Guides · White powder and mould
Why is there white powder or mould on my plant?
It depends where it is. On the leaves, a white, dusty powder that smudges is powdery mildew, a fungus that lives on the leaf. On the compost, a grey or white fuzz is saprophytic mould, which lives on wet compost and does the plant no harm. A hard white crust on the compost or the pot is lime from tap water, and not alive at all. Three things, three fixes, and only one of them is a disease.
- What it is
- Mildew on the leaves; a harmless mould on compost that stays wet; or mineral crust from hard water
- The tell
- Leaves: white powder that wipes off, spreading. Compost: grey fuzz, often after a few days of damp. Crust: hard, white, does not smudge
- Urgency
- Low for mould and crust. Medium for mildew: it spreads leaf to leaf and plant to plant in still air
Powdery mildew — on the leaves
A white to grey powder on the upper surface of the leaves, in patches that spread and join, sometimes on the stems and buds too. It smudges under a finger. Underneath it the leaf yellows and eventually shrivels. It is a fungus, and it is happiest exactly where houseplants are kept in a British autumn: warm days, cool nights, still air, and a plant that has been dry at the roots. Begonias, African violets, rosemary and sage on a kitchen sill, and anything crowded with its neighbours get it first.
What to do. Remove the worst leaves — cut, bag, bin. Move the plant to where the air moves: a room with a window open for an hour a day, or a small fan, and space between it and the next plant. Water at the compost, not over the leaves, and do not let it dry out fully, because a thirsty plant is a mildew plant. Very few fungicides are approved for home use on houseplants in the UK, and for a few leaves you do not need one; for a plant that is covered, a sulphur-based product approved for the purpose is the category to look for, used as the label says. Keep the plant away from others until the new growth comes in clean.
What not to do. Do not mist — mildew spreads in humid, still air. Do not wipe it across the plant with a cloth; you spread the spores. Do not compost the leaves.
Grey or white fuzz on the compost
A soft, fluffy, grey-white growth on the surface of the compost, sometimes with a mushroom or two, usually a few days after watering and in a pot that stays damp. This is a saprophytic mould: it feeds on the decaying organic matter in the compost, not on the plant, and it does the roots no harm. What it tells you is that the compost is staying wet on top for days — which is the condition that does harm roots, and that fungus gnats breed in.
What to do. Scrape the top centimetre off and bin it. Let the surface dry between waterings; water less often, or from below. Improve the air around the pot. A layer of grit or sand on the surface keeps it dry and stops it coming back. If the pot smells sour as well, read the root rot guide — that is a different problem.
What not to do. Do not repot for it alone; it is a symptom of the watering, and it comes back in fresh compost watered the same way. Do not reach for a fungicide.
White crust on the compost or the pot
A hard, white or off-white crust on the compost surface, around the rim of the pot, or as a tide-mark on a terracotta pot. It does not smudge and does not grow. It is lime and other minerals left behind as tap water evaporates, and in the hard-water areas of southern and eastern England every pot gets it within a year. Harmless in itself; over time the minerals build up in the compost and show as brown leaf tips.
What to do. Scrape it off. Water with rainwater or filtered water for the sensitive plants — dracaena, spider plant, calathea, peace lily — and flush the compost through every couple of months so the salts run out of the bottom. Repot into fresh compost every couple of years. The brown leaf tips guide has the fuller case.
Telling them apart, in one look
| Where | What it looks like | What it is | Serious? |
|---|---|---|---|
| Leaves, upper surface | White powder, smudges, spreading | Powdery mildew | Yes — treat and isolate |
| Compost surface | Grey-white fuzz, soft, after watering | Saprophytic mould | No — dry the surface |
| Compost or pot rim | Hard white crust, does not smudge | Lime from tap water | No — flush, change water |
| Leaves, sticky with black film | Black sooty coating | Sooty mould on honeydew | The insect above it is — see pests |
Plants this often affects
Mildew goes for the soft-leaved and the crowded — rex begonia, African violet and kitchen-sill rosemary — while compost mould turns up in any pot watered on a schedule, and lime in any pot in the south-east.
- OleanderNerium oleander
- Polka Dot BegoniaBegonia maculata
- Rex BegoniaBegonia rex
- RosemarySalvia rosmarinus
- Bay LaurelLaurus nobilis
- BorageBorago officinalis
- Busy LizzieImpatiens walleriana
- CucumberCucumis sativus
- Grape IvyCissus rhombifolia
- HopsHumulus lupulus
- Iron Cross BegoniaBegonia masoniana
- Lemon BalmMelissa officinalis
- Madagascar JasmineStephanotis floribunda
- Mapleleaf BegoniaBegonia cleopatra
- MarigoldTagetes
- Metallic-Leaf BegoniaBegonia metallica
- PeonyPaeonia officinalis
- Pot MarigoldCalendula officinalis
- PrimrosePrimula vulgaris
- RoseRosa
- SageSalvia officinalis
- StrawberryFragaria
- ZinniaZinnia species
Common questions
Is white mould on soil bad for plants?
The fuzzy grey-white mould that grows on the compost surface is not — it feeds on the compost, not the plant. It is a sign the compost is staying wet on top for days, and that is what harms roots. Scrape it off, let the surface dry between waterings, and it does not return.
How do I get rid of powdery mildew on a houseplant?
Cut off the worst leaves and bin them, move the plant into moving air with space around it, water at the compost rather than over the leaves, and do not let it go dry at the roots. For a plant that is covered, a sulphur-based fungicide approved for home use is the category; for a few leaves, removing them and changing the air is enough.
Can I use vinegar or milk on powdery mildew?
Milk sprays and vinegar are home remedies with mixed results and no approval for use on plants in the UK; vinegar can scorch leaves. Removing affected leaves and improving the air does more, and the approved products are cheap if it comes to that.
What is the white crust on my plant pot?
Lime and other minerals from tap water, left behind as the water evaporates — the same deposit as in a kettle. It is harmless on the pot; in the compost it builds up over months and browns the leaf tips. Scrape it off, flush the compost through occasionally, and use rainwater or filtered water for the sensitive plants.
Should I repot a plant with mould on the soil?
Not for the mould alone. It comes from the compost staying wet, and fresh compost watered the same way grows the same mould. Scrape the surface, water less often or from below, and add a layer of grit. Repot only if the compost has broken down or the pot smells sour.
Still not sure?
The RHS's pages on powdery mildews cover the fungus in the garden; this page is the houseplant case, and the two look-alikes that are not diseases at all. Sabz's Check-up asks where the white is — on the leaves or the compost — and how the plant has been watered, because a mould on the surface in a pot watered every Sunday has one answer, and it is not a fungicide.
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