Acer Leaves Curling

Sycamore aphid

Sycamore aphid (Drepanosiphum platanoidis) is one of the most abundant and ecologically important aphids in UK gardens and parks, colonising the undersides of sycamore (Acer pseudoplatanus) and closely related large-leaved maples in very large numbers from spring through summer. The aphids are large, pale green, and conspicuous on the leaf undersides, and they produce enormous quantities of honeydew that drips from the tree canopy and coats everything beneath in a sticky, sooty-mould-supporting film. The lobed leaves of sycamore cup and curl downward along their length as the aphid population builds and the weight and feeding stress of the colony increase, and the overall appearance of a heavily infested sycamore in summer, with its cupped, sticky leaves and black sooty mould, is one of the most recognisable symptoms of aphid infestation among UK garden trees. Unlike many aphid species that migrate to alternate hosts in summer, sycamore aphid remains on Acer throughout the season, with population peaks in spring and early summer and again in autumn after a mid-summer trough. The ecological importance of sycamore aphid is significant: it is a key food source for blue tits and other insectivorous birds during the spring nestling period, for hoverfly larvae, and for ladybirds and lacewings that aggregate on sycamore to exploit the abundant food source.

Tolerating sycamore aphid on large established trees is the realistic approach for most gardens, as treating a large sycamore with insecticides is impractical, expensive, and would harm the substantial invertebrate community that depends on the aphid colonies. The honeydew problem can be managed by hosing down affected surfaces regularly. On smaller ornamental acer specimens where the honeydew drip is unacceptable, a fatty acid or neem oil spray applied to all leaf undersides in spring before the population peaks can reduce colony sizes significantly. Natural predators, particularly blue tits and hoverflies, provide important control on established trees and the colonies typically reduce substantially by midsummer without intervention.

Verticillium wilt

Verticillium wilt (Verticillium dahliae and Verticillium albo-atrum) is one of the most serious diseases affecting acer in UK gardens, particularly the ornamental maples and sycamore, which are among the woody plants most susceptible to this soil-borne vascular disease. The fungus infects the root system and colonises the water-conducting xylem vessels of the plant, producing toxins and physical blockages that prevent water and nutrient movement to the affected branches. The above-ground symptoms appear with little warning: one or more branches wilt suddenly in warm weather, the leaves curl downward and inward, brown from the margins inward, and remain attached to the dead shoot in the characteristic manner of vascular wilts. The branch may die back over several days or weeks as the infection progresses. Inspection of the sapwood of a dying branch, by peeling away the bark or making a cross-cut, reveals the characteristic dark brown, olive, or greenish discolouration of the infected xylem that confirms verticillium wilt as the cause. In some acer specimens, particularly large established trees, the disease kills one or a few branches and then appears to stabilise as the tree compartmentalises the infection; in others, particularly young trees and those in infected soil under drought stress, the disease progresses to kill the entire tree over one to several seasons.

Remove all affected branches by cutting well below the visible wilting and discolouration into wood that shows no sapwood staining, sterilising tools between cuts. There is no curative chemical treatment for verticillium wilt. Maintain tree vigour through consistent watering and mulching, as drought stress significantly worsens the disease outcome. Avoid wounding the roots with cultivation near the tree and avoid nitrogen-heavy feeding that stimulates susceptible soft growth. If the whole tree dies from verticillium wilt, avoid replacing it with another acer or other highly susceptible species (cotinus, catalpa, cercidiphyllum, strawberry tree) in the same position; the fungus persists in soil for many years. Replacing with a more resistant species such as Betula, Quercus, or Fraxinus is a more durable solution in infected soil.

Powdery mildew

Powdery mildew on acer produces a white or pale grey floury coating on the upper surfaces of the lobed leaves and causes the margins and sometimes the whole leaf to curl upward as the infection develops in late summer. Erysiphe species cause powdery mildew on various acer species, with field maple (Acer campestre) and Norway maple (Acer platanoides) particularly susceptible in UK conditions. The disease develops most actively on the young growth of shoot tips in warm conditions with dry soil and limited air circulation. Heavily shaded acer, densely planted acer hedges, and acer under drought stress are most susceptible to powdery mildew. The white mildew coating and upward leaf curl is sometimes alarming in appearance on the normally clean, lobed leaves of field maple, which is widely used as a hedging plant in UK gardens and which shows powdery mildew readily in clipped, densely planted hedges where air circulation is poor.

Improve air circulation through appropriate pruning or thinning of dense acer hedges to reduce the humid microclimate that favours mildew. Maintain soil moisture during dry periods, as drought stress amplifies susceptibility. Remove and destroy affected shoot tips where practical and apply a potassium bicarbonate fungicide at the first sign of white coating. In acer hedging, annual hard clipping in late summer removes the most mildew-susceptible new growth and stimulates a fresh flush of clean growth before winter that reduces the carryover of infection into the following season.

