Why does my onion have brown spots?

The 4 most likely causes

The cause of onion brown spots usually narrows to one of the items below, ranked by how often we see each in Growli's diagnostic chats. Work down the list — most readers find their answer in the top two.

1. Fungal or bacterial leaf spot (Possible)
   Brown spots with yellow halos appearing on onion, especially after water sat on the leaves overnight, are most often fungal or bacterial leaf spot. Remove affected leaves, water only at the soil line, improve airflow, and apply a copper fungicide if it spreads.
2. Sunburn or intense direct sun (Possible)
   Bleached patches, papery brown spots, or crisped leaf surfaces on the south-facing side of onion are sunburn. Acclimatise it more gradually after a move, or filter midday sun with a sheer curtain. Sunburn damage doesn't heal — wait for new growth.
3. Overwatering or poor drainage (Likely)
   In most homes overwatering is more often a drainage problem than a frequency problem. Onion needs a pot with a drainage hole, a chunky free-draining mix, and a watering rhythm of about an inch of water per week until the bulbs swell. Soggy soil drowns the roots and the first symptom you see above ground is yellowing or wilting foliage.
4. Pests sucking sap (Possible)
   Sap-sucking pests (aphids, spider mites, mealybugs, scale) all cause similar symptoms: yellow stippling, distorted new growth, sticky residue. Inspect leaf undersides and stem joints with strong light. Most are controlled with weekly insecticidal soap or a horticultural oil spray.

Why onion in particular

Onion foliage is prone to fungal leaf spotting once conditions turn damp. Purple blotch, from Alternaria, starts as small water-soaked flecks that develop white centres, then enlarge into sunken brown-to-purple lesions ringed with yellow, often girdling and toppling the leaf. Downy mildew instead greys and collapses the older leaves in cool, wet spells, and thrips scars give these fungi an easy way in. Crowded, overhead-watered rows with long leaf-wetness suffer the worst spotting.

Normal turnover, or real trouble?

Onion foliage wears a pale, blue-grey waxy bloom, and scuffs where wind, hail or a passing hand rubbed that wax thin can mimic pale blotching without being disease. True leaf-spot is more organised: defined lesions, often faintly target-ringed with a yellow margin, that enlarge and can pinch the tube until it topples. A mark with structure and a coloured edge, spreading in damp weather, is fungal; a dull scuff is not.

How to diagnose in 60 seconds

Run these quick checks before you change anything — the right fix depends on what you find.

• Are the spots dry and crisp or wet and spreading? Crisp = sunburn, fluoride, or old damage; wet = active fungal or bacterial spot.
• Do the spots have a yellow halo? Halos are classic for bacterial and fungal leaf spot.
• Are they only on the sunny side of the plant? That points to sunburn.
• Are the leaf tips uniformly brown while the rest is healthy? Crispy tips on onion usually mean low humidity or tap-water minerals.

The fix — step by step

Get the leaves drying faster before anything else. Water at the soil in the morning, never over the tops in the evening, so blades aren't left wet overnight, and thin or weed crowded rows so air moves through the canopy. Strip off the worst-spotted leaves and clear them away rather than composting them beside the bed. Keeping thrips down helps too, since their feeding scars are the wounds these leaf fungi slip through.

This is the recovery sequence Growli walks users through for onion with brown spots. Work through the steps in order; skipping ahead is the most common reason a plant fails to bounce back.

1. Quarantine if you suspect disease. If spots look wet, are spreading, or have a yellow halo, move onion away from your other plants until you have a diagnosis. Disinfect your tools between plants.
2. Remove the worst-affected leaves. Snip off heavily spotted leaves at the base. Don't compost them — bag and bin to stop spore spread. Never remove more than 30% of foliage in one go.
3. Switch to filtered or rain water. If you suspect tap-water damage, water with rainwater, distilled water, or tap water that has stood uncovered for 24 hours. Continue for 4-6 weeks before re-evaluating.
4. Improve airflow and watering technique. Water at the soil line only — never on the leaves. Space onion so leaves are not touching neighbouring plants. A small fan in stagnant rooms makes a surprising difference.
5. Apply a treatment if needed. For active fungal spread, a copper-based fungicide applied weekly per the label is the standard remedy. For sunburn or fluoride damage, no treatment helps — just remove damaged leaves and wait for new growth.

When this can't be saved

Most cases of onion brown spots are recoverable, but a few red flags point to a plant that has gone past the point of return. If you spot any of these, consider propagating a clean cutting and starting over.

• Spots are spreading by several per day across multiple leaves — active aggressive disease.
• The whole crown of the plant is blackening from a central point outward.
• Healthy-looking neighbouring plants are starting to show the same symptoms.

Prevention

For onion, the single biggest preventative is matching its native rhythm: about an inch of water per week until the bulbs swell, 6+ hours of sun, and a free-draining pot with a working drainage hole. Water at the soil line, not the leaves, especially in the evening when leaves can't dry before nightfall. Switch to filtered or collected rainwater for sensitive species — it pays for itself in fewer crispy edges. Improve airflow around densely planted shelves with a small clip-on fan.

Common questions

Related guides

• What's wrong with my plant? — the diagnosis apps compared
• Brown spots on plants — the full diagnostic guide across species
• What's wrong with my plant? — Growli's flagship diagnosis index
• Onion — yellow leaves
• Onion — curling leaves
• Onion — drooping
• Monstera — brown spots