Rhus Leaves Curling
Powdery mildew and Verticillium wilt are the most common reasons rhus leaves curl. This guide shows you how to identify each cause and keep sumac vibrant through the season.
1. Powdery mildew
Powdery mildew is the most frequently seen disease on rhus in UK gardens. The fungal spores spread through the air and settle on leaves during warm, dry weather, particularly in late summer. Rhus typhina and R. glabra are both susceptible, with young growth at the shoot tips affected most severely.
What to look for
A white or grey powdery coating appears on the upper surface of leaves, most visibly on the newest growth at the tips of stems. Affected leaflets curl upward and inward and may appear slightly stunted or distorted. In severe infections whole fronds of pinnate leaves may appear almost entirely white. The infection can spread rapidly through the canopy in dry, warm conditions.
What to do
Remove and bin affected shoot tips as soon as you see them. Avoid watering foliage from above; water at the base of the plant in the morning. Improve air circulation by thinning stems at the centre of the plant in winter. A fortnightly spray of diluted milk solution (one part whole milk to nine parts water) applied from the first signs of infection is effective and safe. For persistent mildew, a sulphur-based fungicide applied preventively from late spring gives good protection. Avoid high-nitrogen fertiliser in late summer as it encourages the soft new growth most susceptible to infection.
2. Verticillium wilt
Verticillium wilt is caused by the soilborne fungi Verticillium dahliae and V. albo-atrum, and is one of the most significant threats to rhus in the garden. The fungi enter through roots and colonise the vascular system, progressively blocking water movement upward through the stem. Rhus is notably susceptible and is one of the plants most commonly cited in connection with this disease.
What to look for
One or more stems wilt suddenly during the growing season, with leaves curling, turning yellow, then brown while remaining attached rather than dropping cleanly. The wilting affects one side or one stem at a time in the early stages. If you cut through a wilting stem, you may see olive-brown or grey discolouration in the outer ring of wood (sapwood) just inside the bark. Unlike drought, the affected stems do not recover with watering. Shoots emerging from the roots below may appear healthy initially, confusing the diagnosis.
What to do
Cut out all wilting stems immediately, pruning at least 30 cm below any visible discolouration in the wood and sterilising tools between cuts. The fungus persists in the soil for many years, so avoid replanting rhus or other susceptible plants (including tomatoes, aubergines, and many ornamentals) in the same spot. Feed remaining healthy stems with a potassium-rich fertiliser to encourage vigour and disease resistance. Many rhus plants survive Verticillium wilt by regenerating vigorously from the root system even as individual stems die. Keep the plant as healthy as possible through appropriate watering and mulching.
3. Sumac aphid and leaf galls (Melaphis rhois)
The sumac gall aphid (Melaphis rhois) has a fascinating but complex lifecycle that involves alternating between rhus and moss. On rhus, the first generation causes the plant to form distinctive hollow, smooth, red or greenish galls on individual leaflets. These galls can look alarming but are not a serious threat to the health of established plants.
What to look for
Smooth, rounded, hollow galls develop on individual leaflets of the compound rhus leaf in spring and early summer. The galls are initially green and fleshy, turning reddish or brownish by midsummer. Affected leaflets curl around the gall and may appear distorted. If you cut a gall open it will contain numerous small, pale aphids. Later generations migrate to moss and return to rhus in autumn. More general leaf curl and honeydew deposit (without the distinctive galls) indicates other aphid species feeding more conventionally on the undersides of leaves.
What to do
No treatment is needed for gall aphid damage on healthy established rhus; the galls are cosmetic and do not significantly harm the plant. If the appearance bothers you, remove individual galled leaflets by hand and bin them. For general aphid colonies causing leaf curl and honeydew, a horticultural soap spray directed at the underside of leaves is effective. Natural predators including ladybirds and parasitic wasps keep populations in check through the season.
4. Drought stress
Rhus is a tough, drought-tolerant genus once established in the ground, but young plants in their first two or three years and those grown in containers are more vulnerable to drought stress. In very dry summers even established specimens can show signs of stress, particularly those in poor, thin soil.
What to look for
Leaflets roll inward and the whole frond droops in the heat of the day. The plant may take on a slightly dull, grey-green tone rather than its usual rich green. In more severe drought the leaf margins and tips turn brown. In some cases the plant may begin to colour up prematurely for autumn weeks before other plants show any colour change, which is a stress response rather than an early autumn signal. The soil beneath the plant will feel very dry.
What to do
Water young plants deeply once or twice a week during dry spells in their first three summers. Mulch the root zone with a 100 mm layer of wood chip to conserve soil moisture and reduce soil temperature fluctuations. Established rhus rarely needs watering except during prolonged drought or heatwaves. Container specimens need regular watering throughout the growing season; check the compost daily in hot weather and water thoroughly when the top few centimetres feel dry.
5. Leaf spot
Various fungal leaf spot diseases can affect rhus, particularly during wet summers or in gardens where fallen leaves are left to accumulate beneath the plant. The most common is caused by Cercospora species and related fungi, which produce characteristic small dark spots on foliage.
What to look for
Small, circular or angular dark brown or black spots appear on individual leaflets, often with a yellow halo around each spot. As infection progresses the spots enlarge and the surrounding leaf tissue yellows. Severely affected leaflets curl at the margins and drop early, sometimes causing significant premature defoliation by late summer. The disease is worst in wet conditions and on plants with poor air circulation.
What to do
Collect and bin all fallen leaves promptly to prevent spores from overwintering and re-infecting the plant the following year. Prune to improve air circulation through the canopy. Avoid overhead watering. If leaf spot is a recurring problem, a preventive copper-based fungicide spray applied from bud burst and repeated every two to three weeks during wet weather provides good protection. Improving the general health of the plant through appropriate feeding and watering reduces susceptibility in future seasons.
Frequently asked questions
Why are my rhus leaves curling and turning white?
White powdery coating with curling leaves on rhus is caused by powdery mildew. It is especially common in warm, dry summers and on plants in sheltered positions. The mildew rarely kills rhus but reduces vigour and can spoil the late-season colour display. Remove affected shoots and improve air circulation to manage it.
Why is my sumac dying back stem by stem?
Stem-by-stem die-back with wilting and leaf curl is the hallmark of Verticillium wilt, one of the most serious diseases of rhus. The soilborne fungus blocks the vascular system of individual stems. Prune out affected growth well below any discolouration in the wood and maintain good tree vigour with feeding and watering. Many rhus survive for years with the disease by producing new growth.
Are rhus leaves poisonous?
Rhus typhina (staghorn sumac) and Rhus glabra are generally considered safe to handle, but some Rhus species and closely related Toxicodendron species (poison ivy, poison oak, poison sumac) can cause severe contact dermatitis. Always wash hands after handling any rhus plant and avoid touching your eyes or face. Check the exact species identification before pruning.
Why does my sumac spread so aggressively?
Rhus typhina spreads by root suckers that arise from the wide-spreading lateral root system. Cutting the plant back actually stimulates more suckering. To control spread, sever suckers close to their connection with the parent root using a sharp spade or mattock. Mowing over suckers in grass repeatedly weakens them over time.
Can I grow rhus in a container?
Yes, rhus can be grown in a large container (minimum 50 cm diameter) which also controls its tendency to sucker. Use a loam-based compost such as John Innes No. 3 and ensure excellent drainage. Container specimens need watering more regularly than garden plants and will benefit from an annual feed in spring. Repot every two to three years to prevent the roots from becoming pot-bound.
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