White powder on houseplants tends to be powdery mildew. This is a fungal infection that grows on the surface of leaves and stems rather than inside the tissue. It looks like a dusty white or grey coating, often starting in patches before spreading. Unlike many fungal diseases, powdery mildew does not need wet conditions to spread. In fact, it thrives in warm, dry air with poor airflow.
The good news is that it rarely kills an otherwise healthy plant, and caught early it is manageable with the right treatment. The harder question is why it appeared, because recurring powdery mildew usually points to a preventable environmental cause.
What Does Powdery Mildew Look Like on Houseplants?
The coating is typically white to pale grey, dry and powdery to the touch. It starts as small circular patches on the upper surface of leaves, then spreads to cover entire leaves and eventually stems. In advanced cases, affected leaves turn yellow, dry out, and drop. New growth is often most affected because it is softer and more vulnerable.
Powdery mildew is most common in late summer and autumn in UK homes, when warm days and cooler nights create the humidity fluctuations the spores prefer.
Which Houseplants Are Most Susceptible?
Some plants are significantly more prone to powdery mildew than others. Among common houseplants, Begonias are the most frequently affected: certain species and hybrids seem almost predisposed to it in UK indoor conditions.
Plants stressed by incorrect care are more vulnerable for example, a Begonia in poor light, with congested growth, or in dry centrally-heated air is far more likely to develop mildew than the same plant in good conditions.
What Causes Powdery Mildew Indoors?
Poor air circulation: Stagnant air is the most common cause in UK homes. Plants packed too closely together, placed in corners without airflow, or kept in very still rooms are much more susceptible. Mildew spores spread easily through still air and settle on nearby leaves.
Temperature fluctuations: Cool nights after warm days create condensation on leaf surfaces and raise localised humidity, ideal for spore germination. Windowsills near single-glazed windows in autumn are particularly risky.
Overcrowded growth: Dense, bushy plants with leaves touching each other or overlapping retain moisture in the canopy. Regular pruning to open up the plant improves airflow through the leaves significantly.
Incorrect watering: Watering in the evening, or getting water on the leaves, can create the surface moisture that helps mildew take hold. Water at the base of the plant in the morning where possible.
How to Treat Powdery Mildew on Houseplants
Act as soon as you notice it. Early-stage mildew is much easier to control than a well-established infection.
Step 1: Isolate and remove affected growth
Move the affected plant away from others immediately. Mildew spores spread to adjacent plants. Remove the worst-affected leaves by cutting them at the base, place them in a bag, and dispose of them in general waste (not compost). Do not shake the plant while handling it; that dislodges spores.
Step 2: Improve airflow
Before reaching for a spray, improve the plant's position. Move it somewhere with better air movement. Thin out congested growth by removing crossing stems and leaves touching each other. In many mild cases, this alone prevents the spread.
Step 3: Apply a treatment
For established mildew, a fungicide is the most effective option. FungusClear Ultimate 2 Ready to Use is effective against powdery mildew and is straightforward to apply directly to affected plants. For a larger plant or multiple plants, the FungusClear concentrate is more economical.
For a natural approach, neem oil diluted at 5ml per litre of water with a few drops of washing-up liquid works as a preventive spray and mild treatment. It needs to be reapplied every five to seven days and is more effective at prevention than cure for established infections.
SB Plant Invigorator is another option: a contact spray that coats the leaf surface and creates unfavourable conditions for fungal spores. It works best as a preventive measure or on light infections.
Whichever product you use, spray in the morning and ensure good ventilation after treatment. Coat both sides of the leaves thoroughly. Repeat every seven to ten days until no new growth appears. Browse the full Fungal and Pest Control range for other options.
Does Powdery Mildew Spread to Other Plants?
Yes, but usually only between plants of related species. Powdery mildew is host-specific: the strain affecting your Begonia is unlikely to infect your Monstera. That said, isolating an affected plant is still a good practice because mildew indicates environmental conditions that could trigger problems in other susceptible plants nearby.
Preventing Powdery Mildew
Prevention is more effective than treatment. The key measures:
- Space plants so leaves do not touch between pots.
- Open windows or run a fan periodically to keep air moving around your plants.
- Water in the morning and at the base of the plant, not over the foliage.
- Prune Begonias and other susceptible plants to maintain an open shape.
- Quarantine new plants for two to three weeks before placing them near existing plants.
If you find yourself treating the same plant repeatedly year after year, it is worth asking whether the position suits it. Relocating a mildew-prone Begonia from a cool windowsill to a brighter, better-ventilated spot often solves the problem permanently.