Castanea Leaves Curling

Oriental chestnut gall wasp

Oriental chestnut gall wasp (Dryocosmus kuriphilus) is an invasive pest originating in China that has caused severe damage to chestnut trees across continental Europe since its accidental introduction there in the 1990s and has now established in the UK, representing the most significant new pest threat to Castanea sativa in Britain. The wasp is parthenogenetic, with females laying eggs in chestnut buds during summer without mating, and the eggs overwinter within the bud tissue before hatching in spring as the buds begin to expand. The hatching larvae trigger a host-plant gall response, causing the expanding bud and young shoot tissue to form distinctive, round or elongated galls instead of developing normally. The galls are typically two to three centimetres across, often initially red or pink-tinged before turning green as they mature, and develop at the bud position, along the leaf midrib, or on the petiole and young stem tissue. The new leaves emerging from galled buds are severely distorted and stunted, curling around the gall structures and failing to expand to their normal size and shape. In heavy infestations, which can occur rapidly as the wasp populations build over successive seasons, the majority of the new spring growth may be galled, dramatically reducing the leaf area available for photosynthesis and, in the case of Castanea sativa cultivars grown for nut production, severely reducing the catkin and nut production that depends on healthy new growth each spring.

Biological control using the parasitic wasp Torymus sinensis, which is the natural enemy of the oriental chestnut gall wasp in its native range, has been introduced and released in affected European countries with encouraging success in reducing gall wasp populations to manageable levels over several years. Torymus sinensis is being used as a biocontrol agent in the UK, and gardeners with affected trees should check with the relevant plant health authorities about the availability of this approach for private gardens. No chemical treatment is effective against the larvae within the galls, as they are completely protected by the gall tissue. Remove and destroy galled material where practical on young trees to reduce the population developing within the galls, though this is labour-intensive and only partially effective. Pruning out galled shoots in late spring before the adult wasps emerge from the galls in early summer reduces the following year's population. Monitoring the spread of oriental chestnut gall wasp and reporting new infestations to tree health authorities supports the national effort to manage this significant new pest.

Powdery mildew

Powdery mildew on castanea produces a white or pale grey floury coating on the upper surfaces of the long, lanceolate, heavily toothed leaves and causes the leaf margins and tips to curl upward as the infection develops in warm, dry late summer conditions. The primary species responsible for powdery mildew on sweet chestnut in the UK is Erysiphe alphitoides, the same species that causes oak powdery mildew and that infects castanea as a related member of the Fagaceae family. The disease develops most actively on the vigorous new growth produced after coppicing, a traditional management practice with sweet chestnut that is still widely used for producing poles and fencing material, where the very large and soft leaves produced on the long, rapidly growing coppice shoots are particularly susceptible to mildew infection in warm, dry conditions. On young castanea seedlings and saplings, which have a high proportion of soft, mildew-susceptible growth relative to the total canopy, powdery mildew can affect a large proportion of the leaves by late summer. The long, prominently toothed leaves of sweet chestnut display the white mildew coating clearly on their upper surface, and the upward curl of the large, distinctive leaf margin and leaf tip is readily visible.

Apply a potassium bicarbonate or sulphur fungicide at the first sign of white coating on the leaves of young castanea or coppiced sweet chestnut where mildew has been severe in previous seasons. Maintain adequate soil moisture through dry periods to prevent the drought stress that amplifies mildew susceptibility. For coppiced sweet chestnut, the timing of the coppice cut to late winter or early spring rather than later in the season reduces the coincidence of the flush of soft new growth with the peak mildew season. Ensuring adequate air circulation between coppice stools reduces the microclimate that favours mildew development in densely grown coppice areas. Established large castanea trees managed as standards rather than coppice are less severely affected by powdery mildew, as the proportion of soft, susceptible new growth in the overall canopy is smaller.

Chestnut blight

Chestnut blight (Cryphonectria parasitica) is a devastating fungal disease that has killed virtually the entire American chestnut (Castanea dentata) population in North America, reducing a once-dominant forest tree to a functionally extinct species, and has caused significant mortality of Castanea sativa in affected parts of continental Europe. The disease enters through wounds, insect damage, and bark crevices, producing cankers that spread through the bark and cambium, girdling branches and stems to cause the characteristic dieback of all growth above the canker. The cankers are initially visible as sunken, discoloured areas of bark, often with orange or yellow fruiting bodies of the fungus visible on the surface, and the leaves on branches above a girdling canker wilt, curl, and die as water transport above the canker is cut off. Chestnut blight has been found at a small number of sites in the UK but has not yet established as a widespread problem in British sweet chestnut populations, and Castanea sativa shows considerably more tolerance to the disease than the American species, often producing epicormic growth below cankers that allows the tree to survive.

Report any suspected chestnut blight, identified by orange-yellow canker lesions with fruiting bodies on sweet chestnut bark and associated branch dieback, to the Forestry Commission or the relevant devolved authority plant health team. Early detection and reporting of new sites is critical to limiting the spread of this potentially devastating disease. Remove and destroy infected branches by cutting into healthy wood well below the visible canker margin, sterilising tools between cuts. There is no effective chemical treatment for established chestnut blight. Research into biological control of chestnut blight using hypovirulent strains of the fungus that reduce the virulence of the pathogen through viral infection is ongoing; this approach has shown promise in continental Europe and is under active investigation for use in UK conditions. The relative tolerance of Castanea sativa to the disease compared to the American chestnut provides some grounds for cautious optimism about the long-term prospects for sweet chestnut in the UK landscape.

