Why Are My Plum Leaves Curling?
Plum trees (Prunus domestica and related species) are a staple of UK kitchen gardens, producing reliable crops of fruit from mid to late summer. Two aphid species cause the majority of leaf curling problems on UK plum trees: leaf curl plum aphid, which attacks the growing tips in spring and causes tight, dramatic leaf curling, and mealy plum aphid, which colonises leaf undersides from late spring and produces enormous amounts of sticky honeydew. Both are manageable with the right timing.
Leaf curl plum aphid
Leaf curl plum aphid (Brachycaudus helichrysi) overwinters as eggs on plum bark and hatches from bud burst in April. The small, pale yellow-green aphids colonise the growing tips of plum shoots in dense colonies. As the colony builds, the young leaves at the shoot tip curl tightly inward around the aphids, and the shoot becomes stunted and distorted. This tight leaf curling is the defining characteristic: by May, the growing tips of badly infested shoots are virtually enclosed in curled, distorted leaves with the aphid mass inside. The aphid migrates to its summer hosts (members of the daisy family, including tansy and ragwort) by midsummer, and the damage is done. Young trees and trees with vigorously growing new shoots are worst affected.
What to do
- Inspect plum shoot tips weekly from bud burst in April. The critical window is before the leaves have curled: treat any visible colonies immediately with insecticidal soap, pyrethrum, or plant oil spray.
- Once leaves have curled tightly around the colonies (usually by late April to May), contact sprays cannot reach the aphids inside. Pinch out and destroy the most severely infested shoot tips. Accept the cosmetic damage to the remaining shoots and wait for natural migration in midsummer.
- Apply a plant-based oil winter wash in January to smother overwintering eggs and reduce the following year's infestation.
Mealy plum aphid
Mealy plum aphid (Hyalopterus pruni) is a medium-sized, pale green aphid coated with a characteristic whitish, waxy, flour-like powder that distinguishes it from other aphid species. It colonises the underside of plum leaves on the older parts of the shoot from late spring and can build up to enormous population densities: in a bad year, the entire underside of leaves may be covered in a mass of mealy, pale insects. The honeydew produced falls onto fruit, shoots, and lower leaves, encouraging sooty mould. Heavily infested trees show yellowing and early leaf drop, and the fruit quality is significantly reduced by contamination with honeydew and mould.
What to do
- Allow natural predators (ladybirds, parasitoid wasps, hoverfly larvae) to develop: mealy plum aphid populations are often naturally regulated by midsummer. Avoid broad-spectrum insecticides that kill beneficial insects.
- If populations reach damaging levels before natural predators have built up (typically in May to June), apply insecticidal soap to the underside of leaves. The soap must contact the aphids to be effective.
- Remove and destroy the worst-affected shoots. By midsummer, most colonies migrate to common reed (Phragmites) and the tree recovers.
Silver leaf disease
Silver leaf disease (Chondrostereum purpureum) does not cause leaf curling, but causes a silvery sheen on the leaves of infected branches that can be confused with other leaf problems. It is a serious fungal disease of plum and other Prunus species, entering through pruning wounds and damaged bark. Affected branches eventually die, and the tree can be killed if the infection is not controlled by removing infected branches.
What to do
- Prune plum only in summer (May to August). Remove infected branches to at least 10 centimetres beyond the last brown-stained wood visible on the cut surface. Do not leave pruning stubs.
Plum rust
Plum rust (Tranzschelia pruni-spinosae) is a fungal disease that causes small, yellow-orange spots on the upper surface of plum leaves and powdery, orange-brown pustules on the underside, which may cause the leaves to curl slightly and drop early. It appears from midsummer and is most prevalent in warm, humid conditions. It is more common on plum grown in sheltered, warm positions and in southern England.
What to do
- Collect and destroy fallen infected leaves in autumn. Good airflow in the canopy reduces humidity and slows spread. Fungicide application is rarely necessary in the UK on an established tree.
Frequently asked questions
Why are my plum leaves curling?
Plum leaves curl most commonly because of leaf curl plum aphid (Brachycaudus helichrysi) or mealy plum aphid (Hyalopterus pruni). Leaf curl plum aphid colonises the growing tips of plum shoots in spring and causes the young leaves to curl tightly, the shoots to become distorted, and the growing tip to be covered in dense aphid colonies. The colonies are pale yellow-green in colour. The leaves curl so tightly around the aphids that contact insecticides applied after this stage cannot reach the insects inside. Mealy plum aphid is pale green and coated with a whitish, waxy powder; it colonises the undersides of leaves in large numbers from spring through to summer, causing yellowing, distortion, and copious honeydew.
How do I control leaf curl plum aphid?
Leaf curl plum aphid (Brachycaudus helichrysi) must be controlled before the leaves curl around the aphid colonies. The effective treatment window is early spring, from bud burst to the point when the first leaves are just opening. At this stage the aphids are visible on the shoot surface and have not yet triggered the leaf-curling response. Inspect plum shoots carefully from April and treat any colonies promptly with insecticidal soap, pyrethrum, or plant oil spray. Once the leaves have curled (typically by May), contact sprays cannot penetrate. At that point, remove and destroy the worst-affected shoot tips if practical. The aphid naturally declines and migrates in midsummer. Apply a plant-based oil winter wash in January to smother overwintering eggs on the bark.
What is mealy plum aphid?
Mealy plum aphid (Hyalopterus pruni) is a medium-sized, pale green aphid coated with a whitish, waxy, flour-like powder. Unlike leaf curl plum aphid, which attacks the growing tips in spring, mealy plum aphid colonises the underside of leaves on the older parts of the shoot, typically from late spring onward. Large colonies can build up through summer, covering the leaf underside and producing copious honeydew that drips onto leaves and fruit below, encouraging sooty mould. The leaves may yellow, curl slightly, and drop early in heavy infestations, and the fruit can become contaminated with honeydew and mould. In midsummer, the aphid migrates to its secondary host (common reed, Phragmites) and the plum tree recovers. Control: natural predators (ladybirds, parasitoid wasps) provide effective biological control from late spring; insecticidal soap applied to the leaf underside controls populations if they reach damaging levels before predators establish.
Can I prune plum in winter?
No. Plum (and all Prunus species including cherry, nectarine, and peach) should not be pruned in autumn, winter, or early spring. Pruning wounds made in these seasons are very susceptible to infection by the spores of silver leaf disease (Chondrostereum purpureum) and bacterial canker (Pseudomonas syringae), both of which can cause severe damage or kill the tree. The correct time to prune plum is in summer, from late May to mid-August, when the tree is actively growing and the wound-healing process is fastest, and when silver leaf spores are less abundant in the air. Never leave large pruning wounds unsealed; painting large cuts (over 3 centimetres in diameter) with a wound sealant is recommended for plum and other Prunus species. Always use sharp, sterilised tools to make clean cuts.