Common Onion Pests and Diseases (and How to Control Them)
By Ada Jones · Updated October 2026 · 8 min read
Short answer: the onion pests and diseases that actually cause crop loss for home growers are thrips (tiny sap-sucking insects causing silvery streaks), onion maggots (larvae that tunnel into young bulbs, especially in cool wet spring soil), and the fungal diseases downy mildew, purple blotch, and white rot. Most are manageable with crop rotation, airflow, and catching symptoms early — very few require chemical treatment if you’re growing at a backyard scale.
Common Onion Pests and Diseases at a Glance
| Problem | Symptoms | Control |
|---|---|---|
| Thrips | Silvery-white streaks or blotches on leaves, tiny dark specks (frass), curling tips | Insecticidal soap, reflective mulch, remove weeds that host them, encourage predatory mites/lacewings |
| Onion maggot | Seedlings wilt and die; tunnels visible in young bulbs; worse in cool, wet spring soil | Rotate beds yearly, use floating row covers at planting, avoid planting right after a cover crop turn-in |
| Downy mildew | Pale yellow patches that develop grey-purple fuzz in humid mornings | Improve airflow, avoid overhead watering, copper fungicide at first sign |
| Purple blotch | Small water-soaked spots enlarging into purple-brown zoned lesions on older leaves | Same as downy mildew; remove infected leaves promptly |
| White rot | Yellowing, wilting, white fluffy fungal growth at the base with small black sclerotia | No cure once present — remove and destroy plants, don’t replant alliums in that soil for 8+ years |
| Onion thrips vs. spider mites | Both cause stippling, but thrips leave black frass specks; mites leave fine webbing | Confirm with a hand lens before treating — treatments differ |
Thrips: The Most Common Home-Garden Pest
Thrips are the pest most backyard onion growers will actually encounter. They’re barely visible to the naked eye, feed by rasping the leaf surface, and leave behind silvery or white streaked patches, especially in hot, dry weather. Heavy infestations stunt bulb size since the plant can’t photosynthesize efficiently through damaged leaves.
- Check leaves weekly by tapping them over white paper — thrips will fall off and be visible as tiny moving specks
- Reflective aluminum mulch between rows measurably reduces thrips landing rates
- Insecticidal soap or spinosad spray, applied to the leaf axils where thrips hide, for active infestations
- Keep nearby weeds (especially grasses) mowed, since they’re a common overwintering host
Onion Maggots: A Spring Problem
Onion maggot flies lay eggs at the base of young plants in cool, wet spring conditions, and the larvae tunnel into the developing bulb, often killing seedlings outright or leaving transplants with a soft, rotting base. Because the adult flies are drawn to the smell of previously disturbed onion soil, the single most effective defense is rotation: never plant onions, garlic, or leeks in the same bed two years running.
Floating row covers installed at transplant time, before the adult flies are active, physically block egg-laying and are the most reliable non-chemical control for home gardens.
White Rot: The One Disease With No Cure
White rot is the disease to take most seriously, because its fungal sclerotia can survive in soil for over a decade with no host present. Symptoms are yellowing and wilting above ground with a white, fluffy fungal mat and small black poppy-seed-like sclerotia at the base below ground. There’s no treatment — infected plants should be removed (not composted) and that bed avoided for all allium crops for at least 8 years. If you buy sets or transplants, inspect the base before planting, since white rot is most often introduced on infected planting stock.
FAQ
Do I need to spray my onions preventatively?
Most home gardens don’t need routine spraying. Crop rotation, airflow, and weekly visual checks catch the majority of problems before they need chemical intervention.
Can I still eat onions with minor pest damage?
Yes — cosmetic leaf damage from thrips doesn’t affect the edibility of the bulb. Discard any bulb with soft rot, mold, or tunneling from maggots.
How do I tell thrips damage from sunscald or drought stress?
Thrips leave small dark frass specks within the silvery streaks; drought/sunscald damage is more uniform tan-brown without the black specks. A hand lens makes the difference obvious.
Final Thoughts
Rotation and airflow prevent more onion problems than any spray. Identify what you’re actually seeing before treating — see our yellow leaf diagnosis guide if the symptom is yellowing without visible pests or lesions.
How This Guide Was Checked
Compiled from university extension pest/disease fact sheets and grower-reported cases on Reddit and gardening forums. Not sure what’s affecting your plants? Contact us with a photo.
Related Onion-Growing Guides
- How to Harvest, Cure, and Store Onions for Months
- Why Are My Onion Leaves Turning Yellow? Diagnosis Guide
- Why Onions Bolt (and What to Do When It Happens)
- Why Your Onions Aren’t Forming Bulbs (or Are Too Small)
See the full How to Grow Onions guide for the complete seed-to-harvest walkthrough.