Look at the edge of one spot. A yellow halo around it means infection. A dry papery edge with no halo usually means sun or heat. Soft dark spots on a plant in wet soil mean the roots.
Read the edge of the spot
Colour tells you little, because almost everything ends up brown. What separates the causes is the boundary and the texture.
- Yellow ring around a brown centre: fungal leaf spot, the most common. Spreads slowly, leaf by leaf.
- Dark, soft, water-soaked looking, spreading fast: bacterial. Acts in days rather than weeks and needs decisive removal.
- Pale then papery brown patch, dry, no halo, on the side facing the window: sunburn. Does not spread once the plant is moved.
- Soft dark blotches with yellowing, on a plant in wet soil: the roots, showing up in the leaves.
The four causes
Fungal leaf spot
- What it looks like
- Round or irregular brown spots, often with a yellow halo, sometimes with tiny dark dots in the centre. Appears on lower and inner leaves first, where air moves least.
- Why it happened
- Water sitting on leaves with nowhere to evaporate: misting in a cool room, overhead watering in the evening, or crowded plants with no airflow.
- What to do
- Remove affected leaves and bin them. Stop wetting the foliage and water the soil only. Space plants out and get some air moving. Treat what remains with a houseplant fungicide or neem oil if it keeps spreading after those changes.
Bacterial leaf spot or blight
- What it looks like
- Dark spots that look wet or greasy rather than dry, sometimes with a translucent edge, spreading noticeably within days. May smell unpleasant on soft-leaved plants.
- Why it happened
- Warm, wet, still conditions, usually after water splashed between plants or a leaf was damaged.
- What to do
- Act the same day. Cut affected leaves off well below the damage, sterilising the scissors between cuts, and bin them. Isolate the plant. Keep the foliage completely dry. Bacterial problems do not respond to fungicide, and a badly affected plant is often not worth saving next to your healthy ones.
Sunburn or heat scorch
- What it looks like
- Bleached pale patches that dry to thin brown, on the surfaces that face the light, with no halo and no spread. Often after a plant was moved, or after the season changed the angle of the sun.
- Why it happened
- Direct sun through glass on a plant adapted to shade, or leaves touching a hot window. Wet leaves in strong sun make it worse.
- What to do
- Move it out of the direct beam, or diffuse the light with a sheer curtain. The damaged tissue will not recover, but nothing new should appear. Reintroduce plants to brighter spots over a couple of weeks, not in one move.
Overwatering showing in the leaves
- What it looks like
- Soft brown or black blotches, often starting at the base of the leaf or the stem rather than the tip, with lower leaves yellowing at the same time.
- How to confirm
- Soil wet days after watering, pot heavy, stem soft at soil level. This is a root problem that appears in the foliage, so treating the leaves achieves nothing.
- What to do
- Stop watering, empty the saucer, and check the roots if the stem is soft. Brown mushy roots need cutting away and the plant needs fresh free-draining soil. See drooping and wilting for the full sequence.
What not to do
- Do not mist a spotted plant. Every common cause here except sunburn is made worse by wet leaves.
- Do not compost the removed leaves if you suspect fungus or bacteria. Bin them.
- Do not reach for fungicide first. If the cause is bacterial or the roots, it will not help, and you lose the week that mattered.
Related symptoms
- White spots or powder, the other spotting pattern, usually fungal or insect.
- Yellow leaves, which alongside soft spots points at the roots.
- All plant symptoms.
General guidance, not a diagnosis of your plant. Fungal and bacterial spotting are genuinely hard to separate by eye, and they want different responses on different timescales.
Not sure which one it is?
This page narrows it down. A photo settles it: Bloom names the species, scores its health out of 100, and pairs each problem it finds with the treatment for it.
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