Plant problems

Why Are My Worcesterberry Leaves Curling?

Q: Why are my worcesterberry leaves curling?

Worcesterberry (Ribes divaricatum, also called coast black gooseberry or spreading gooseberry) leaves curl most commonly in the UK because of aphid infestation, gooseberry sawfly (Nematus ribesii), or American gooseberry mildew (Podosphaera mors-uvae). Worcesterberry is a North American wild gooseberry grown in UK gardens for its small, dark purple-black berries; it shares the pest and disease susceptibilities of its close relatives gooseberry (Ribes uva-crispa) and blackcurrant (Ribes nigrum). The currant-lettuce aphid (Hyperomyzus lactucae) and blackcurrant aphid (Cryptomyzus ribis) colonise the undersides of the leaves in spring, causing them to curl tightly upward and inward; the distortion is similar to that seen on blackcurrant and jostaberry. Gooseberry sawfly (Nematus ribesii) larvae are not a direct cause of leaf curl but produce a rapid, dramatic defoliation of the plant in late spring and summer; when defoliation is severe, the plant produces a new flush of growth that sometimes shows curling caused by the stress of regrowth. American gooseberry mildew produces a white powdery coating on the young leaves and shoot tips, which curl and may die back.

Q: What is a worcesterberry?

Worcesterberry is the common UK garden name for Ribes divaricatum, a species of wild gooseberry native to the Pacific coast of North America (British Columbia south to California). Despite the name suggesting a connection to the English city of Worcester, the origin of the common name is not certain; some sources suggest it derives from the fact that the plant was popularised in UK gardens via Worcester-based nurseries in the early twentieth century. Ribes divaricatum is not a cultivar or hybrid but a distinct species; it is sometimes confused with Worcestershire-bred gooseberry cultivars ('Worcester Red' is an unrelated cultivar), which adds to the naming confusion. The plant is a large, vigorous, very thorny shrub reaching 1.5 to 2 metres that produces small, dark purple-black to nearly black berries in July; the berries are similar in size to a large blackcurrant or small gooseberry, with a flavour that combines the tartness of gooseberry with slight blackcurrant notes. One of worcesterberry's notable characteristics is its significantly greater resistance to American gooseberry mildew compared to most gooseberry cultivars; this makes it useful in gardens where mildew has been a persistent problem. It is available from specialist soft fruit nurseries in the UK.

Q: How do I grow worcesterberry in the UK?

Worcesterberry (Ribes divaricatum) grows in the same conditions as gooseberry and blackcurrant: any reasonably fertile, well-drained UK garden soil in full sun or partial shade. It is fully frost-hardy, vigorous, and self-fertile. Plant in autumn or late winter; space at 1.5 metres. Worcesterberry produces very long, thorny canes and grows into a large, sprawling shrub; for ease of management, train it as an open goblet shape with 6 to 8 main canes, cutting out the weakest and oldest wood each winter to encourage new fruiting shoots. The small, dark berries ripen in July and August; they can be picked when soft and used in jam, pies, and wine (where the flavour is excellent, with more body than pure gooseberry). The thorns are sharp and long; wear gloves when pruning and picking. Worcesterberry is propagated easily from hardwood cuttings taken in winter (cuttings 25 to 30 cm long, planted to two-thirds of their depth in a sheltered position, root readily by spring). It requires less attention than standard gooseberry varieties and rarely suffers from American gooseberry mildew, which is its principal advantage over gooseberry in gardens where mildew is persistent.

Q: How do I control gooseberry sawfly on worcesterberry?

Gooseberry sawfly (Nematus ribesii) can defoliate worcesterberry very rapidly if not caught early. The pale green, black-spotted caterpillar-like larvae hatch in April and May from eggs laid on the undersides of the leaves in the centre of the bush; they begin feeding on the leaf edges and, if undetected, can strip a worcesterberry of all its leaves within a week. To control: check the centre of the bush thoroughly from April onward; the first signs are small holes at the leaf margins. Pick off larvae by hand (they drop to the ground when disturbed, so lay a sheet underneath and shake the branches). Apply insecticidal soap or a pyrethrin-based spray when larvae are first detected; treat the undersides of the leaves in the centre of the bush where the larvae feed. There can be up to three generations per summer; check again in June and late July. Natural predators (birds and parasitoid wasps) are important; avoid broad-spectrum insecticides that kill the parasitoid wasps that control sawfly populations. Ground-feeding birds (blackbirds, thrushes) eat fallen larvae and pupae; mulching around the base of the bush encourages them.

Worcesterberry (Ribes divaricatum) is a North American wild gooseberry species grown in UK gardens for its small, intensely flavoured, dark purple-black berries and its notably better resistance to American gooseberry mildew compared to standard gooseberry cultivars. A large, vigorous, very thorny shrub, it requires the same conditions as gooseberry and can be pruned to an open goblet shape for easier picking. The aphid and sawfly problems it shares with gooseberry are the most common causes of leaf distortion and damage in UK gardens.