Aphids

Aphids colonise the new shoot-tip growth of castanea in spring, establishing colonies on the undersides of the expanding young leaves and on the soft new stems, causing the youngest growth to curl slightly around the feeding insects. Several aphid species feed on sweet chestnut in the UK, with pale green or yellowish species most commonly found on the shoot tips in spring as the buds break and the new growth expands. The large, prominently toothed leaves of castanea display the slight inward curling of aphid-affected shoot tips less dramatically than the smaller leaves of some other garden trees, and in most garden situations aphid pressure on established sweet chestnut trees is a minor and short-lived spring feature. Honeydew production from aphid colonies on sweet chestnut contributes to the sticky coating on surfaces beneath the canopy in early summer, though castanea aphid colonies are generally less productive of honeydew than the notorious lime aphid on tilia.

Natural predators including ladybirds, hoverfly larvae, lacewing larvae, and parasitic wasps establish on castanea aphid colonies rapidly once they build up in spring and reduce populations without intervention in most garden situations. A strong jet of water directed at the affected shoot tips dislodges colonies before the leaves curl around them. Fatty acid spray provides contact chemical control where necessary on young trees. The vigorous growth of castanea, particularly on coppiced or managed trees that produce rapid new growth, means that even significant spring aphid pressure is quickly outgrown and the mature summer canopy shows little evidence of the spring distortion. Where aphid pressure coincides with oriental chestnut gall wasp damage in affected areas, the combined effect on the new growth may be more significant in the short term, but the tree's natural vigour allows recovery through summer.

Drought

Drought stress on castanea causes the long, prominently toothed leaves to curl inward along their central midrib and lose the deep, lustrous green of healthy summer growth, developing a pale, slightly greyish-green tone before the margins brown and the leaves drop prematurely. Castanea sativa is native to the Mediterranean region and southern Europe and, once established in UK conditions, is considerably more drought-tolerant than many other large ornamental trees, performing well in the dry, free-draining, acid soils of southern England and Wales where it naturalises most successfully. The shallow, fibrous root system of sweet chestnut is effective at extracting moisture from the upper soil layers once established, and the tree's natural association with dry, well-drained heathland and woodland soils in its UK native and naturalised range reflects its adaptation to summer drought. Newly planted castanea and young trees in their first two to three growing seasons are considerably more vulnerable to drought stress than established specimens, as the limited root system cannot access the deeper soil moisture reserves available to large, mature trees.

Water newly planted sweet chestnut during dry periods through the first two to three growing seasons. Established castanea in free-draining or average soil requires no supplementary irrigation in typical UK summers and manages prolonged dry periods better than most other large ornamental trees in the UK flora. Apply a mulch over the root zone of newly planted trees to retain moisture during establishment. The drought tolerance of established sweet chestnut is one of its most valuable attributes for planting in dry, difficult sites where other large trees struggle, and the combination of drought tolerance, coppice regeneration, and excellent autumn colour and wildlife value makes it one of the most versatile and ecologically valuable trees available for planting in appropriate UK garden and landscape conditions.

Frequently asked questions

Why are my sweet chestnut leaves curling?

Oriental chestnut gall wasp (Dryocosmus kuriphilus) is an invasive pest now established in the UK that causes distinctive round or irregular galls to develop on the buds, shoot tips, and leaf midribs of castanea, causing the affected leaves to curl and distort around the developing galls and severely stunting the new growth. Powdery mildew (Erysiphe alphitoides and related species) produces a white coating on the long, toothed leaves and causes the leaf margins to curl upward in late summer. Drought stress causes the lance-shaped, heavily toothed leaves to curl inward and lose their deep green colour in prolonged dry conditions.

What is the oriental chestnut gall wasp?

Oriental chestnut gall wasp (Dryocosmus kuriphilus) is an invasive pest originating in China that has spread through Europe and reached the UK, where it is now established and spreading. The wasp is parthenogenetic, meaning females lay eggs without mating, and eggs laid in chestnut buds in summer hatch the following spring as the buds expand, triggering the formation of distinctive, often reddish or green, round galls on the buds, leaf midribs, and shoot tips. The galled tissues are stunted and distorted, and the leaves emerging from affected buds are curled, distorted, and do not develop normally. In heavy infestations, much of the new spring growth is galled, significantly reducing the tree's photosynthetic capacity and productivity.

Does castanea get powdery mildew?

Yes, sweet chestnut is susceptible to powdery mildew, most commonly caused by Erysiphe alphitoides (the same species that causes oak powdery mildew, which also infects castanea as a member of the Fagaceae). The white floury coating develops on the upper surfaces of the long, lanceolate, toothed leaves and causes the leaf margins to curl upward in late summer. Powdery mildew on castanea is most severe on young trees and on the new growth produced by coppiced or pollarded sweet chestnut, where the soft young leaves produced after cutting are most susceptible to infection in warm, dry conditions with limited air circulation.

What is chestnut blight and is it in the UK?

Chestnut blight (Cryphonectria parasitica) is a devastating fungal disease that has killed virtually all American chestnut trees in North America and has caused significant damage to Castanea sativa in continental Europe. The disease enters through wounds and produces cankers that girdle and kill branches and eventually the trunk above the canker. Chestnut blight has been found at isolated sites in the UK but has not yet spread to become a widespread problem in British conditions, and Castanea sativa is considerably more tolerant of the disease than the American chestnut. Monitoring for the disease and reporting suspected cases to tree health authorities is important given the catastrophic impact it has had elsewhere.