Why Are My Leek Leaves Curling?

Leek rust

Leek rust (Puccinia allii) is the most common disease of leeks in UK gardens and is also the most visually striking: the bright orange, powdery pustules on the flat leaf surfaces are unmistakable. The pustules erupt through the leaf surface, and leaves curl and yellow around the infection sites as the rust spreads. Rust is most severe in late summer and autumn, when the combination of warm days and cool, dewy nights provides ideal conditions for spore germination. It affects all allium family crops and spreads readily via airborne spores from neighbouring infected plants.

What to do

• Remove and destroy the worst-affected outer leaves to reduce the spore load. The inner leaves and the white shaft of the leek are typically less affected, and peeling back outer leaves at harvest usually reveals clean, rust-free inner tissue.
• Space leeks adequately: 15 to 20 centimetres between plants in the row and 30 centimetres between rows. Crowded plantings create the humid, poorly ventilated conditions that favour rust development.
• Rotate allium crops each year to different beds, and remove all plant debris at the end of the season to reduce the rust spore load overwintering in the garden.
• Some leek varieties show better rust tolerance than others: varieties bred for autumn and winter use such as 'Musselburgh', 'Bandit', and 'Oarsman' are generally considered more tolerant in UK conditions.

Leek moth

Leek moth (Acrolepiopsis assectella) is an increasingly significant pest of leeks, garlic, onions, and chives in UK gardens, now established across much of England and spreading northward. The small, pale-brown moth lays eggs on the leaves of allium plants, and the caterpillars mine into the leaf tissue, creating pale, papery, windowed patches that cause the leaf to curl around the damaged area. In severe attacks, caterpillars burrow into the central leaves and the stem, causing the plant to yellow and collapse. There are two generations per year in the UK: April to May and July to September.

What to do

• Cover leek crops with fine insect mesh (maximum 0.8mm aperture) from transplanting through to harvest. This is the most effective preventive measure, excluding the adult moth from laying eggs on the plants.
• Remove and destroy affected leaves to reduce caterpillar numbers. Inspect the growing point of affected plants and remove caterpillars found there before they cause further damage to the central stem.
• In areas with established leek moth populations, prioritise mesh protection from July onward when the second generation of moths is active and the leeks are approaching their most vulnerable stage.

Downy mildew

Onion downy mildew (Peronospora destructor) affects leeks as well as onions and garlic. It produces pale oval patches on the leaf surface, developing into a grey or purple-grey fuzzy coating as the spores emerge. Affected leaf tips yellow and curl, and the disease progresses down the leaf rapidly in wet conditions. Downy mildew on leeks is most severe in cool, wet springs and autumns, and in plantings with poor airflow.

What to do

• Remove and destroy affected leaves and plants promptly to reduce spore spread. The disease moves rapidly via airborne spores and swift removal of affected material slows its progression through the crop.
• Ensure adequate plant spacing and weed control to improve airflow and reduce the humidity within the crop. Avoid overhead irrigation; water at the base of plants if possible.
• Rotate allium crops each year: the downy mildew oospores can persist in the soil and on plant debris for several years, and rotation reduces the soil inoculum over time.

Onion thrips

Onion thrips (Thrips tabaci) rasp the surface cells of leek leaves, causing white or silvery streaking and stippling on the flat leaf surfaces. In heavy infestations, the leaves may curl and the overall vigour of the plant is reduced. Thrips populations build up in hot, dry summer conditions and are most damaging from July to September.

What to do

• Spray with insecticidal soap or pyrethrin-based spray in the early morning when thrips are least active, covering the leaf surfaces thoroughly. Repeat every 5 to 7 days for 2 to 3 weeks to address the damage before populations build further.
• Maintaining adequate soil moisture during hot, dry spells reduces the severity of thrips damage; stressed, drought-affected plants are more vulnerable to thrips than well-watered ones.

Drought

Drought causes leek leaves to lose their upright posture and curl or droop. The broad, flat leaves have significant surface area and lose moisture readily in warm, dry conditions. Drought stress during the period of stem development (the white shaft) reduces the final thickness and length of the edible portion and reduces overall yield.

What to do

• Water leeks consistently during dry spells from transplanting through to the end of the growing season. Leeks benefit from deep watering every 10 to 14 days in dry conditions rather than frequent shallow watering.
• Apply a mulch of compost between the rows to conserve soil moisture and suppress weeds that compete for water. Mulching also improves soil structure over time, which aids moisture retention in subsequent seasons.

Frequently asked questions

Why are my leek leaves curling and going yellow?

Leek leaves curl and yellow most often from leek rust, leek moth damage, or onion downy mildew. Leek rust (Puccinia allii) is the most widespread leek disease in UK gardens, producing bright orange pustules on the flat leaves that cause them to yellow and curl around the infection sites. Leek moth (Acrolepiopsis assectella) caterpillars mine into the leaf tissue, leaving pale, papery patches that cause the leaf to curl around the damaged area. Onion downy mildew (Peronospora destructor) causes the leaf tips to yellow and collapse with a grey-purple fungal coating in cool, wet conditions.

Is leek rust serious?

Leek rust is rarely fatal to leek crops but can significantly reduce the quality and appearance of the harvest, particularly on the outer leaves. Mild to moderate rust infections on otherwise healthy leeks have relatively little effect on the edible white shaft of the plant, and the outer leaves can be peeled back to reveal clean, uninfected inner leaves at harvest. Severe rust infections that develop early in the season (before August) can reduce plant vigour and the development of the white shaft. There are no approved fungicides for home gardeners targeting leek rust in the UK; cultural management is the primary approach.

How do I grow leeks successfully in the UK?

Leeks are sown from January to April (in a greenhouse or on a windowsill for the earliest sowings, or outside from March) and transplanted to their final growing position from June to July when the seedlings are pencil thickness. Plant into holes made with a dibber, dropping the seedling into the hole and filling it with water rather than soil, which blanches the lower stem and creates the characteristic white shaft. Space 15 to 20 centimetres apart in the row and 30 centimetres between rows. Leeks are cold-hardy and can be harvested from October through to April the following year depending on variety.

What are the pale, papery patches on my leek leaves?

Pale, papery patches on leek leaves are most likely caused by leek moth caterpillars (Acrolepiopsis assectella) mining between the layers of leaf tissue and consuming the internal tissue while leaving the outer skin intact. This creates characteristic windowed patches where the outer leaf is transparent and papery over the excavated area. Leek moth is now established across much of southern and central England and is spreading northward. The damage may be accompanied by small entry or exit holes where the caterpillar has moved between leaves. Fine insect mesh covering the crop from transplanting prevents the adult moths from laying eggs on the plants.