If you've spotted white, dusty or powdery patches on your plant leaves, you're almost certainly dealing with powdery mildew — one of the most common fungal diseases affecting garden plants, greenhouse crops, and ornamentals alike. The good news: caught early, it's very treatable. And with the right environmental controls in place, it's entirely preventable.
What Is Powdery Mildew?
Powdery mildew is a fungal disease caused by several species of fungi in the order Erysiphales. Unlike most fungal diseases, it doesn't need wet conditions to spread — in fact, it thrives in warm, dry air with high humidity. This makes it particularly common in late summer and early autumn in the UK, and in poorly ventilated greenhouses and growing spaces year-round.
It affects a wide range of plants including tomatoes, courgettes, cucumbers, squash, roses, and many ornamentals.
How to Identify Powdery Mildew
- White or grey powdery coating on the upper surface of leaves
- Affected leaves may curl, yellow, or brown at the edges over time
- Stems and buds can also be affected in severe cases
- Unlike downy mildew, the white coating is on the top of the leaf, not the underside
- Starts as small circular patches and spreads rapidly if untreated
What Causes Powdery Mildew?
Powdery mildew spores are airborne and present in most growing environments — the disease only takes hold when conditions favour it. The key triggers are:
- Poor airflow — stagnant air allows spores to settle and germinate on leaf surfaces
- High humidity combined with warm temperatures — the ideal range for powdery mildew is 60–80% relative humidity at 15–30°C
- Overcrowded plants — dense canopies restrict airflow between plants
- Stressed plants — plants under nutrient stress or water stress are more susceptible
- Temperature fluctuations — cool nights following warm days create ideal conditions
How to Treat Powdery Mildew
Act quickly — powdery mildew spreads fast once established. Remove and dispose of heavily affected leaves first (do not compost them), then treat the plant.
Recommended treatment: CX Horticulture PM Treatment is a dedicated powdery mildew control spray formulated specifically for plants. Apply as a foliar spray, covering both the upper and lower leaf surfaces thoroughly. Repeat every 5–7 days until symptoms clear.
For broader plant defence and disease resistance, Dr FeelGoods Chitinase Magic stimulates the plant's natural immune response, making it more resistant to fungal attack.
Home remedy option: A dilute solution of sodium bicarbonate (baking soda) at 1 tsp per litre of water with a few drops of washing-up liquid can provide temporary relief, though dedicated treatments are significantly more effective.
How to Prevent Powdery Mildew
Prevention is far easier than treatment. Focus on environmental control:
Improve airflow: Good air circulation is the single most effective preventative measure. A circulating fan keeps air moving around the canopy, preventing the stagnant conditions powdery mildew needs to establish. For enclosed growing spaces, an inline extraction fan maintains fresh air exchange and controls humidity.
Control humidity: Keep relative humidity below 60% during the day and avoid sharp drops at night. If you're growing in a greenhouse or enclosed space, a climate controller such as the GrowHub E25 Controller can automate fan and heating settings to maintain optimal conditions.
Space your plants: Avoid overcrowding. Good spacing allows air to move freely through the canopy and reduces the chance of spores transferring between plants.
Water at the base: Avoid wetting foliage when watering. Water at the base of the plant, ideally in the morning so any splashes dry quickly.
Keep plants healthy: Well-fed, unstressed plants resist disease far better. Maintain a consistent feeding programme and keep pH in the correct range to ensure full nutrient availability.
Quick Reference: Powdery Mildew
- Looks like: White/grey powdery coating on upper leaf surfaces
- Caused by: Airborne fungal spores, favoured by warm temps + high humidity + poor airflow
- Treat with: CX Horticulture PM Treatment or Chitinase Magic
- Prevent with: Good airflow, humidity control, plant spacing, healthy feeding