Crataegus Leaves Curling

Hawthorn aphid

Hawthorn aphid (Dysaphis crataegi) produces one of the most characteristic and dramatic leaf-curling symptoms of any aphid on UK garden and hedging plants, causing the lobed, deeply cut leaves of crataegus to fold and curl tightly around dense colonies of dark green, powdery-coated aphids in spring. The aphid overwinters as eggs on hawthorn bark and hatches in spring to coincide precisely with the emergence of the new foliage. The young, soft leaves curl tightly along their central vein and roll along their margins, enclosing the aphid colony in a compact, protective tube of leaf that prevents rain from reaching the insects and deters some natural predators. The distorted, rolled leaves are clearly visible on hawthorn hedging and ornamental crataegus specimens from April onwards, producing a distinctive crinkled, curled appearance on the affected shoots that is very different from the normal, deeply lobed hawthorn leaf. In heavy infestations, a large proportion of the new spring growth can be affected, particularly on the outer shoots of hawthorn hedging where the young leaves are most exposed. Honeydew produced within the rolled leaves drips onto the surfaces below and accumulates on the inner leaves of the hedge, supporting sooty mould development. The hawthorn aphid migrates to plantain (Plantago species) as an alternate host in early to midsummer, so the infestation on crataegus is concentrated in the spring period and largely resolves by June or July as the winged forms fly off.

Natural predators provide the most effective control of hawthorn aphid in most garden and hedging situations. The spring emergence of hawthorn aphid colonies is an important food source for blue tit and great tit nestlings, and the accessible position of hawthorn hedging shoot tips makes predator access easy for birds and beneficial insects. The infestation on the tree is typically short-lived, resolving naturally by early to midsummer as the aphids migrate. Annual clipping of hawthorn hedging in late summer removes the most heavily infested shoot tips and incidentally disrupts the overwintering egg population on the clipped material. For ornamental crataegus specimens where the spring leaf curl is aesthetically unacceptable, a strong jet of water directed at the curled shoot tips before the roll is complete provides effective physical control; once the leaves are tightly rolled, spray penetration is severely limited.

Powdery mildew

Powdery mildew (Podosphaera clandestina) is a common and persistent disease on crataegus in UK gardens, producing a white or pale grey floury coating on the upper surfaces of the lobed leaves and causing the leaf margins to curl upward as the infection develops in late summer. The disease is particularly prevalent on the new, soft growth produced by hawthorn after clipping in summer, which is the most susceptible growth of the season. Hawthorn hedging that is clipped repeatedly through summer produces successive flushes of soft growth each of which is vulnerable to mildew until it hardens, and the result in warm, dry late summers can be a persistent mildew problem across the outer clipped surface. On ornamental crataegus, including the popular Crataegus laevigata Paul's Scarlet with its double pink-red flowers and Crataegus laevigata Rosea Flore Pleno, powdery mildew is one of the most common problems and can produce a significantly whitened appearance on the new growth that detracts from the ornamental value of these popular garden trees.

Time hawthorn hedge clipping to avoid the hottest, driest periods when the new growth produced after clipping is most susceptible to mildew. Maintain adequate soil moisture through dry periods to prevent the drought stress that amplifies susceptibility. Remove affected growth where practical and apply a potassium bicarbonate fungicide at the first sign of white coating. For formal hawthorn hedging, a late August or September clip produces new growth that has less time to develop heavy mildew before the cooler autumn weather reduces infection pressure.

Hawthorn rust

Hawthorn rust (Gymnosporangium clavariiforme) causes orange-yellow spots on the upper surfaces of crataegus leaves from early summer, with the corresponding undersides of affected leaves developing the characteristic orange, horn-like or tubular spore-producing structures (aecia) from midsummer onwards. The tissue around each rust spot puckers and distorts slightly as the developing fungal structures displace the leaf cells, and in heavy infections the distortion of the leaf blade around multiple rust pustules can cause significant overall leaf curling and premature drop. Hawthorn rust has the same type of two-host life cycle as pear rust and rowan rust, alternating between crataegus and juniper species; the rust overwinters as orange gelatinous galls on juniper stems and releases spores in spring that infect hawthorn, while the summer spores from the hawthorn infection fly back to reinfect junipers. The disease is most severe in gardens or localities where ornamental junipers are planted in close proximity to hawthorn hedging or ornamental crataegus.

