GrowingHQ

Calathea: spreading brown or black patches on leaves and stems

2 causes GrowingHQ holds specifically for calathea, what separates them, and what is safe to do before you are sure.

What you are looking at

Dark, water-soaked-looking lesions that enlarge over days, often with a pale halo, sometimes with a white fuzz on the leaf underside in damp weather. On potatoes and tomatoes in a wet summer this is the sign that matters most.

What it is likely to be on calathea

  1. Leaf scorch

    Candidate 1common on this plant
    source

    Sourced guidancemoderate confidence

    • Royal Horticultural Society: Houseplants — light

    Checked 2026-08-13

    Dry, brown, brittle patches or margins where tissue has died, typically on the side facing the light and on the most exposed leaves. Unlike a disease it does not spread, and unlike drought it affects the exposed leaves rather than the whole plant.

    Points towards it

    • · scorch follows a move into stronger light within days
    • · only the leaves facing the light are marked

    Argues against it

    • · damage spread evenly argues against direct sun, which comes from one direction

    Check this next

    Check whether the patches are on the leaves nearest the window and whether they appeared after a move. Scorch is sharply defined, pale brown and papery, and confined to the exposed side; watering problems mark the whole plant.

    What to do now

    Move it out of direct sun or diffuse the light with a blind. Scorched tissue never recovers, so the test is whether new leaves come through unmarked rather than whether the damaged ones improve.

    source

    Sourced guidancehigh confidence

    • Royal Horticultural Society: Houseplants — light

    Checked 2026-08-13

  2. Grey mould

    Candidate 2occasional on this plant
    source

    Sourced guidancemoderate confidence

    • Royal Horticultural Society: Grey mould

    Checked 2026-08-13

    Soft fruit turning brown and collapsing under a fuzzy grey coat, often overnight in wet weather. It is the single biggest cause of strawberry losses in a wet British summer, and it infects at flowering rather than when the mould appears.

    Points towards it

    • · a soft grey coating over the damage is botrytis
    • · botrytis needs sustained wetness and still air

    Argues against it

    • · damage that appeared straight after a move into brighter light is scorch rather than a fungus that takes days to establish

    Check this next

    Check whether the brown area is furry and whether the air around the plant moves. Botrytis needs still, damp air and produces visible grey growth; scorch is dry and papery, and appears on the side facing the light rather than in the congested middle of the plant.

    What to do now

    Remove affected leaves and open the plant up so air moves through it. Space congested clumps and stop watering over the foliage; a plant in still damp air will keep producing this whatever is removed.

    source

    Sourced guidancemoderate confidence

    • Royal Horticultural Society: Grey mould — control

    Checked 2026-08-13

Narrow it down

This page lists everything at once, which is the right shape for reading and the wrong shape for deciding. The interactive tool asks one question at a time, starting with whichever would separate these causes fastest, and stops as soon as further answers would not help.

Work through it for your calathea

Opens with the plant and the symptom already set. Nothing you answer is stored or sent anywhere — the answers live in the address bar and nowhere else.

What this does not cover

These are the causes GrowingHQ holds rules for on calathea — not every cause there is. Where several problems look alike, the honest answer is often that they cannot be told apart by eye, and a page that named one would be guessing. If none of the above fits, the general spreading brown or black patches on leaves and stems page lists causes across all plants, and the Calathea profile lists the problems it is prone to.