Alnus Leaves Curling

Phytophthora alder disease

Phytophthora alder disease (Phytophthora x alni) is a hybrid Phytophthora water mould that has caused the death of large numbers of native common alder (Alnus glutinosa) along UK rivers, streams, and wetlands since its identification in Britain in the 1990s, representing one of the most serious tree health emergencies in the UK after Dutch elm disease. The pathogen is particularly aggressive in waterside conditions where the constantly moist or flooded soil creates ideal conditions for the spread of Phytophthora zoospores through water movement, and affected trees are typically those growing with their roots or lower trunks at or below the waterline. The disease destroys the feeder roots and attacks the cambium and inner bark of the trunk, particularly at and below the waterline, producing the characteristic bleeding canker lesions that are the most visible diagnostic symptom: rust-coloured, dark, or pinkish fluid exudes from cracks in the bark of the lower trunk, staining the bark surface below the wound. As the root system and lower trunk are progressively destroyed, the tree's capacity to take up water is diminished, and the leaves become small, pale, and sparse before wilting, curling, and dropping prematurely. Infected trees also characteristically produce an abnormally large number of catkins and seeds in the season or two before death, as the stressed tree makes a final reproductive effort.

There is no effective chemical treatment for Phytophthora alder disease in established infected trees. The most important management action is early identification of infected trees to prevent spread, as the pathogen travels rapidly downstream in watercourses. Report suspected Phytophthora alder disease to the Forestry Commission or to the relevant devolved authority tree health team, as the disease is a notifiable pest in the UK and monitoring data are important for tracking its spread. Infected trees that are structurally unsafe should be removed to prevent risk to people and to reduce the disease burden in the watercourse. Equipment used for any work on or near infected trees should be thoroughly cleaned and disinfected before being moved to other sites to prevent mechanical spread of the pathogen. Where new alder planting is planned along watercourses in areas where Phytophthora x alni is known to be present, using Alnus incana (grey alder) or Alnus cordata (Italian alder), which show somewhat higher tolerance to the disease than common alder, reduces the risk of rapid loss.

Alder leaf beetle

Alder leaf beetle (Agelastica alni) is a shiny, dark blue-violet or almost black beetle that is a common and often abundant pest of alnus throughout the UK, particularly on trees in riverside and wetland positions that provide the moist conditions in which the beetle thrives. Both the adult beetles and their larvae feed on the leaves of alnus, but cause quite different damage: the adults chew irregular, rounded holes through the leaf blade, while the larvae feed on the underside of the leaf, removing the soft green tissue while initially leaving the upper epidermis intact to produce a translucent, window-pane effect that dries, collapses, and tears as the season progresses. The larvae feed gregariously in early stages, producing patches of skeletonised leaf tissue that are clearly visible as pale or brown areas on the upper surface, and the remaining leaf tissue curls and dries around the dead, skeletonised zones. In heavy infestations, which can be dramatic on riverside alders in summer, the majority of the outer leaf canopy may be affected, giving the tree a scorched or bleached appearance by late August. The adults overwinter in the leaf litter at the base of the trees and emerge in spring to begin feeding on the new growth before laying eggs in clusters on the leaf undersides.

Established alder trees tolerate even significant alder leaf beetle damage without lasting impact on long-term health, and no treatment is warranted or practical on large established riverside trees. The beetle has natural predators and parasites in UK conditions, and populations fluctuate naturally between years. On young or newly planted alder where the leaf beetle is causing stress during the critical establishment period, pyrethrin-based insecticide sprays applied to the undersides of the leaves when the first-instar larvae are seen provide contact control; the larvae are most vulnerable in their early gregarious feeding stage before they disperse across the canopy. Removing and destroying fallen leaves in autumn reduces the overwintering adult population to some degree, though adults also shelter in soil and litter beyond the immediate leaf fall zone. The beetle is a native insect associated with alder as part of the characteristic waterside invertebrate community, and its presence on established trees should be viewed as a normal feature of alnus ecology rather than a problem requiring intervention.

Aphids

Several aphid species colonise alnus in UK gardens and riverside plantings, with the alder aphid (Pterocallis alni) and related species forming colonies on the undersides of the leaves and the shoot tips in spring and early summer, causing the youngest growth to curl slightly around the feeding insects. Alder aphids are typically pale yellowish-green and produce honeydew that drips onto the surfaces below, supporting sooty mould development on the leaves below the colony. In riverside and wetland positions, the honeydew falling onto the water surface below alder trees is visible as a sheen on calm water and is an important food source for invertebrates and fish. Alder aphid colonies are typically most abundant in May and June before the leaf hardens, and the infestation declines naturally through summer as the leaves mature and natural predators including ladybirds and hoverfly larvae exploit the colonies. The impact of aphid feeding on established alders is generally minor, and the rolling and curling of individual shoot tips around aphid colonies resolves as the colony declines and the leaves grow out of their distorted posture.

No treatment is warranted for aphids on established alnus in most garden and riverside situations. Natural predators provide effective biological control in the two to four weeks following the peak of the aphid infestation. A strong jet of water directed at the affected shoot tips dislodges colonies before the leaves have rolled tightly around them. The ecologically important role of alder in waterside habitats means that pesticide use on riverside alders is particularly undesirable, as chemical runoff into watercourses affects aquatic invertebrates. Maintaining alnus in vigorous health through appropriate site selection and adequate soil moisture produces strong growth that is less severely affected by aphid colonisation than stressed or poorly established plants.

