White powder on the leaves
Powdery mildew looks like flour and wipes off. Hard water leaves a similar film that does not. Telling them apart takes a damp cloth.
What it usually is
1 Powdery mildew
| How to tell | A white or grey dusty coating that starts as small round patches and joins up, on the upper surface first and often on the newest growth. It wipes off with a damp cloth and comes back within days, which limescale does not. |
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| The fix | Remove the worst leaves, move the plant somewhere with better air movement, and stop wetting the foliage. A spray of one part milk to nine parts water, or a proprietary fungicide for ornamentals, checks it. Crowded plants in still damp air are what feed it. |
2 Limescale from hard water
| How to tell | A fine white film, often in the shape of dried droplets, on plants that are misted or watered from above in a hard water area. It wipes off and does not return until the next misting. |
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| The fix | Wipe the leaves with a damp cloth, and mist with rainwater or filtered water if you mist at all. It is cosmetic and does the plant no harm beyond blocking a little light. |
3 Mealybug wax mistaken for powder
| How to tell | White that is fluffy rather than dusty, gathered in the joints where a leaf meets a stem and under the leaf, rather than spread evenly across the surface. |
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| The fix | Treat it as mealybug rather than mildew: dab each one with a cotton bud dipped in surgical spirit, then wash the plant, and repeat weekly for a month. |
Through the year. Powdery mildew wants warm days, cool nights and still humid air, which is autumn indoors and late summer outdoors. It is one of the few fungal problems that does not need the leaf to be wet.
Three quite different things look like white powder on a leaf, and the fastest way to sort them out is a damp cloth and a week of watching.
Wipe a patch. If it comes away and does not return, it was limescale from hard water, which is cosmetic and only worth removing so the leaf gets its light. If it comes away and reappears in the same place within a few days, it is powdery mildew, a fungus growing on the surface and feeding through the leaf. If it does not really wipe so much as smear, and it is concentrated in the joints rather than spread across the surface, look closely: mealybugs coat themselves in white wax and are often mistaken for mildew by people who have not met them before.
Powdery mildew is unusual among fungal diseases in not needing wet leaves. It wants warm days, cool nights, still air and moderate humidity, which is exactly a room in autumn with the window shut. The practical remedies are therefore about air rather than about water: space the plants further apart, open the door, run a fan in the room for an hour a day if the plants are crowded, and stop misting.
Where treatment is needed, remove the worst affected leaves first, because they carry the spores. Milk diluted one part in nine has repeatedly been shown to check powdery mildew on ornamentals, and proprietary fungicides for ornamental plants work as well. Both are preventive rather than curative: they stop the fungus taking new tissue and do nothing for the leaves already coated.
Mildew rarely kills a houseplant. What it does is cover the surface the plant lives by, so a badly affected plant slows down, drops its worst leaves and looks poor. Catching it early costs one leaf and an open window; leaving it costs a season of growth.
Entries that name it
What the journal says about this symptom on each plant.
| African violet | Powdery mildew, which follows still damp air. Improve the air movement around the plant, remove the worst leaves, and avoid wetting the foliage. |
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| Blackcurrant | Mildew on crowded growth in a still, dry season. Prune out a third of the oldest wood each winter so light and air reach the middle, and water the soil deeply in dry spells. |
| Cherry laurel | Powdery mildew, most often on the leaf underside in warm, humid, still conditions. Trim off the worst-affected leaves. Avoid a hard prune while it is active, since that adds stress the plant does not need on top of the mildew. |
| Courgette | Powdery mildew on the older leaves, which is inevitable by late season. Water the roots deeply, remove the worst leaves, and accept it. A plant usually crops well for weeks after mildew appears. |
| Cucumber | Powdery mildew, which arrives with the first cool nights. Keep the roots moist, give the plant air, and remove the worst leaves. Resistant varieties are worth choosing. |
| Gooseberry | American gooseberry mildew, powdery grey-white patches on leaves and shoot tips that turn brown as it progresses, the most serious disease this plant gets. Prune to an open shape for good airflow, avoid feeding it too much nitrogen, and remove infected shoot tips and fallen leaves rather than leaving them. Resistant cultivars exist where mildew is a recurring problem locally. |
| Grape ivy | Powdery mildew in still, crowded conditions. Thin the growth so air moves through it and water the compost rather than the leaves. It is the one real weakness of this plant. |
| Mint | Powdery mildew, which follows dry roots and still crowded air. Water more evenly, thin the stems, and cut a badly affected plant right down so it regrows clean. |
| Peas | Powdery mildew in dry soil and still air. Water the roots well, mulch, and give the row space. Late sowings mildew worst. |
| Photinia | Powdery mildew, an occasional secondary problem in humid, still conditions. Improve airflow around the plant and avoid wetting the leaves when watering. It is rarely as persistent here as the leaf spot, and usually clears once the weather changes. |
| Polka dot begonia | Powdery mildew, which begonias are unusually prone to in still damp air. Improve the air movement, water the compost rather than the leaves, and take off the worst affected foliage. |
| Pot marigold | Powdery mildew late in the season on a plant that has run out of water. Cut the whole plant back by half and soak it. It will produce a fresh flush of clean growth within a fortnight. |
| Purple shamrock | Powdery mildew in still crowded growth. Thin the stems, improve the air movement, and water at the base. |
| Rex begonia | Powdery mildew, which follows still humid air around a crowded plant. Improve the air movement, remove affected leaves, and keep water off the foliage. |
| Rose | Powdery mildew, from dry roots and stagnant air. Soak the ground properly and thin the centre of the bush so air can move through it. Mildew is a drought signal more than a damp one. |
| Rosemary | Powdery mildew, which follows still humid air and crowded growth. Thin the plant to let air through, water at the base, and remove the worst shoots. |
| Sage | Powdery mildew in still, humid air. Give it space and sun. Mildew on sage is nearly always a crowding problem rather than a watering one. |
| Scarlet sage | Powdery mildew on plants that were allowed to go dry, then crowded together. Space the plants further apart, water the soil rather than the leaves, and remove the worst affected growth. |
| Sweet pea | Powdery mildew late in the season on plants with dry roots. Soak the ground deeply and mulch it. Mildew on sweet peas is nearly always a signal of drought at the root rather than damp air. |
Questions from the margin
- What is the white powder on my plant's leaves?
- Wipe it with a damp cloth. If it comes off and returns within days it is powdery mildew, a fungus growing on the leaf surface. If it comes off and does not return it was limescale from hard water. If it is fluffy and gathered in the joints, it is mealybug.
- How do I treat powdery mildew on a houseplant?
- Remove the worst leaves, move the plant somewhere with better air movement, and stop wetting the foliage. Milk diluted one part in nine, or a fungicide for ornamentals, checks it. Neither repairs a leaf already coated.
Checked against RHS, Powdery mildews; University of California IPM, Powdery mildew on ornamentals.