Removing ornamental junipers from the vicinity of hawthorn significantly reduces rust pressure by breaking the alternate host cycle, though spores from distant junipers can still cause infection. Collect and destroy infected hawthorn leaves in autumn. No fungicide provides complete protection against rust on hawthorn, but copper-based sprays applied from bud burst in spring reduce infection severity on ornamental specimens where rust has been a persistent problem. On hawthorn hedging in rural areas, rust is a normal and expected occasional occurrence that the plants tolerate without significant harm.

Fireblight

Fireblight (Erwinia amylovora) affects crataegus as a member of the Rosaceae family, causing the flowers, leaves, and shoot tips to blacken and wilt in the characteristic shepherd's crook formation during and after the flowering period in spring. Hawthorn is a host of fireblight but is generally less severely affected than pear or apple in garden conditions, and established hawthorn hedging that has been naturalised in the garden for many years often shows a degree of tolerance to the disease. The spring flowering of crataegus in May provides the entry point for fireblight infection, and the warm, wet weather conditions around flowering time that are most favourable to infection coincide with one of the most common UK weather patterns at that time of year. Individual shoots and small branches die back from the tip in the characteristic fashion, and the dead, blackened shoots with their wilted, attached leaves are conspicuous on otherwise healthy hedging.

Remove all fireblight-infected shoots promptly by cutting into healthy wood well below the visible symptoms, sterilising tools between cuts. Do not compost infected material. Established hawthorn hedging often compartmentalises fireblight infections effectively without further spread if infected material is removed promptly, and the vigorous growth habit of crataegus means that the removed shoots are rapidly replaced. Preventive copper sprays at bud burst and flowering reduce infection risk in gardens where fireblight has caused significant problems in previous seasons.

Drought

Drought stress causes crataegus leaves to curl inward, lose their deep green colour, and develop brown, scorched margins before dropping prematurely in prolonged dry conditions. Hawthorn is considerably more drought-tolerant than most ornamental trees and garden shrubs, and its native distribution across the chalky downs and dry, exposed positions of the UK countryside reflects its adaptation to conditions that would stress less hardy plants. Established hawthorn hedging in average garden soils manages UK summer drought without supplementary watering in most years. The situations where drought stress is most likely on crataegus are in newly planted specimens in their first two seasons before root establishment, in very free-draining sandy or shallow chalk soils, and in formal ornamental crataegus in garden positions where competition with surrounding plants depletes soil moisture rapidly.

Water newly planted crataegus during prolonged dry spells in its first two growing seasons. Once established, hawthorn requires no supplementary watering in typical UK gardens. Apply mulch at the base of ornamental crataegus specimens to retain soil moisture and reduce temperature extremes in the root zone. The exceptional drought tolerance of established crataegus is one of the reasons it has been the traditional British hedging plant for exposed, difficult positions where other species would struggle.

Frequently asked questions

Why are my hawthorn leaves curling?

Hawthorn aphid (Dysaphis crataegi) is the most common cause of severe leaf rolling on crataegus, causing the leaves to fold and curl tightly around dense colonies of dark green aphids in spring. Powdery mildew (Podosphaera clandestina) produces a white floury coating on the lobed leaves and causes the margins to curl upward in late summer. Hawthorn rust (Gymnosporangium clavariiforme) causes orange spots on the leaf surface and distortion of the affected tissue.

What is hawthorn aphid?

Hawthorn aphid (Dysaphis crataegi) is a dark green or olive-green aphid that colonises the shoot tips and leaves of crataegus from overwintered eggs in spring, causing very tight, characteristic leaf rolling that is one of the most distinctive aphid symptoms on any UK garden plant. The leaves fold along their central vein and roll along their margins to enclose the aphid colony completely. The migration of winged forms to plantain as an alternate host in early summer means the infestation on hawthorn is largely confined to spring.

Does hawthorn get rust?

Yes, hawthorn rust (Gymnosporangium clavariiforme) affects crataegus, causing orange-yellow spots on the upper leaf surface and swollen, tube-like spore structures on the undersides in summer. The rust has a two-host life cycle requiring both hawthorn and juniper to complete its cycle. Removing junipers from the vicinity of hawthorn reduces but does not eliminate rust pressure. The disease causes premature leaf drop in severe cases but does not significantly threaten the long-term health of established hawthorn trees or hedges.

Is crataegus a good hedging plant?

Yes, Crataegus monogyna (common hawthorn) is one of the best native hedging plants for UK gardens and the traditional choice for boundary hedges in the British countryside. It is extremely hardy, fast-growing, tolerant of almost any soil and position, and produces a dense, impenetrable barrier that provides outstanding wildlife value through its spring flowers for insects, summer leaves for caterpillars, and autumn berries for birds. The thorny stems make it one of the most effective stock-proof and security hedges available.