Wipe a patch with a damp cloth. If it comes away clean and does not return, it was hard water. If it smears and returns, it is fungal. If it is fluffy and sits in the leaf joints, those are insects.
The wipe test
Before treating anything, rub one affected area gently with a damp cloth or a cotton bud.
- Comes off cleanly, leaf is green underneath, does not come back: mineral deposit from hard water or from misting. Cosmetic, harmless.
- Smudges like flour and reappears over days: powdery mildew, a fungus. It spreads and needs treating.
- Comes away as small waxy tufts, maybe leaving a sticky film: mealybugs. Insects, and they will move to your other plants.
- Fine pale speckling rather than white coating, with faint webbing: spider mites. Look for movement under a phone camera zoom.
What each one needs
Powdery mildew
- What it looks like
- A dusty white or grey film, usually starting on the upper surface of older or lower leaves, spreading in patches that merge. Leaves may curl or yellow beneath the film.
- Why it happened
- Still, humid air and poor airflow, often crowded plants or a room that never gets ventilated. Unlike most fungal problems it does not need wet leaves to take hold.
- What to do
- Remove the worst-affected leaves and bin them rather than compost them. Improve airflow and space the plants out. Treat the rest with a suitable fungicide, or a horticultural oil such as neem, repeating on the label's schedule. Stop misting, and water the soil rather than the foliage.
Mealybugs
- What it looks like
- Small cottony white tufts tucked where leaves meet stems, on new growth, and on the undersides. Often a sticky residue on leaves below, and sometimes a black sooty mould growing on that residue.
- Why it happened
- Almost always arrived on a new plant. They spread slowly by crawling, so a single untreated plant infects a whole shelf over months.
- What to do
- Isolate the plant away from the others today. Dab each visible insect with a cotton bud dipped in rubbing alcohol, then wash both sides of every leaf. Follow with insecticidal soap or neem oil weekly for at least three weeks: the eggs hatch after the first treatment, and one pass never clears them. Check neighbouring plants.
Hard water or mineral residue
- What it looks like
- Flat white speckles or a chalky film, often in the pattern of dried droplets, with matching crust on the soil surface or the pot rim. The leaf underneath is healthy.
- Why it happened
- Misting or overhead watering with hard tap water. The water evaporates and the dissolved minerals stay behind.
- What to do
- Nothing urgent. Wipe the leaves with a damp cloth. To stop it recurring, water the soil rather than the leaves, and use filtered or stood-out water on plants you like to keep glossy.
Why getting this right matters
These three want opposite responses. Raising humidity helps a plant with mineral spotting and actively feeds powdery mildew. Treating limescale with a fungicide does nothing except stress the leaf. And leaving mealybugs while you adjust the humidity costs you the rest of your plants.
Related symptoms
- Yellow leaves, which pests cause as fine stippling rather than whole-leaf yellowing.
- Brown, crispy tips, another symptom that salt build-up produces.
- All plant symptoms.
General guidance, not a diagnosis of your plant. Fungus and insects are easy to confuse at a glance, and treating the wrong one wastes the weeks that matter.
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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