Onion Pests and Diseases: Onion Fly, Thrips, Mildew and Rot

The most common onion pests are onion maggots (larvae of the onion fly) and thrips. Key diseases are downy mildew in cool, wet weather and neck rot in storage. Rotation, good spacing and proper curing prevent most problems.

Pests

• Onion maggot / onion fly: wilting seedlings and tunnels in bulbs; cover with fine mesh in spring, rotate beds.
• Thrips: silvery streaks on leaves in hot, dry weather; spray water or insecticidal soap.
• Leek moth (Europe): caterpillars mining leaves; fine mesh.

Diseases

• Downy mildew: pale, grey-purple fuzz on leaves; space plants and avoid overhead watering.
• Neck rot (Botrytis): soft, grey mouldy necks in storage; cure properly.
• White rot: white fluffy growth and black specks on the base; long rotation.
• Purple blotch: purple lesions on leaves in warm, humid weather.

Prevention

• Rotate alliums every 3–4 years.
• Avoid excess nitrogen and wet foliage.
• Harvest and cure promptly.

More Onion Guides

• How to Grow Onions: The Complete Guide
• When to plant onions
• Short-day vs long-day onions
• Onion sets vs seeds and planting

Frequently Asked Questions

What are the white streaks on my onion leaves?

Usually thrips feeding damage.

Why do onions rot in storage?

Most often neck rot from poor curing or damp storage.