Why does my lettuce have brown spots?

The 4 most likely causes

The cause of lettuce 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 lettuce, 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 lettuce 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. Lettuce needs a pot with a drainage hole, a chunky free-draining mix, and a watering rhythm of shallow daily watering — lettuce roots are shallow. 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 lettuce in particular

Because lettuce leaves are thin, soft, and quick to grow, they show fungal and bacterial spotting readily. In cool, wet spells look first at downy mildew: angular yellow blotches that brown with age, backed by pale mould underneath. Circular tan lesions with concentric rings suggest anthracnose, while water-soaked spots that dry papery point to bacterial leaf spot. Brown flecking confined to the margins of young inner leaves, however, is tipburn, a calcium disorder rather than an infection.

Normal turnover, or real trouble?

Not every mark is disease. Reddish-brown flecking or russeting along midribs and edges, and the rusty tinge that appears where a leaf has been cut or bruised, is oxidation, natural to many lettuce types rather than an infection. Wind, hail, and handling leave brown nicks at the wound. The test is behaviour: static, dry marks with no mould, halo, or ring stay cosmetic, while spots that enlarge or weep are the ones to act on.

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 lettuce usually mean low humidity or tap-water minerals.

The fix — step by step

For fungal or bacterial spotting that thrives on wet leaves, the first move is to stop wetting the foliage: switch to base or drip watering, water early so leaves dry before nightfall, and thin or space plants so air moves through. Pick off spotted leaves promptly, and on a fast crop it's usually better to harvest the clean leaves and clear badly hit plants than to spray. If it's tipburn margins, that's physiological, not disease.

This is the recovery sequence Growli walks users through for lettuce 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 lettuce 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 lettuce 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 lettuce 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 lettuce, the single biggest preventative is matching its native rhythm: shallow daily watering — lettuce roots are shallow, 4-6 hours of sun (afternoon shade in heat), 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
• Is lettuce toxic to cats and dogs? — pet-safe, per the ASPCA
• How often to water lettuce — the watering brief
• When to repot lettuce — pot size and timing
• Brown spots on plants — the full diagnostic guide across species
• What's wrong with my plant? — Growli's flagship diagnosis index
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