Why does my golden hop (Humulus lupulus Aureus) have white powder on its leaves?

White powdery coating on the leaves and shoots of Humulus lupulus 'Aureus' is almost certainly powdery mildew, specifically Podosphaera macularis, the hop powdery mildew. This is the most serious disease of hops and the golden hop is not immune. The fungus thrives in warm dry conditions with poor airflow around the foliage, which is common when the plant is growing densely against a fence or through a pergola with congested growth. The white coating is made up of fungal mycelium and spores; the infection causes leaf distortion, curling, yellowing, and early leaf drop. Remove affected leaves and shoots promptly. Improve airflow by thinning out congested growth and training the plant with spacing between the stems. Apply a sulphur-based fungicide or a potassium bicarbonate spray as a contact treatment. Avoid overhead watering. Good sun exposure and consistent airflow are the best long-term preventive measures.

Hops Leaves Curling — Why It Happens and How to Fix It

Q: What is hop aphid and how do I control it?

Hop aphid (Phorodon humuli) is a specialist aphid that feeds exclusively on hops during the summer and overwinters as eggs on Prunus species, particularly plum, sloe, and blackthorn. In spring, winged females migrate from Prunus onto hops and rapidly establish large colonies on the undersides of leaves and on growing shoots. The colonies cause severe downward leaf curl, yellowing, and distorted growth as they extract sap and inject saliva that disrupts cell development. Honeydew deposits from the aphids allow sooty mould to establish, further reducing the plant's ability to photosynthesize. In commercial hop gardens hop aphid is a major economic pest requiring careful monitoring and timely intervention. In garden settings, monitor from May when migration from Prunus begins. Knock colonies off with a strong jet of water. Apply insecticidal soap or a pyrethrin-based spray to the undersides of affected leaves. Biological controls including parasitic wasps and ladybirds are effective if broad-spectrum insecticides are avoided. Growing hops away from Prunus species reduces the nearby overwintering population, though winged aphids can migrate from a distance.

Q: Will my hops recover if the early shoots are killed by frost?

Yes. Humulus lupulus is a robust perennial that dies back completely to the root every winter and re-shoots each spring from a large established root system. If late spring frost catches the emerging shoots in May or June, the frosted growth will blacken and die back, which can look alarming. However, the root is unaffected and the plant will push new shoots within a few weeks. Protect emerging shoots from late frosts with horticultural fleece if frost is forecast after they have emerged. Once the replacement shoots get going, the plant typically grows vigorously and still produces a substantial amount of foliage and, in the case of female plants, hop cones by late summer. The main consequence of frost damage is a shortened season for that year; the plant itself is not at risk.

Q: How do I tell hop powdery mildew from hop downy mildew?

Hop powdery mildew (Podosphaera macularis) and hop downy mildew (Pseudoperonospora humuli) cause different symptoms that are reasonably easy to distinguish once you know what to look for. Powdery mildew produces a white, dry, powdery or dusty coating on the upper leaf surface and on young shoots; the white material can be rubbed off with a finger. The leaves curl, distort, and yellow and the disease is worst in warm dry weather with poor airflow. Downy mildew produces angular yellow or brown blotches on the upper leaf surface that are bounded by the leaf veins; turning the leaf over reveals a grey or whitish downy coating on the underside beneath those patches. The downy material on the underside is the key diagnostic feature that distinguishes it from powdery mildew. Downy mildew is worst in cool wet conditions, the opposite of powdery mildew. Both cause leaf curl and yellowing but through different mechanisms and in different weather conditions.

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At a glance

Severe downward curl with colonies on leaf undersides: Hop aphid (Phorodon humuli); treat with insecticidal soap or pyrethrin, monitor from May
White powdery coating with distortion and curl: Powdery mildew (Podosphaera macularis); improve airflow, apply sulphur or potassium bicarbonate
Brown blotches on upper surface, grey fuzz underneath: Downy mildew (Pseudoperonospora humuli); remove affected growth, improve drainage and airflow
Stippling, bronzing, and fine webbing in hot dry weather: Spider mite; spray undersides with insecticidal soap, increase humidity
Wilting large leaves in dry spells: Drought stress; water at the base, apply mulch
Blackened shoot tips on emerging growth in spring: Late frost damage; let the plant re-shoot from the root, protect with fleece if frost is forecast

Why hops leaves curl

Humulus lupulus, the common hop, is one of the most vigorous perennial climbers available to UK gardeners. The ornamental golden hop (H. lupulus 'Aureus') is among the most popular garden climbers for covering a pergola, arch, or unsightly fence quickly, capable of reaching 5 to 6 metres in a single season from its large overwintering root. The traditional hop grown in Kent, Herefordshire, and Worcestershire for brewing is the same species; only female plants produce the hop cones used in beer, and garden plants are often grown for foliage alone. The large, deeply lobed leaves are the main ornamental feature, and any problem that causes them to curl or distort is immediately visible.

