Why Are My Yam Leaves Curling?

Yam mosaic virus

Yam mosaic virus (YMV) is a potyvirus spread by aphids and one of the most damaging diseases of Dioscorea species globally. On infected yam vines, YMV causes a mosaic and mottling of the leaf (irregular yellow-green patches), chlorotic streaking along or between the veins, and distortion and curling of the younger leaves at the growing tip. Affected plants are stunted. The virus is most commonly introduced through infected tuber pieces used as planting material; aphids then spread the virus from plant to plant within a growing area. Infected plants do not recover.

What to do

• Remove and destroy infected plants. Source new planting material from healthy, known-clean tubers. Control aphids to reduce transmission risk. Do not save and replant sections from infected tubers.

Anthracnose

Anthracnose (Colletotrichum gloeosporioides) causes irregular brown to dark grey lesions on the leaves and stems of yam, particularly in warm, humid conditions. The lesions have a darker border and may develop concentric rings; in humid conditions, orange-pink spore masses appear within the lesion. As lesions expand and merge, the affected tissue dies and curls, giving the leaf a scorched, crumpled appearance. Anthracnose spreads rapidly in wet weather and can cause severe defoliation of yam vines in tropical production; in UK greenhouse conditions, it can appear during periods of high humidity.

What to do

• Remove and destroy affected leaves and stems promptly. Improve air circulation around the plants. Avoid overhead watering and keep foliage dry. There are no fungicides approved specifically for anthracnose on yam in UK home settings, but a copper-based fungicide used as a preventive treatment in high-risk conditions may slow spread.

Red spider mite

Red spider mite (Tetranychus urticae) colonises the undersides of yam leaves in warm, dry greenhouse and conservatory conditions. The mites produce fine webbing and cause pale, whitish stippling on the upper leaf surface; heavily infested leaves yellow, curl downward, and drop. The rapid reproduction of spider mite in warm, dry conditions means infestations can escalate quickly.

What to do

• Increase humidity around the plants by misting and placing pots on gravel trays with water. Introduce Phytoseiulus persimilis predatory mites for biological control. Remove the most heavily infested leaves. Treat with a plant-safe miticide spray if biological control cannot be established quickly enough to contain a severe infestation.

Cold and draught

Yam vines are tropical plants that slow and shut down below 15°C and suffer damage below 10°C. Cold draughts from open windows or doors in UK homes cause sudden leaf curl and yellowing. In winter, yam vines naturally die back to the tuber as temperatures fall and day length shortens; this die-back is normal and expected, not a sign of disease. The tuber survives in a cool (but frost-free), dry state until warmth and longer days trigger new growth in spring.

What to do

• Grow yam in the warmest, most sheltered position available. Allow the vine to die back naturally in autumn and winter; reduce watering to a minimum. Store the tuber in slightly dry compost in a frost-free position through winter; restart watering and warmth in March to April to encourage resprouting.

Frequently asked questions

Why are my yam leaves curling?

True yam (Dioscorea species, including Dioscorea alata, D. rotundata, and D. cayenensis) leaves curl most commonly because of yam mosaic virus (YMV), anthracnose (caused by Colletotrichum gloeosporioides), or red spider mite in dry indoor conditions. Yam mosaic virus is a potyvirus spread by aphids; infected yam plants show mosaic and mottling on the leaves, distortion, chlorotic streaking along the veins, and curling of the leaf blade, particularly on young leaves. Anthracnose causes irregular brown lesions on the leaf surface and margins; as the lesions expand in humid conditions, the affected tissue dies and curls, giving the leaf a scorched, curled appearance. Red spider mite colonises the undersides of the heart-shaped yam leaves in warm, dry greenhouse conditions, producing pale stippling and eventual leaf curl and drop. Cold temperatures (below 15°C) cause yam leaves to pale, yellow, and curl as the tropical vine shuts down growth.

How do I grow yam in the UK?

True yam (Dioscorea species) can be grown in the UK as a conservatory or heated greenhouse plant. The large, starchy tubers sold in UK Afro-Caribbean and Asian grocery shops are typically Dioscorea alata (water yam or winged yam) or D. rotundata (white Guinea yam); a section of an undamaged, fresh yam tuber can be planted and will sprout. Plant tuber pieces in pots of rich, well-drained compost in spring at a temperature of 20 to 25°C; the vines emerge and grow rapidly in warm conditions, climbing strongly if given a support. Yam requires high temperatures (above 20°C), high humidity, and a very long frost-free growing season (9 to 12 months) to produce a harvestable tuber. UK outdoor growing is not feasible for most of the country; in a heated greenhouse, yam can be grown as an ornamental vine with attractive heart-shaped leaves, though tuber yields are modest compared with tropical production. Reduce watering when the vine begins to die back in autumn; this signals the end of the growing season and the beginning of tuber dormancy.

Is yam the same as sweet potato?

No, true yam (Dioscorea species) is botanically unrelated to sweet potato (Ipomoea batatas), despite the widespread use of 'yam' as an informal name for sweet potato in the United States. True yams are monocots in the family Dioscoreaceae, with twining vines that can reach 10 metres or more in tropical conditions, large, usually heart-shaped or arrowhead-shaped leaves, and large starchy tubers that can weigh up to 70 kilograms in commercial production. Sweet potatoes are dicots in the family Convolvulaceae (the morning glory family), with trailing or twining stems, lobed or heart-shaped leaves, and orange, yellow, white, or purple-fleshed roots. In the UK, all the brown, rough-skinned, firm-fleshed starchy tubers sold as 'yam' in Afro-Caribbean groceries are true Dioscorea yams; the orange-fleshed tubers sold as 'sweet potato' in UK supermarkets are Ipomoea batatas.

What is yam mosaic virus?

Yam mosaic virus (YMV) is a potyvirus that infects Dioscorea species and is one of the most widespread viral diseases of yam in West Africa, where yam is a staple food crop. YMV is transmitted non-persistently by aphids (primarily Aphis gossypii, the cotton-melon aphid). Infected plants show a range of symptoms including mosaic and mottling (irregular yellow-green patches on the leaves), vein clearing (the veins appear paler than the surrounding leaf tissue), chlorotic streaking, leaf distortion and curling (particularly on young leaves), and plant stunting. Infected planting material (tuber pieces) is the primary route of long-distance spread; aphids spread the virus within a growing area. There is no cure for YMV; infected plants should be removed. Using certified virus-free seed yam and controlling aphids to slow local spread are the main management strategies. In UK conservation and research collections of Dioscorea, YMV testing of new arrivals is standard practice.