Why Are My Longan Leaves Curling?

Red spider mite

Red spider mite (Tetranychus urticae) is the most common pest of longan in UK greenhouse conditions. The mites colonise the undersides of the leaves in warm, dry conditions, producing pale, bronze stippling on the upper leaf surface and fine, silky webbing on the undersides; the leaves curl downward, lose their glossy green colour, and eventually yellow and drop. Spider mite populations build up rapidly in hot, dry conditions and can cause significant damage before the webbing becomes visible enough to alert the grower. Regular inspection of leaf undersides (particularly on the middle and lower canopy) is the best way to catch infestations early.

What to do

• Increase humidity around the plant by misting the foliage (particularly leaf undersides) and using a gravel tray with water. Introduce Phytoseiulus persimilis predatory mites as soon as spider mite is detected. Treat with insecticidal soap or a plant-safe miticide in severe cases. Move the tree outdoors to a sheltered position in summer, where fresh air, occasional rain, and natural predators reduce mite pressure significantly.

Witches' broom disease

Longan witches' broom disease (LWBD) is caused by a phytoplasma and produces the most visually distinctive symptom in the longan genus: dense clusters of abnormally small, distorted, pale or chlorotic leaves at the shoot tips. The growing tips proliferate into bunches of short, stunted shoots bearing tiny, misshapen leaves, giving an appearance like a broom head or a bird's nest. Affected shoots fail to extend normally; the tree's vigour and fruiting capacity decline as more growing tips become infected. The phytoplasma is spread by leafhopper insects feeding on infected plants.

What to do

• Remove and destroy all visibly affected branches, cutting back well into healthy wood. Control leafhoppers with insecticide sprays to limit further spread. Do not propagate from infected plants. There is no chemical cure for phytoplasma diseases; severely infected trees must be removed and destroyed. Source longan plants only from reputable suppliers with clean planting material to avoid introducing the disease.

Scale insects

Soft scale (Coccus hesperidum) and occasionally hard scale species colonise longan stems and leaf undersides in UK greenhouse conditions. The insects are often overlooked until populations are large enough to produce visible honeydew accumulation and sooty mould. Feeding scale insects cause yellowing and curling of the affected leaves and may cause premature leaf drop if populations become high.

What to do

• Scrub scale off stems and leaves with a toothbrush and insecticidal soap. Apply a horticultural oil spray to smother scales and eggs. Use the parasitoid wasp Metaphycus helvolus for biological control in a heated glasshouse. Inspect the plant routinely in winter (when scale populations can build up unnoticed) by checking the undersides of leaves and the stem junctions.

Cold and humidity

Longan requires temperatures above 15°C for comfortable growth and is damaged by temperatures below 5°C. Cold causes the leaves to turn yellow at the margins and curl; sudden temperature drops can trigger significant leaf drop. Low humidity in UK winter conditions causes the leaves to dry at the tips and margins, with the tips curling. Longan is somewhat more cold-tolerant than lychee and can survive a short period at 10°C without lasting damage, but sustained cold causes leaf damage and can kill young growth.

What to do

• Maintain temperatures above 15°C in the growing season and above 10°C in winter. Increase humidity with a gravel tray and misting. Reduce watering in winter but do not allow the rootball to dry completely. Keep the plant away from cold draughts near vents and glass panes. Cold-damaged leaves do not recover but the tree will produce new growth in spring as temperatures rise.

Frequently asked questions

Why are my longan leaves curling?

Longan (Dimocarpus longan) leaves curl most commonly in UK greenhouse cultivation because of red spider mite, longan witches' broom disease (caused by a phytoplasma), scale insects, low humidity, or cold temperatures. Red spider mite is the most common pest, colonising the undersides of the leaves in warm, dry conditions and producing pale stippling, fine webbing, and leaf curl. Longan witches' broom disease produces dense clusters of abnormally small, distorted, pale leaves at the shoot tips; the growing tip proliferates into a broom-like mass of stunted shoots. Scale insects and low humidity also cause leaf curl and yellowing.

Can I grow longan in the UK?

Longan (Dimocarpus longan) can be grown in the UK as a heated greenhouse plant. It requires temperatures above 15°C, high humidity, full sun, and a period of cooler temperatures in winter to initiate flowering. Seeds from fresh longan fruit (available from Chinese supermarkets) germinate readily when sown fresh; the seed must be planted promptly as longan seeds lose viability within a few days of removal from the fruit. Grafted plants from specialist nurseries fruit much sooner than seed-grown trees. The glossy, pinnate leaves are ornamental and make longan a rewarding greenhouse foliage plant even without fruit.

What is longan witches' broom disease?

Longan witches' broom disease (LWBD) is a serious disease of longan caused by a phytoplasma, producing dense clusters of abnormally short, bunched shoots bearing numerous tiny, distorted, pale or yellowish leaves at the growing tips. The shoot tips fail to extend to normal length and the plant's overall vigour declines. The phytoplasma is spread by leafhoppers. There is no chemical cure; management focuses on removing and destroying affected branches, controlling the leafhopper vector, and not propagating from infected plants. Always source longan plants from reputable suppliers with clean planting material.

What is the difference between longan and lychee?

Longan (Dimocarpus longan) and lychee (Litchi chinensis) are closely related tropical fruit trees in the soapberry family that produce similar-looking fruits. Lychee fruit has a rough, red or pink bumpy skin; longan fruit has a smoother, tan or light brown skin and is generally slightly smaller. The flavour of lychee is floral and intensely sweet; longan has a more musky, smoky-sweet flavour with a deeper, caramel-like quality. Both are available fresh or canned in Chinese supermarkets. Longan is considered slightly easier to grow in UK conditions than lychee, tolerating marginally cooler temperatures.