Tar spot

Tar spot (Rhytisma acerinum) is a fungal disease that produces large, distinctive, shiny black, raised tar-like blotches on the upper surfaces of sycamore and field maple leaves from midsummer, making it one of the most visually striking and unmistakable leaf diseases in UK gardens. The black blotches begin as small, yellowish-green spots in early summer that gradually enlarge and blacken as the fungus develops through summer, eventually reaching one to two centimetres across or more. The tissue around the black spots may yellow and the leaf may curl slightly around the affected areas as the disease progresses, though the leaf distortion is generally less dramatic than with aphid or mildew damage. Tar spot is very common on sycamore in UK gardens and parks, particularly in areas with cleaner air, as the disease was severely suppressed by sulphur dioxide air pollution in industrial areas in previous centuries and has become more widespread as air quality has improved. Despite its alarming appearance, tar spot causes negligible harm to the health or vigour of established acer trees, which tolerate even very heavy infections without significant impact on their growth or the following season's performance.

No treatment is necessary or recommended for tar spot on established acer trees. Collecting and destroying fallen sycamore leaves in autumn, rather than leaving them to decompose on the soil surface, reduces the overwintering spore reservoir and can reduce infection severity in subsequent seasons. This is the only practical management action and its effect is gradual rather than dramatic. Tar spot will not kill or significantly weaken the tree; it is a cosmetic condition that gardeners can tolerate without concern.

Gall mites

Several species of gall mite cause distinctive galls on acer leaves that can cause puckering, distortion, and apparent curling of the leaf blade. On field maple (Acer campestre), the mite Aceria campestricola causes numerous small, reddish, hairy pimple galls on the upper leaf surface, and the corresponding depressions on the underside filled with white or cream hairs. On sycamore, the mite Aceria pseudoplatani causes similar nail galls. The galls are the result of the mite feeding within the leaf tissue and stimulating the plant to produce abnormal growth around the feeding site. Where many galls are clustered closely together on a single leaf, the leaf blade can become noticeably buckled, puckered, and distorted as the gall tissue contracts the surrounding leaf surface. The effect is most pronounced on the young leaves of spring and early summer, where multiple closely spaced galls can cause significant apparent curling of the leaf.

Gall mites on acer cause no significant harm to the tree and no treatment is required or recommended. The galls are part of the natural ecology of acer in UK gardens and are tolerated without impact on the tree's health, vigour, or autumn colour. The mites cannot be effectively controlled by surface-applied sprays as they live within the leaf tissue inside the galls, and attempting to control them would be more harmful to the garden ecosystem than the mites themselves.

Frequently asked questions

Why are my acer leaves curling?

Sycamore aphid (Drepanosiphum platanoidis) is the most common cause of leaf distortion and honeydew problems on Acer pseudoplatanus (sycamore) and related large-leaved maples, colonising the leaf undersides and causing the lobed leaves to cup and curl around the feeding colonies. Verticillium wilt causes individual branches to wilt and die suddenly, with the leaves curling and browning on affected shoots. Powdery mildew causes white coating and upward leaf curl on many acer species in late summer.

What are the black spots on my sycamore leaves?

Black spots on sycamore (Acer pseudoplatanus) leaves are almost certainly tar spot (Rhytisma acerinum), a fungal disease that causes large, distinctive, shiny black raised blotches on the upper surface of the leaves. Despite its dramatic appearance, tar spot is a largely cosmetic problem that does not significantly affect the tree's health or vigour. The disease overwinters in fallen infected leaves; collecting and destroying fallen sycamore leaves in autumn reduces but does not eliminate the disease in subsequent seasons.

Does acer get verticillium wilt?

Yes, acer is among the woody plants most susceptible to verticillium wilt (Verticillium dahliae and V. albo-atrum). The soil-borne fungus infects the root system and spreads through the water-conducting vessels, causing individual branches or entire trees to wilt and die suddenly. The characteristic symptom is sudden wilting and death of one or more branches, with the leaves curling and browning on the affected shoots while remaining attached. Dark brown or olive discolouration in the sapwood of affected branches confirms the diagnosis.

Why are my field maple leaves curling?

Field maple (Acer campestre) leaves curling most commonly indicates powdery mildew, which causes a white floury coating on the lobed leaves and upward curling of the margins in late summer. Aphid colonies on field maple shoot tips in spring cause the youngest leaves to curl around the feeding insects. Gall mites (Aceria campestricola) cause small, reddish pimple galls on the upper surface of field maple leaves that can cause slight puckering and distortion of the leaf blade, though the plant's health is not affected.