling, with the affected leaves becoming crumpled, narrow, and chlorotic (yellow-green); stems are stunted and the overall plant is greatly reduced in size and productivity. Tuber yields from SPVD-infected plants can be reduced by 80 to 90%.

What to do

• Remove and destroy infected plants immediately. Source new planting material (slips) only from certified virus-free suppliers. Control both aphids and whitefly to reduce transmission risk. Do not propagate slips from infected plants. Grow certified clean planting material each season rather than saving slips from the previous year's crop unless it was visibly healthy.

Whitefly

Glasshouse whitefly (Trialeurodes vaporariorum) on plants grown under cover, and silverleaf whitefly (Bemisia tabaci) in warmer outdoor conditions, both infest sweet potato. Whitefly cause direct feeding damage (yellowing, curling leaves, sticky honeydew deposits) and, more importantly, the silverleaf whitefly transmits SPCSV, one of the two viruses in the SPVD complex. Yellow sticky traps placed near sweet potato plants in a greenhouse will alert you to early whitefly presence before populations build.

What to do

• Use yellow sticky traps for monitoring and population reduction. Introduce Encarsia formosa parasitoid wasps for biological control of glasshouse whitefly in heated growing spaces. Apply insecticidal soap to adult and nymph populations on the undersides of leaves. Remove whitefly-infested plants that also show viral symptoms.

Cold temperatures

Sweet potato is highly sensitive to cold. Temperatures below 10°C cause the leaves to yellow, develop purple stress-pigment colouration, and curl as the plant shuts down. Even brief cold spells slow growth significantly, and plants planted out too early in UK springs may sit and sulk for several weeks rather than establishing. UK growers should not plant sweet potato slips outdoors until the soil temperature is consistently above 15°C and all risk of frost has passed.

What to do

• Plant out only when the soil is warm (late May to June). Warm the soil in advance with black polythene mulch film, which also suppresses weeds and reflects heat onto the developing tubers. Use cloches or fleece to extend the season at each end. In a cold UK summer, a cold frame or unheated greenhouse significantly improves yield over outdoor growing.

Aphids

Aphids (particularly peach-potato aphid, Myzus persicae) colonise sweet potato shoots, causing the youngest leaves at the growing tips to curl and pucker around the feeding colonies. More significantly, Myzus persicae transmits SPFMV, one of the two viruses in the SPVD complex. Heavy aphid pressure on sweet potato in the UK is less common than on other crops, but controlling aphids is important to reduce the risk of virus introduction to a healthy crop.

What to do

• Apply insecticidal soap to aphid colonies on growing tips. Encourage natural predators by avoiding broad-spectrum insecticides. Inspect new plants closely before planting and reject any with signs of aphid infestation. Remove severely aphid-infested growing tips to reduce the source of virus-spread within the crop.

Frequently asked questions

Why are my sweet potato leaves curling?

Sweet potato (Ipomoea batatas) leaves curl most commonly because of sweet potato virus disease (SPVD), whitefly infestation, or cold temperatures. Sweet potato virus disease is a synergistic infection of two viruses (sweet potato feathery mottle virus and sweet potato chlorotic stunt virus) that together cause far more severe symptoms than either virus alone: infected plants show severe leaf distortion, puckering, curling, chlorosis, and stunting, with the affected leaves often appearing yellow-purple and crumpled. Whitefly (particularly glasshouse whitefly on plants under cover, or silverleaf whitefly in warmer outdoor conditions) cause leaf curling and yellowing through feeding damage and by transmitting the viruses that cause SPVD. Cold temperatures below 10°C cause sweet potato leaves to yellow, purple (from anthocyanin stress pigments), and curl as the tropical plant shuts down; cool summers are the most common reason UK-grown sweet potatoes perform poorly.

How do I grow sweet potatoes in the UK?

Sweet potatoes (Ipomoea batatas) can be grown in the UK, but they need a long, warm growing season to produce a good harvest. Start slips (rooted cuttings) indoors in a heated propagator at 20 to 25°C from March to April, or buy certified slip plants from a reputable supplier in May. Plant out after all risk of frost has passed (late May to early June in most of the UK) into a very warm, sheltered, sunny position; a sheltered south-facing border against a wall, a raised bed, or a cold frame or cloche are all good options. Use a rich, free-draining soil or compost. Water regularly in dry spells; sweet potatoes need warmth more than anything else. Harvest in October before the first frost, or when the foliage is killed by frost. UK yields are lower than tropical production but a warm summer can produce a very respectable harvest of small to medium tubers. Grow named UK-adapted varieties ('Beauregard' and 'Bonita' perform well in UK conditions) for the best results.

Can you eat sweet potato leaves?

Yes, sweet potato leaves (Ipomoea batatas) are edible and widely consumed in many parts of the world, including West Africa, Southeast Asia, the Pacific Islands, and parts of East Asia. The young shoot tips and leaves are the most tender and are used as a leafy vegetable in stir-fries, soups, stews, and salads; older leaves are tougher and better suited to cooked dishes than raw consumption. Sweet potato leaves have a mild, slightly sweet, earthy flavour and are nutritious, containing vitamin A (from beta-carotene), vitamin C, calcium, and iron. In the UK, sweet potato is grown almost exclusively for the tubers, and the leaves are rarely used as a vegetable, but harvesting the shoot tips during the growing season does not significantly reduce tuber yield if done in moderation. The leaves should be harvested from healthy, untreated plants.

Why are my sweet potato leaves turning purple?

Sweet potato leaves turning purple is most commonly caused by cold stress or phosphorus deficiency. When sweet potato plants are exposed to cool temperatures (below 15°C), they produce anthocyanin stress pigments that turn the leaves and stems purple or red-purple; this is a defence response to cold and the purple colour usually fades as temperatures warm. The same purple pigmentation can appear during cold spells in UK summers. Phosphorus deficiency also causes purple discolouration of sweet potato leaves; this is more likely in cold, wet soils where phosphorus is less available to the roots, or in very acid soils. Some sweet potato varieties naturally have purple or reddish-purple leaves (particularly ornamental and edible-leaf varieties); in these cases, the purple is normal. Viral infections (particularly sweet potato virus disease) can also cause reddish-purple discolouration alongside leaf distortion.