Powdery mildew

Powdery mildew on alnus produces a white or pale grey floury coating on the upper surfaces of the broadly oval leaves and causes the leaf margins to curl upward as the infection develops in warm, dry late summer conditions. The disease typically affects alnus in conditions that are unusually dry for the species, as the natural waterside habitat of alder provides the consistently moist air conditions that suppress powdery mildew development, and alder in garden settings away from water or in dry years shows more mildew than trees in their natural waterside habitat. Erysiphe species cause powdery mildew on alnus, developing most actively on the new growth at the shoot tips in late summer. On established riverside alders, powdery mildew is relatively uncommon as the high humidity of waterside environments suppresses mildew development. On ornamental alders including Alnus incana Aurea (golden-leaved grey alder) planted in garden borders away from water, mildew can be more prevalent in dry late summers.

Ensure adequate soil moisture for alder planted in garden positions away from their natural waterside habitat, as the drought stress that amplifies mildew susceptibility is the primary predisposing factor. Apply a potassium bicarbonate or sulphur fungicide to ornamental alders where mildew has been severe in previous seasons. Planting alder in positions with adequate natural soil moisture or with access to supplementary irrigation reduces both drought stress and mildew. The most significant management action for powdery mildew on alder is ensuring the tree is in an appropriately moist site; alder in correctly chosen, adequately moist conditions rarely develops significant mildew, and persistent mildew on an established alder is often an indicator that the site is too dry for the species to perform at its best.

Drought

Drought stress on alnus is most likely to occur on ornamental specimens planted in garden positions away from their natural waterside habitat or during exceptional dry summers that deplete even the normally moist soils associated with riverside and wetland plantings. Alnus glutinosa is natively a riparian and wetland tree adapted to consistently moist, sometimes flooded conditions, and its root system is less effective at accessing soil moisture reserves in dry conditions than trees native to better-drained habitats. In a prolonged summer drought, alder growing in garden borders or in positions away from a permanent water supply responds with leaf curl, yellowing, and premature drop that can begin in July or August, several weeks before normal leaf fall. The leaves curl inward along their midrib and develop a pale, dull green colour before turning yellow or brown and dropping, and the overall canopy thins more rapidly than on well-sited trees. Alnus incana and Alnus cordata are somewhat more tolerant of dry conditions than common alder and are better choices for garden sites where consistent soil moisture is not guaranteed.

Site alnus in positions where consistent soil moisture is available, ideally at or near a water feature, pond, or in the moist margins of a garden where the water table remains accessible to the roots through summer. Water newly planted alder consistently through dry periods in the first two to three growing seasons until the root system is established. In garden positions away from permanent water, apply a mulch over the root zone and water deeply during dry periods. Alnus cordata (Italian alder) is the recommended choice for garden planting in well-drained positions where consistent moisture is not available, as it is considerably more drought-tolerant than common alder while providing similar ornamental and ecological value. The spectacular yellow autumn foliage and persistent decorative catkins of ornamental alders are best displayed on trees that have maintained strong summer growth through adequate moisture.

Frequently asked questions

Why are my alder leaves curling?

Phytophthora alder disease (Phytophthora x alni) is the most serious cause of alder decline in the UK, affecting the root system and lower trunk to produce a general wilting and curling of the leaves as water uptake is disrupted, typically accompanied by bleeding canker lesions on the bark near the waterline. Alder leaf beetle (Agelastica alni) feeds on the leaves and causes significant skeletonising damage that leaves the leaf veins intact but removes the green tissue, causing the remaining leaf to curl and dry around the empty vein framework. Aphids colonising the shoot tips in spring cause the youngest leaves to curl around the feeding colonies.

What is Phytophthora alder disease?

Phytophthora alder disease (Phytophthora x alni) is a devastating hybrid Phytophthora water mould that has killed large numbers of native alder trees along UK rivers and wetlands since its identification in the 1990s. The pathogen attacks the roots and the trunk of alnus, typically entering at or below the waterline and causing bleeding canker lesions on the bark that exude a rust-coloured or dark fluid. The destruction of the root system and lower trunk prevents normal water uptake, causing the leaves to wilt, curl, and discolour as the tree declines progressively. Infected trees typically show sparse, small, yellowing leaves and abnormal cone production before eventual death.

What is the alder leaf beetle?

Alder leaf beetle (Agelastica alni) is a shiny, dark blue or violet-black beetle that feeds extensively on the leaves of alnus and related species throughout the UK. Both the adults and the larvae feed on the leaf surface, with adults making small, rounded holes through the leaf and larvae skeletonising the leaf underside by removing the green tissue while leaving the upper epidermis intact. The damaged leaves dry, turn brown, and curl around the remaining vein skeleton. In heavy infestations, which are particularly common on alder in riverside and wetland positions, the majority of leaves on the outer canopy may be affected by late summer. The damage is cosmetic and does not significantly harm established trees.

Does alnus grow in waterlogged soil?

Yes, alnus glutinosa (common alder) is one of very few large native UK trees that tolerates and indeed prefers waterlogged, poorly drained, and periodically flooded soils, making it invaluable for planting along riverbanks, in wetland gardens, and in low-lying areas where drainage is poor. Alnus has root nodules containing nitrogen-fixing bacteria that allow it to thrive in the nutrient-poor conditions of waterlogged soils. It is less well suited to dry, free-draining soils where it performs poorly compared to its natural waterside habitat, and in very dry conditions established alder shows drought stress symptoms including leaf curl and premature drop.