White powdery coating on leaves
A dusty white film on the upper surface of leaves, looking as though the plant has been sprinkled with flour. Most familiar on courgettes, cucumbers and peas from midsummer onwards.
Described view
Two courgette leaves compared at the same scale: one with the loose white flour-like powdery mildew lying on the upper surface and rubbing off under a thumb, and one turned over to show downy mildew — angular yellow blocks on the upper surface bounded by the veins, with grey-purple fuzz on the underside directly beneath them
What causes it, and how to tell them apart
Work down the list making the observation under each heading. GrowingHQ will not guess which one you have — the observation is the answer.
- Powdery mildewvery common
source
Sourced guidancehigh confidence
- Royal Horticultural Society: Powdery mildews
Checked 2026-08-13
How to tell
It rubs off between finger and thumb, sits mainly on the upper leaf surface, and appears in warm dry spells rather than wet ones. Silvery-white patterning that does NOT rub off is likely to be natural leaf marking — many courgette and cucumber cultivars are mottled silver-white by nature and are regularly mistaken for mildew.
source
Sourced guidancehigh confidence
- Royal Horticultural Society: Powdery mildews — identification
- UC Statewide Integrated Pest Management Program — University of California Agriculture and Natural Resources: Powdery mildew on vegetables
Checked 2026-08-13
If it is this, do first
Water thoroughly at the base and mulch. Dry roots in humid air are what brings it on, and the response is at the root rather than on the leaf.
source
Sourced guidancehigh confidence
- Royal Horticultural Society: Powdery mildews — cultural control
Checked 2026-08-13
Where GrowingHQ holds more for one plant
This page lists the causes across every plant. The plants below are the ones where something about that plant changes the answer — different candidates, or a different observation that separates them.
Vegetables
- Courgette: white powdery coating on leaves — 2 candidates specific to courgette
- Cucumber: white powdery coating on leaves — 2 candidates specific to cucumber
- Pea: white powdery coating on leaves — 2 candidates specific to pea
- Pumpkin: white powdery coating on leaves — 2 candidates specific to pumpkin
Fruit
- Grape: white powdery coating on leaves — 2 candidates specific to grape
- Melon: white powdery coating on leaves — 2 candidates specific to melon