Juglans Leaves Curling

Bacterial blight

Walnut bacterial blight (Xanthomonas arboricola pv. juglandis) is the most common and damaging disease of Juglans regia in UK gardens, causing dark brown to black, water-soaked spots on the leaflets, leaf stalks, young shoots, and developing nuts that are most severe in wet spring conditions when prolonged rain enables rapid bacterial spread through the canopy. The disease is present in virtually all walnut-growing regions of the UK and affects most walnut trees to some degree in years with wet springs, particularly during the period from leaf emergence through to midsummer when the plant tissue is most susceptible and rainfall is most likely to provide the moisture films on which the bacteria move and infect. The spots begin as small, water-soaked, dark green or pale marks on the leaflet surface that enlarge rapidly and turn dark brown to black as the infected tissue dies, with the dead zone often surrounded by a pale yellow halo in early stages. The dead tissue in each spot contracts as it dries, pulling the surrounding live leaflet tissue into a curl or pucker around the edge of the lesion, and in heavy infections the entire leaflet may be distorted by the combined pulling effect of multiple lesions. On the developing nuts, bacterial blight causes black, sunken lesions on the husk that penetrate to the kernel and destroy the nut completely, making nut production negligible in a severe blight year.

Apply copper-based fungicide (bordeaux mixture or copper oxychloride) at bud burst as the buds begin to open in spring, before the expanding tissue is exposed, and repeat at ten to fourteen day intervals during wet weather through leaf expansion and the flowering period. This preventive programme, timed to maintain a protective copper deposit on the expanding tissue before infection can occur, is the most effective management approach for walnut bacterial blight. Avoid wetting the foliage unnecessarily, and if possible do not irrigate overhead during wet spring periods. Collect and destroy fallen infected leaves in autumn to reduce the bacterial reservoir from which spring infections originate. Pruning to open up the canopy and improve air circulation reduces the prolonged leaf wetness that enables the bacteria to move through the canopy. Some walnut cultivars, particularly those selected for commercial production in the UK, show lower susceptibility to bacterial blight than the common unimproved walnut, and choosing these cultivars for new plantings reduces the management burden.

Walnut leaf blotch

Walnut leaf blotch (Ophiognomonia leptostyla, formerly Gnomonia leptostyla, with the anamorph Marssonina juglandis) is a fungal disease that causes brown blotches with a diffuse yellow halo on the leaflets of Juglans regia from late spring through summer, often occurring simultaneously with bacterial blight and contributing to an overall picture of severe leaf distortion and discolouration that makes distinguishing the two diseases difficult without close examination. The fungal spots are typically slightly different from bacterial blight spots in producing a more defined, rounder blotch with a more pronounced yellow margin before the centre darkens, but in wet years when both diseases are active simultaneously, the combined damage can make individual lesion distinction impractical. In wet seasons, the blotches enlarge and coalesce to cover large areas of the leaflet surface, and the accumulated dead tissue causes significant overall leaf distortion as the large necrotic areas contract and curl. Premature leaf drop in late summer is a consistent feature of severe walnut leaf blotch infections, and in successive wet years the repeated defoliation can reduce the vigour of the tree and reduce nut production by limiting the photosynthetic capacity of the canopy before the nuts are fully developed.

Collect and destroy fallen walnut leaves in autumn, as the leaf blotch fungus overwinters in infected leaf debris and produces spores from fallen leaves in spring that initiate the following year's infection. This is the single most effective cultural management action for walnut leaf blotch and also reduces the bacterial blight reservoir simultaneously. Apply copper-based sprays from bud burst through leaf expansion in spring on the same schedule used for bacterial blight management, as copper provides some protective activity against both the bacterial and fungal pathogens affecting walnut leaves. Improving air circulation by appropriate pruning reduces the prolonged leaf wetness that enables fungal spore germination and leaf penetration. Where walnut leaf blotch has been severe in previous years, the preventive copper programme in spring, combined with autumn leaf collection, provides the most comprehensive reduction in annual disease severity.

Walnut aphid

Walnut aphid (Chromaphis juglandicola) forms colonies on the undersides of walnut leaflets in spring and summer, typically along the main veins of the leaflets where the insects feed on the phloem sap and cause the young growth to curl slightly around the feeding insects. The aphids are pale yellow or yellowish-green and relatively small, and colonies build up from overwintered eggs on the bark and young wood. The honeydew produced by walnut aphid colonies drips onto the surfaces below, creating sticky deposits on the lower leaves, on garden furniture, and on any surfaces beneath the tree's canopy, and sooty mould developing on the honeydew creates a black coating on the leaf surfaces. A separate species, the dusky-veined walnut aphid (Callipterinella calliptera), is a dark-coloured aphid that feeds on walnut and causes characteristic dark discolouration of the main leaf veins that is visible on the upper surface of the leaflet. In most garden situations, walnut aphid pressure is moderate and short-lived, peaking in late spring before declining as the leaves harden and natural predators establish.

Natural predators including ladybirds, parasitic wasps, and hoverfly larvae provide effective biological control of walnut aphid in most garden situations. The colonies typically peak in late spring and early summer before declining naturally without intervention. A jet of water directed at the undersides of the leaflets dislodges colonies effectively. Fatty acid spray provides contact chemical control where aphid colonies are causing visible distortion on young or newly planted trees. The combination of aphid pressure in spring and simultaneous bacterial blight and leaf blotch infections can make walnut appear badly affected by late spring, but the tree's vigorous growth means that new, undistorted leaflets are produced through summer as the older, distorted growth is succeeded by stronger regrowth.