Aphids

The currant-lettuce aphid (Hyperomyzus lactucae) and blackcurrant aphid (Cryptomyzus ribis) cause the most characteristic leaf curl on worcesterberry in spring: the young leaves at the shoot tips curl tightly upward and inward into small, cup-like rolls, with colonies of small green aphids visible on the undersides. The distortion is most pronounced on the first flush of new growth in April and May. The currant-lettuce aphid migrates to sow thistle in summer and returns in autumn to lay overwintering eggs on Ribes plants; removing sow thistles from the vicinity reduces reinfestation in subsequent years.

What to do

Check the shoot tips from April. Pinch off and destroy the most tightly curled leaves. Apply insecticidal soap or neem oil spray to the shoot tips and leaf undersides, repeating every 7 days. Encourage natural predators. Apply a winter oil wash to dormant stems in December to January to smother overwintering aphid eggs. Remove sow thistles from the garden to reduce reinfestation.

Gooseberry sawfly

Gooseberry sawfly (Nematus ribesii) larvae can defoliate worcesterberry very rapidly; they hatch in April and May from eggs laid on leaf undersides in the centre of the bush and feed outward from there, beginning with small holes at the leaf margins and quickly progressing to complete stripping of the foliage. A bush can be entirely defoliated within a week of first noticing the problem. Following defoliation, worcesterberry produces a new flush of growth; this regrowth can show leaf curl and distortion caused by the stress of rapid regrowth and secondary aphid colonisation.

What to do

Check the centre of the bush from April for the small holes at leaf margins that indicate early larval feeding. Pick off larvae by hand (lay a sheet under the bush and shake the branches; larvae drop readily). Apply insecticidal soap or pyrethrin-based spray to the leaf undersides in the centre of the bush at first detection. Check again in June and late July for second and third generation larvae. Encourage ground-feeding birds by mulching around the base.

Powdery mildew

One of worcesterberry's main advantages over standard gooseberry is its significantly better resistance to American gooseberry mildew (Podosphaera mors-uvae). However, it is not completely immune; in a warm, dry summer with poor air circulation, a mild white powdery coating can appear on the young shoot tips, which curl and may die back. The resistance means that infections on worcesterberry are typically less severe and easier to control than on susceptible gooseberry cultivars.

What to do

Prune to an open, goblet shape to improve air circulation. Apply a potassium bicarbonate or sulphur-based fungicide at the first sign of infection. Avoid high-nitrogen feeding in spring. On worcesterberry, mildew infections rarely require aggressive treatment; good air circulation usually limits the problem without chemical intervention.

Big bud mite

Big bud mite (Cecidophyopsis ribis) can affect worcesterberry as it does other Ribes species; infested buds swell to a rounded, bloated shape rather than the elongated, pointed shape of healthy buds, and when they open in spring they produce stunted, distorted, curled leaves. Worcesterberry's resistance profile against big bud mite relative to blackcurrant and gooseberry is not well documented; treat any suspected infestation with the same approach as for blackcurrant.

What to do

Inspect buds carefully in late winter before growth begins. Remove and destroy any swollen, rounded buds. If more than 10 to 15% of buds are affected, consider removing and replacing the plant with certified virus-free stock. There is no effective chemical control for home gardeners.

Frequently asked questions

Why are my worcesterberry leaves curling?

Worcesterberry (Ribes divaricatum) leaves curl most commonly in the UK because of aphid infestation, gooseberry sawfly defoliation, or powdery mildew. The currant-lettuce aphid and blackcurrant aphid colonise the young shoot tips in spring, causing leaves to curl tightly upward into cup-like rolls. Gooseberry sawfly larvae defoliate the plant rapidly from the centre outward; regrowth after defoliation can show stress-induced curl. Powdery mildew causes a white coating and curling on young shoot tips.

What is a worcesterberry?

Worcesterberry is the common UK name for Ribes divaricatum, a wild gooseberry species from the Pacific coast of North America. It produces small, dark purple-black berries in July with a gooseberry-plus-blackcurrant flavour. Its main advantage in UK gardens is significantly better resistance to American gooseberry mildew than standard gooseberry cultivars. A large, vigorous, very thorny shrub; available from specialist soft fruit nurseries.

How do I grow worcesterberry in the UK?

Worcesterberry grows in the same conditions as gooseberry: any reasonably fertile, well-drained UK soil in full sun or partial shade. Fully frost-hardy and self-fertile. Train as an open goblet shape with 6 to 8 main canes; prune out old wood each winter. The small, dark berries ripen in July and August and are excellent for jam, pies, and wine. Thorns are sharp; wear gloves. Propagates easily from hardwood cuttings taken in winter.

How do I control gooseberry sawfly on worcesterberry?

Check the centre of the bush from April for small holes at the leaf margins. Pick off larvae by hand (shake branches over a sheet). Apply insecticidal soap or pyrethrin spray to the leaf undersides at first detection. Check again in June and late July for second and third generation larvae. Encourage ground-feeding birds by mulching around the base; they eat fallen larvae and pupae. Avoid broad-spectrum insecticides that kill the parasitoid wasps that naturally control sawfly.