Hops are tough but they have two well-known specialist adversaries: the hop aphid and hop powdery mildew. Both are closely associated with hops and both can cause rapid, serious damage. Understanding which problem you are dealing with is straightforward once you know what to look for, and most causes are controllable with prompt action.

Cause 1: Hop aphid (Phorodon humuli)

Signs: The leaves are curling severely downward and inward, particularly on growing shoots and the newest leaves. Turning a curled leaf over reveals dense colonies of small, pale-green or greyish soft-bodied insects packed onto the underside and clustered around the growing tips. The affected shoots may be so heavily infested that the aphids are visible from a distance. Sticky honeydew deposits coat the leaves below the colonies and black sooty mould often follows on the honeydew. Growth is distorted and the shoot tips may fail to extend. The problem appears suddenly in late spring or early summer and can escalate very quickly in warm weather.

Why it happens: Phorodon humuli is a specialist aphid that has a two-host life cycle. It overwinters as eggs on Prunus species, particularly plum, damson, sloe (Prunus spinosa), and blackthorn, then in spring produces winged females that migrate to hops. On hops the population explodes rapidly through multiple asexual generations in warm conditions, with each female producing dozens of offspring. A light infestation in May can become a severe one by June. In UK gardens planted near Prunus hedging or near sloe-rich hedgerows the problem tends to be worse because the overwintering population is close by, though winged migrants can travel considerable distances.

Fix: Start monitoring from May by checking leaf undersides on the first emerging shoots. Knock early colonies off with a strong jet of water from a hose; this is most effective before colonies become very large. Apply insecticidal soap thoroughly to both leaf surfaces and growing tips, targeting the undersides of leaves where aphids concentrate. Pyrethrin-based sprays are also effective. Repeat every seven to ten days while the infestation is active. Avoid broad-spectrum persistent insecticides, which destroy the parasitic wasps, ladybirds, and lacewings that provide significant natural control once populations build. Growing hops well away from Prunus species reduces the nearby source of overwintering aphids. Sooty mould on honeydew deposits wipes off with a damp cloth once the aphid population is controlled and is not itself harmful once the underlying cause is gone.

Cause 2: Powdery mildew (Podosphaera macularis)

Signs: A white or pale grey powdery coating appears on the upper surfaces of leaves and on young shoots. Affected leaves curl, distort, and yellow; in severe cases they drop early. The white powder is made up of fungal mycelium and spores and can be rubbed off a leaf surface with a finger, distinguishing it from the permanent white colouring of some variegated plants. The disease typically starts on young growth at the shoot tips and spreads. In bad years the whole plant can be affected and growth is significantly stunted. The golden hop's vivid foliage becomes disfigured and loses its ornamental value.

Why it happens: Podosphaera macularis is the specialist powdery mildew of hops and is the most serious disease the plant faces commercially. Unlike most powdery mildews, hop powdery mildew is capable of infecting in a relatively wide range of conditions, though it is worst in warm dry summers with poor airflow around the foliage. Dense, congested growth against a wall or through a tight pergola creates the low-airflow microclimate the fungus exploits. Spores spread on air currents and dry weather conditions favour rapid infection.

Fix: Remove and dispose of heavily affected leaves and shoots as soon as the disease is noticed; do not compost affected material. Improve airflow around the plant by thinning out congested stems and training the remaining shoots with space between them. Apply a sulphur-based fungicide or a potassium bicarbonate spray as a contact fungicide; both are effective against powdery mildew and are approved for garden use. Repeat at seven to ten day intervals while conditions favour the disease. Avoid overhead watering, which spreads spores. For the golden hop, growing it in a position with good air movement and not against a solid wall with no air circulation significantly reduces the severity of the problem in subsequent years.

Other causes of leaf curling in hops

Downy mildew (Pseudoperonospora humuli)

Downy mildew is a different disease from powdery mildew, caused by a water mould rather than a true fungus. The symptoms are distinct: angular yellow to brown blotches appear on the upper leaf surface bounded by the leaf veins, and turning the leaf over reveals a grey or white downy growth on the underside beneath those patches. The leaves curl and yellow and affected shoots can die back. Downy mildew is worst in cool, wet conditions, the opposite of powdery mildew. Remove affected growth, improve drainage and airflow, and avoid wetting the foliage. Copper-based fungicides provide some control. The different weather preference and the diagnostic grey fuzz on the underside of the leaf distinguish it clearly from powdery mildew.

Spider mite

In hot, dry summers, two-spotted spider mite (Tetranychus urticae) can establish on hops. The signs are pale stippling on the upper leaf surface as mites pierce individual cells, combined with a bronzed or dusty appearance and fine silken webbing between leaves and stems. The large hop leaves provide a substantial food source and populations grow quickly in hot dry conditions. Spray thoroughly with insecticidal soap or neem oil, covering the undersides of leaves where mites live and feed. Increasing humidity around the plant through ground-level watering and mulching makes the environment less hospitable for mites. Repeat applications are needed to break the life cycle.

