Plant problems

Why Are My Grape Vine Leaves Curling?

Q: How do I tell powdery mildew from downy mildew on my grape vine?

Distinguishing grape powdery mildew (Erysiphe necator) from grape downy mildew (Plasmopara viticola) is one of the most important diagnostic skills for UK grape vine growers, because the two diseases have different causes, different optimal conditions, and require somewhat different management approaches. Powdery mildew: the white to grey powder is on the UPPER surface of the leaf (and also on shoots and fruit clusters); the powder looks like a fine dusting of chalk or talc; the disease is most severe in warm, dry conditions; the affected leaf surface under the powder may have yellowish-brown patches; the leaf curls and the shoot tips distort and the leaves become pale and bleached; the over-wintering buds are an important source of spring reinfection. Downy mildew: the most diagnostic sign of downy mildew is a white, fluffy, cottony growth on the UNDERSIDE of the leaf; the underside growth corresponds exactly with pale, oily, yellow-green 'oil spot' patches on the UPPER surface of the leaf; the oil spots on the upper surface look waxy and slightly translucent; the white fluffy growth on the underside is very characteristic and unmistakable if you have seen it before; the disease is most severe in cool, wet conditions (so more common in wet UK summers or in an irrigated vineyard). Management differences: for powdery mildew, sulphur-based fungicide sprays and good ventilation are the primary controls; for downy mildew, copper-based fungicide sprays (copper oxychloride, Bordeaux mixture) and reducing leaf wetness (avoiding overhead irrigation and ensuring good air circulation) are the primary controls. Both diseases can be present simultaneously on the same vine in a wet UK summer. The white growth on the underside of the leaf is the key to identifying downy mildew; if the white growth is on the upper surface, it is powdery mildew.

Vitis vinifera (grape vine) is widely grown on UK walls, pergolas, and in greenhouses. The large, palmate, prominently lobed leaves curl, show white powdery coatings, develop oily patches with cottony undersides, or show bright yellow between the veins most often from grape powdery mildew (the most serious and most persistent disease of UK grapevines); from grape downy mildew in wet UK summers; from magnesium deficiency causing the very characteristic interveinal yellowing; or from vine weevil notch damage on container-grown vines.

Powdery mildew

Erysiphe necator (grape powdery mildew) is one of the most serious diseases of UK grapevines. White to grey powder on the upper surfaces of leaves, young shoots, and fruit clusters; affected shoot tips and leaves curl and distort; leaves turn pale and bleached under the powder; developing grapes turn grey, split, and develop off-flavours. Most severe in warm, dry conditions and in unventilated UK greenhouses. The fungus over-winters in dormant buds, reinfecting the vine each spring from within.

What to do

Apply sulphur-based fungicide spray (not at temperatures above approximately 27°C as sulphur can scorch vine foliage in heat; check the product label for temperature restrictions) starting at bud burst in spring and repeat every 10 to 14 days through to fruit set; ensure excellent ventilation in a greenhouse vine by opening vents and doors on warm days; remove and dispose of very badly affected shoots; choose mildew-resistant varieties where possible for new plantings in UK gardens with a history of severe powdery mildew.

Downy mildew

Plasmopara viticola (grape downy mildew) requires wet conditions and is most destructive in cool, wet UK summers. The very diagnostic symptom is pale, oily, yellow-green oil-spot patches on the upper leaf surface with a corresponding white, fluffy, cottony growth on the underside directly below each oil spot. Affected leaves curl at their margins, turn brown, and fall prematurely. Severe infections can cause very significant defoliation and devastate developing fruit clusters.

What to do

Apply copper-based fungicide sprays (copper oxychloride, Bordeaux mixture) at bud burst in spring and repeat through the growing season following the product label interval; copper-based sprays are effective against downy mildew; avoid overhead irrigation of vine foliage; ensure good air circulation around the vine canopy; remove and dispose of severely affected leaves and fallen leaf litter (the over-wintering spores survive in fallen infected leaf litter).

Magnesium deficiency

A very common nutritional disorder of UK grapevines; characteristic bright yellow patches appear between the prominent leaf veins while the veins themselves remain dark green, typically beginning on the older, lower leaves first. Most common on sandy or chalk soils, on heavy-cropping vines, or where high potassium fertiliser use has suppressed magnesium uptake.

What to do

Apply foliar magnesium sulphate (Epsom salts: 20g per litre of water) spray directly onto the yellowing leaves in the early morning or evening on a dry day; repeat every two weeks through the growing season until deficiency symptoms resolve; for longer-term correction apply powdered dolomitic limestone around the base of the vine in autumn or early spring (useful on acid soils where both magnesium and calcium may be low); avoid over-applying high-potassium fertilisers to vines already showing magnesium deficiency symptoms.

Vine weevil

Otiorhynchus sulcatus adults cut very characteristic irregular scalloped-notch shapes from the margins of vine leaves at night; the distinctive notch damage to leaf margins is a reliable early warning sign of adult vine weevil activity. Larvae are most damaging to container-grown vines; they feed on the roots and can cause wilting and dieback in a heavy infestation.

What to do

Inspect container-grown vine root balls in September to October for cream-coloured C-shaped larvae; remove and destroy any found; replace all container compost; apply entomopathogenic nematodes (Steinernema kraussei or Heterorhabditis bacteriophora) watered into moist container compost in late summer (August to October) as a preventative and treatment; both are available from UK biological control suppliers.

