Why Are My Onion Leaves Curling?

Onion fly

Onion fly (Delia antiqua) is the most destructive pest of onions in UK gardens. The adult fly, which resembles a small grey house fly, lays its eggs at the base of onion plants from late April through August. The white maggot larvae burrow into the base of the bulb, consuming the tissue and roots. The first sign above ground is the outer leaves yellowing and then collapsing limply at the base, while the inner leaves may appear temporarily green before they too collapse. Pulling up an affected plant reveals the white maggots at the base of the bulb and in the rotting root system.

What to do

• Cover onion beds with fine insect mesh (maximum 0.8mm aperture) from planting or sowing until harvest. This is the most reliable preventive measure for onion fly.
• Remove and destroy affected plants immediately: do not compost them as the maggots are still active and will pupate in the compost heap.
• Grow onions from sets rather than seed where possible: sets establish quickly and develop a thicker, more robust base before the peak onion fly season.
• Avoid planting onions in the same bed two years running; crop rotation disrupts the soil pupae cycle.

Downy mildew

Downy mildew of onions (Peronospora destructor) is a serious fungal-like disease that thrives in cool, wet UK growing seasons. It appears initially as pale oval patches on the leaf surface, then develops into a grey or pale purple fluffy coating as the spores emerge. Affected leaves yellow from the tip downward and eventually collapse. The disease spreads rapidly in wet weather and can move from plant to plant via airborne spores within days.

What to do

• Remove and destroy affected leaves and plants promptly: this reduces the spore load available to infect surrounding plants. Do not compost diseased material.
• Improve airflow around plants by spacing adequately (at least 10cm between sets) and removing weeds that increase humidity within the bed.
• Avoid overhead watering which keeps foliage wet; water at the base of plants or use drip irrigation where possible.
• Rotate onion family crops (onions, shallots, leeks, garlic) to different beds each year to avoid building up disease spores and white rot resting bodies in the soil.

Onion white rot

Onion white rot (Sclerotium cepivorum) is a soil-borne fungal disease that causes yellowing and collapse of the leaves combined with a distinctive white, fluffy fungal growth at the base of the bulb, often with small black resting bodies (sclerotia) visible within the white mycelium. It is one of the most persistent and damaging onion diseases because the sclerotia can survive in the soil for 15 to 20 years, making infected beds unsuitable for growing any allium family member for the long term.

What to do

• Remove and destroy all infected plants, taking as much of the surrounding soil as possible with the roots to reduce the sclerotia left behind. Do not compost infected material.
• Once white rot is confirmed in a bed, cease growing all allium family crops (onions, leeks, garlic, shallots, chives) in that soil for as long as possible. There is no practical treatment to eliminate white rot from infected soil in the domestic garden.
• Ensure all tools, boots, and equipment used in an infected bed are cleaned thoroughly before use elsewhere to avoid carrying sclerotia to unaffected beds.

Drought

Drought causes onion leaves to yellow from the tip and eventually collapse. The tubular leaves lose turgor and may droop and curl inward during the hottest part of the day, recovering slightly in cooler evening conditions in mild drought. In severe drought, the leaves die back prematurely and the bulb stops growing. Onions have relatively shallow roots concentrated in the top 20 to 30 centimetres of soil and are more vulnerable to surface drought than deeper-rooted crops.

What to do

• Water onions consistently during dry periods, particularly from early May when the bulbs begin to swell. The period of bulb swelling is when consistent moisture has the greatest impact on final bulb size.
• Stop watering completely once the leaves begin to yellow and fall over naturally in midsummer: at this point the bulbs are maturing and excess moisture can cause disease problems including neck rot during storage.
• Apply a light mulch of compost between the rows to retain soil moisture without burying the bulb necks (which should remain above soil level to prevent rotting).

Thrips

Onion thrips (Thrips tabaci) are tiny, slender insects that rasp the surface of onion leaves, causing silvery-white streaking and speckling on the foliage. In heavy infestations the leaves may curl and distort, and the overall vigour of the plant is reduced. Thrips populations build up in hot, dry conditions and are less active in cool, wet UK summers. They also transmit some viral diseases of alliums.

What to do

• A strong spray of water can dislodge thrips populations from the leaves. For more persistent infestations, insecticidal soap or pyrethrin-based sprays applied to the leaf surface, covering both sides of the leaves, provide effective control.
• Encourage natural predators by avoiding broad-spectrum insecticides. Predatory mites and parasitic wasps provide biological control of thrips populations in gardens where natural enemy populations are maintained.

Frequently asked questions

Why are my onion leaves curling and collapsing?

Onion leaves curl and collapse most often from onion fly damage, downy mildew, or onion white rot. Onion fly (Delia antiqua) larvae feed on the base of the bulb and lower leaves, causing the outer leaves to yellow, wilt, and collapse from the base. Downy mildew (Peronospora destructor) causes the leaves to develop a grey-purple fungal coating and collapse from the tip downward. Onion white rot (Sclerotium cepivorum) causes yellowing and collapse of the leaves combined with a white, fluffy fungal growth at the base of the bulb. All three are common problems in UK onion growing and can occur at any point in the growing season.

What is onion downy mildew and how do I treat it?

Onion downy mildew is a fungal-like disease caused by Peronospora destructor that affects onions, shallots, and leeks in the UK. It appears as a grey or purple-grey fuzzy coating on the leaf surface, particularly in cool, damp conditions. Affected leaves develop pale patches where the mildew is growing, then yellow and collapse from the tip downward. The disease spreads rapidly in wet weather and can devastate an onion crop within weeks of first appearing. There are no approved fungicides for home gardeners effective against onion downy mildew; cultural controls are the primary management tools.

Why are my onion leaves going yellow from the tip?

Onion leaves yellowing from the tip downward most often indicates downy mildew, drought stress, or natural maturation at the end of the growing season. Downy mildew causes tip yellowing combined with a grey fuzzy coating on the leaf surface, and the yellowing progresses rapidly from the tip to the base of the leaf. Drought causes tip yellowing without fungal growth, and watering usually slows the progression. Natural leaf die-back from the tip is normal as onions approach maturity in midsummer, when the leaves progressively collapse and the neck of the bulb shrivels as a signal that the bulb is ready to harvest.

How do I prevent onion fly in the UK?

Onion fly can be prevented most effectively by covering the crop with fine insect mesh (maximum 0.8mm aperture) from planting to harvest. The adult flies are attracted to the smell of onions to lay their eggs, and the mesh barrier prevents access. Growing onions from sets (small bulbs) rather than seed provides a head start that reduces vulnerability during the period when seedlings are most at risk. Transplanting seedlings grown under cover in modules rather than direct-sowing outside also reduces the window of vulnerability. Onion fly populations are lower in cooler springs and in exposed, breezy positions where adult flies struggle to land and lay eggs.