Drought stress

Hops are vigorous plants that produce an enormous amount of growth in a short season, and they need consistent moisture to support that growth. In dry spells the large leaves wilt and curl as the plant cannot supply water fast enough to match transpiration demand. Container-grown hops are particularly vulnerable as their restricted root volume dries out quickly. Water at the base of the plant rather than overhead, soak the root zone thoroughly during dry periods, and apply a mulch to retain soil moisture. Improving drought resilience is particularly important for golden hops grown in containers as ornamentals.

Late spring frost damage

Hops emerge from the root in May or June and the new shoots are vulnerable to late frosts. If frost catches the emerging growth, the shoot tips blacken and die back. This can look serious but the plant is not threatened because the large root system is unaffected. New shoots will push within a few weeks and the plant recovers to produce a full season of growth, though somewhat shortened. If late frost is forecast after shoots have emerged, covering with horticultural fleece overnight prevents damage. No treatment is needed once frost damage has occurred; simply allow the plant to re-shoot.

Prevention

The most effective prevention for the two main problems combines cultural and monitoring practices. Grow hops in full sun with good airflow around the foliage; the golden hop colours best and resists both powdery mildew and aphid problems better in open sunny positions than in shaded, sheltered spots. Begin checking leaf undersides for hop aphid from early May when migration from Prunus begins, and act as soon as colonies appear rather than waiting for them to build. Where possible, avoid planting hops close to established Prunus species such as plum trees, ornamental cherries, sloe hedges, or blackthorn, which serve as the winter host for Phorodon humuli. Water at the base of the plant rather than overhead to keep foliage dry and reduce the humidity that favours mildew. In future seasons, train the plant to maintain spacing between stems rather than allowing solid dense growth that cuts off airflow.

Frequently asked questions

Why are my hops leaves curling?

Hops leaves curl most commonly because of hop aphid (Phorodon humuli) or powdery mildew (Podosphaera macularis). Hop aphid forms large colonies on leaf undersides and growing tips, causing severe downward curl and distortion; colonies build extremely fast in warm weather from May onward after the aphids migrate from their winter host plants. Powdery mildew produces a white powdery coating on leaves and shoots alongside leaf distortion and curling; it is the most serious disease of hops and thrives in warm dry summers with poor airflow. Downy mildew, spider mite, drought stress, and late frost damage can also cause curling in different conditions and with distinct patterns of symptom.

What is hop aphid and how do I control it?

Hop aphid (Phorodon humuli) is a specialist aphid that spends winter as eggs on Prunus species such as plum, sloe, and blackthorn, then migrates to hops in spring. Colonies on the undersides of leaves and on growing shoots cause severe downward leaf curl, yellowing, and distorted growth, and multiply extremely fast in warm weather. Monitor leaf undersides from May and act early. Knock colonies off with a strong water jet, apply insecticidal soap or pyrethrin to the undersides of affected leaves, and repeat every seven to ten days. Avoid broad-spectrum persistent insecticides to preserve natural predators. Growing hops away from Prunus species reduces the nearby overwintering population.

Why does my golden hop have white powder on its leaves?

White powdery coating on the leaves and shoots of Humulus lupulus 'Aureus' is almost certainly powdery mildew, specifically Podosphaera macularis, the hop specialist. It thrives in warm dry conditions with poor airflow, which is common when the plant grows densely against a wall or through a congested pergola. Remove affected leaves and shoots, improve airflow by thinning congested growth, and apply a sulphur-based or potassium bicarbonate fungicide at seven to ten day intervals. Good sun exposure and consistent air movement around the foliage are the best long-term preventive measures.

Will my hops recover if the early shoots are killed by frost?

Yes. Humulus lupulus dies back completely to the root every winter and re-shoots each spring from a large established root system. If late frost blackens and kills the emerging shoots, the root is unaffected and new shoots will push through within a few weeks. The season is shortened but the plant is not at risk. Protect emerging shoots with horticultural fleece if frost is forecast after they have appeared. No other treatment is needed; simply allow the plant to re-shoot and continue growing.

How do I tell hop powdery mildew from hop downy mildew?

Powdery mildew (Podosphaera macularis) produces a white dry powdery coating on the upper leaf surface and on young shoots; the white material can be rubbed off with a finger. It is worst in warm dry weather. Downy mildew (Pseudoperonospora humuli) produces angular yellow or brown blotches on the upper leaf surface bounded by the veins, with a grey or white downy coating on the underside of the leaf beneath those patches. The downy coating on the underside is the key diagnostic feature. Downy mildew is worst in cool wet conditions, the opposite of powdery mildew.