Frequently asked questions

Why are my grape vine leaves curling?

Grape vine leaves curl most commonly because of grape powdery mildew (Erysiphe necator; white to grey powder on upper surfaces of leaves shoots and fruit clusters; affected shoot tips and leaves curl and distort; over-winters in dormant buds and reinfects from within each spring; most severe in warm dry conditions and unventilated UK greenhouses; apply sulphur-based fungicide spray from bud burst in spring repeating every 10 to 14 days through to fruit set; ensure excellent ventilation; choose mildew-resistant varieties for new plantings), grape downy mildew (Plasmopara viticola; very diagnostic combination of pale oily yellow-green oil-spot patches on upper leaf surface with white fluffy cottony growth on underside directly below each oil spot; most destructive in cool wet UK summers; apply copper-based fungicide sprays (copper oxychloride Bordeaux mixture) from bud burst repeating through growing season; avoid overhead irrigation; ensure good air circulation), magnesium deficiency (bright yellow patches between prominent leaf veins while veins themselves remain dark green; most common on sandy or chalk soils or on heavy-cropping vines; foliar magnesium sulphate (Epsom salts: 20g per litre of water) spray on yellowing leaves every 2 weeks through growing season; dolomitic limestone around base of vine for longer-term correction), or vine weevil (characteristic irregular scalloped-notch shapes from vine leaf margins at night; most damaging on container-grown vines; inspect container root balls September to October for cream C-shaped larvae; apply entomopathogenic nematodes to moist container compost August to October).

How do I tell powdery mildew from downy mildew on my grape vine?

Powdery mildew: the white to grey powder is on the UPPER surface of the leaf (and also on shoots and fruit clusters); powder looks like a fine dusting of chalk or talc; disease most severe in warm dry conditions; affected leaf surface under the powder may have yellowish-brown patches; leaf curls and shoot tips distort; over-wintering buds are an important source of spring reinfection. Downy mildew: the most diagnostic sign is a white fluffy cottony growth on the UNDERSIDE of the leaf; the underside growth corresponds exactly with pale oily yellow-green oil-spot patches on the UPPER surface of the leaf; the oil spots on the upper surface look waxy and slightly translucent; the white fluffy growth on the underside is very characteristic and unmistakable; the disease is most severe in cool wet conditions (more common in wet UK summers). Management differences: for powdery mildew sulphur-based fungicide sprays and good ventilation are the primary controls; for downy mildew copper-based fungicide sprays (copper oxychloride Bordeaux mixture) and reducing leaf wetness are the primary controls. Both diseases can be present simultaneously on the same vine in a wet UK summer. The white growth on the UNDERSIDE of the leaf is the key to identifying downy mildew; if the white growth is on the upper surface it is powdery mildew.

Why are my grape vine leaves turning yellow between the veins?

Bright yellow patches appearing between the prominent veins of grape vine leaves while the veins themselves remain dark green is the classic and very characteristic symptom of magnesium deficiency. Why it happens: magnesium (Mg) is the central atom of the chlorophyll molecule; when a vine is deficient in magnesium it is unable to produce sufficient chlorophyll; the deficiency symptoms appear first and most severely in the older lower leaves because the vine preferentially translocates the limited available magnesium to the younger actively growing upper leaves; the yellow interveinal pattern (veins stay green while tissue between them turns yellow) is characteristic of magnesium deficiency rather than nitrogen deficiency (which turns all leaf tissue uniformly pale green or yellow) or iron deficiency (which appears on the youngest leaves first). Contributing factors: sandy or chalk soils have naturally low magnesium content; heavy cropping in a restricted root zone depletes soil magnesium; high potassium levels in soil can interfere with magnesium uptake. Treatment: fastest treatment is foliar spray of magnesium sulphate (Epsom salts: 20g per litre of water) sprayed directly onto yellowing leaves in early morning or evening on a dry day; repeat every 2 weeks through growing season until deficiency symptoms resolve; for longer-term correction apply powdered dolomitic limestone around the base of the vine in autumn or early spring.

How do I grow a grape vine on a UK wall?

Choosing the right wall: the ideal wall for a UK grape vine is a south-facing or south-west-facing brick or stone wall in full sun; at least 1.8 to 2.4 m high for adequate training space; a south-facing wall in a sheltered warm UK garden in the south of England will allow most dessert grape varieties to ripen successfully; in northern UK regions east-west-facing walls or unheated greenhouses give more reliable results for dessert grapes. Choosing the right variety: for outdoor UK walls choose varieties selected for cool UK conditions: tried and tested outdoor UK dessert grape varieties include 'Boskoop Glory', 'Seyval Blanc', 'Madeleine Angevine', 'Regent', 'Rondo', and 'Lakemont' (seedless); for indoor greenhouse growing the classic UK greenhouse dessert grape is 'Black Hambrough' (syn. 'Schiava Grossa'). Training and pruning: the most common training systems for UK wall-grown vines are the modified Guyot system the rod-and-spur system (spur-pruned cordon) or simple espalier; all systems require annual winter pruning (December to February before sap rise) to manage size and to remove previous season's fruiting wood; correct annual pruning is essential for good fruit production and disease management on UK wall-grown vines. Watering and feeding: vines in the open ground rarely need supplementary watering in UK conditions once established; apply a balanced fertiliser or specialist vine fertiliser in early spring; mulch with well-rotted garden compost around the base of the vine (but not touching the stem) in spring.