Onions: Pests & Diseases
Onions are dependable UK kitchen-garden crops when given sun, firm soil and steady moisture. Sets are convenient, while seed-grown plants offer a wider choice of varieties. Good hygiene and a long allium rotation matter because some damaging soil-borne diseases can remain troublesome for many years.
Common pests
Onion fly lays near young plants and its maggots tunnel into bulbs, causing yellowing and collapse. Allium leaf miner larvae make pale mines and brown pupae in leaves or stems; flights commonly occur in spring and autumn. Onion thrips cause silvery streaking in dry weather. Leek moth may damage foliage, while stem and bulb eelworm can distort plants and ruin bulbs.
Common diseases
Onion white rot causes yellowing, wilting and rotting bulbs with white fluffy fungal growth and tiny black sclerotia. It persists for years in soil. Downy mildew creates pale patches and greyish mould on leaves during damp weather. Neck rot can develop in poorly cured stored bulbs. Basal rot, damping off and soft rots are also more likely in wet soil or damaged crops.
Prevention
Grow onions on a long allium rotation, ideally avoiding affected ground for as long as practical after white rot. Never move soil, tools or plants from an infected patch to clean beds. Buy healthy sets and seed, space plants for airflow, and avoid overwatering. Cover crops with fine mesh or fleece during onion fly and allium leaf miner flights. Cure lifted bulbs thoroughly before storage.
Non-chemical control
Keep fine mesh or fleece over crops during the main spring and autumn flight periods, sealing the edges well. Remove and dispose of badly infested or rotting plants without composting them. Hand-weed carefully so roots are not disturbed. Encourage predatory ground beetles and birds nearby. For eelworm concerns, discard affected material, clean tools and avoid replanting alliums in that bed for several years.
What to look for
From spring onwards, check for yellow, floppy seedlings, leaf mines and thrips streaking. Look closely between leaves for brown allium leaf miner pupae. In wet weather, inspect foliage for pale mildew patches. At lifting, examine bulbs and roots for rot, white fungal growth or black sclerotia, then recheck stored onions weekly.
Tips
Keep plants growing steadily but stop watering as bulbs mature. Lift when foliage folds naturally and dries, then cure under cover with good airflow. Store only sound, dry bulbs in a cool, frost-free, airy place and use any damaged ones first.
Onions pests & diseases: frequently asked questions
What are the common pests of onions?
Onion fly lays near young plants and its maggots tunnel into bulbs, causing yellowing and collapse.
What diseases affect onions?
Onion white rot causes yellowing, wilting and rotting bulbs with white fluffy fungal growth and tiny black sclerotia.
How do I prevent pests and diseases on onions?
Grow onions on a long allium rotation, ideally avoiding affected ground for as long as practical after white rot.
General guidance for UK home gardens, focused on prevention and non-chemical control. The RHS or a local garden centre can help with tricky cases. Page updated 18 Sep 2026.