Walnut gall mite

Walnut gall mite (Aceria erinea) is a microscopic eriophyid mite that causes distinctive felt-like or blister-like patches on the upper surface of walnut leaflets, with the corresponding underside developing a pale, woolly or felted mass of distorted leaf hairs (erineum) within which the mite colony lives and feeds. The upper surface of the leaflet above each mite colony becomes raised and thickened into a pale or yellowish-green blister or bump that is clearly visible and gives the leaf an unusual, warty texture. As the mites feed and the gall develops, the surrounding leaf tissue distorts and puckers around the thickened gall areas, causing the leaflet to curl and contort as the irregular thickening disrupts the normal flat surface. Multiple galls on a single leaflet, which is common in years when mite populations are high, can cause significant overall leaflet distortion and give the tree a rough, unusual appearance. Walnut gall mite is extremely common on Juglans regia throughout the UK and the characteristic erineum patches are one of the most consistent features of walnut foliage in British gardens.

Walnut gall mite causes no significant harm to the health of established walnut trees and requires no treatment in the vast majority of garden situations. The galls are a permanent feature of the affected leaves for the season but cause no lasting damage to the tree's vigour, structure, or nut production. Natural predatory mites and insectivorous birds feed on the gall mite colonies and provide some population regulation. No practical chemical treatment is available for gall mites once they are established within the erineum, as the felted covering protects the colony from contact sprays. Dormant oil sprays applied to the bark and buds in late winter can reduce overwintering mite populations before they migrate to the developing leaves in spring, but this treatment is rarely warranted or practical on established large walnut trees. Accepting walnut gall mite as a normal and harmless feature of Juglans regia in UK gardens is the appropriate response for the vast majority of garden owners.

Drought

Drought stress on juglans causes the pinnate leaves to droop and the individual leaflets to curl slightly inward as the plant reduces its transpiration surface in response to soil moisture depletion. Walnut is a substantial tree with high summer water demands from its large canopy, and while established trees in deep, fertile soils manage typical UK summer dry periods satisfactorily, trees in shallow, free-draining soils or in positions with significant competition from adjacent plants may show drought symptoms in prolonged dry spells. The leaflets develop a pale, slightly greyish-green colour before curling inward and the petioles lose their normal firm posture and droop, giving the affected branches a wilted appearance. In severe drought, the leaflets may brown at the margins and drop prematurely, reducing the canopy before the normal leaf fall period. Newly planted walnut in its first two to three seasons is most vulnerable, as the limited root system is confined to the original planting area and cannot access the deep soil moisture reserves available to established trees with wide-spreading root systems.

Water newly planted walnut consistently during dry periods through the first two to three growing seasons. Established walnut in deep, fertile soil requires no supplementary irrigation in typical UK summer conditions. Apply a mulch over the root zone to retain soil moisture and moderate temperature fluctuations. Walnut performs best in deep, fertile, well-drained soil in a sheltered, sunny position with adequate moisture availability; planting in shallow, compacted, or consistently dry soil produces a less vigorous tree with poorer nut production, and the investment in good soil preparation at planting pays dividends in the performance and resilience of the tree through subsequent dry periods. The deep, extensive root system of an established walnut, which is one of the features that makes the tree difficult to move once established, is also the source of its drought resilience in appropriate growing conditions.

Frequently asked questions

Why are my walnut leaves curling?

Walnut bacterial blight (Xanthomonas arboricola pv. juglandis) causes dark brown to black spots on walnut leaflets that cause surrounding tissue to distort and the leaflet to curl around the dead zones, and is the most common disease of Juglans regia in UK gardens in wet springs. Walnut leaf blotch (Ophiognomonia leptostyla) produces brown blotches with a yellow halo on the leaflets that coalesce in wet seasons and cause widespread discolouration, distortion, and premature drop. Walnut aphid (Chromaphis juglandicola) colonises the shoot tips and leaf undersides in spring and early summer, causing the youngest leaflets to curl around the feeding colonies.

What causes black spots on walnut leaves?

Black or dark brown spots on walnut leaves and leaflets are almost certainly caused by walnut bacterial blight (Xanthomonas arboricola pv. juglandis), the most common disease of walnut in UK gardens. The bacterium enters through stomata and wounds, particularly during rain or in humid conditions, producing small, water-soaked spots that enlarge and turn dark brown to black as the infected tissue dies. On the nuts, the same bacterium causes black, sunken lesions that destroy the developing walnut and produce the characteristic black, shrivelled appearance of blight-affected nuts. The disease is most severe in wet springs with prolonged rain coinciding with leaf expansion and the susceptible period of new growth.

Does juglans get aphids?

Yes, walnut aphid (Chromaphis juglandicola) forms colonies on the undersides of walnut leaflets in spring and summer, causing the youngest growth to curl and producing honeydew that drips onto the surfaces below. The pale yellow aphids are typically found in moderate numbers on the underside of the leaves along the main veins, and the colonies rarely reach the extreme densities seen on some other tree species. Natural predators including ladybirds and parasitic wasps provide effective biological control in most garden situations. A separate species, the dusky-veined aphid (Callipterinella calliptera), forms colonies on the undersides of walnut leaflets that cause characteristic dark vein discolouration.

Why is my walnut tree not producing nuts?

Walnut trees typically take ten to fifteen years to begin producing nuts from seed, though grafted varieties on commercial rootstock may produce within five to seven years. Late frosts in spring that damage the male catkins or the emerging female flowers before pollination cause crop failure in years where the blossom is affected. Walnut bacterial blight attacking the developing nuts in wet spring conditions causes them to blacken and drop prematurely. Some walnut cultivars are more reliably productive in UK conditions than others; Juglans regia Buccaneer and Juglans regia Broadview are among the cultivars recommended for reliable UK nut production.