Why are my courgette leaves covered in white powder?
Powdery mildew on cucurbits is close to inevitable by late summer — how to tell it from harmless silver leaf markings, how long it costs you, and what genuinely delays it.
- Difficulty
- beginner
- Time
- 12 min
- Read
- 3 min
Short answer
That is powdery mildew, and on courgettes and squash it is a late-season certainty rather than a disaster. Keep the roots properly watered — dry roots are what lets it in — remove the worst leaves for airflow, and keep picking. Expect it to end the plant a few weeks earlier than it otherwise would.
Almost every courgette plant in Britain gets powdery mildew from August onwards. The mistake is treating it as an emergency: by the time it appears the plant has usually produced most of what it is going to produce, and the practical question is how many more weeks you can get.
Step by step
- Check it is mildew and not variety markings.Many courgette and squash varieties have natural silver-grey mottling along the leaf veins, which alarms people every year. Mildew is a chalky white dusty coating that sits on the leaf surface and rubs off on a finger; the markings do not.
- Water the roots, thoroughly and often.Powdery mildew, unlike most fungal diseases, is worse on plants with dry roots — it does not need wet leaves to spread. A well-watered plant resists it far longer. This is the most useful thing you can do.
- Water at the base, in the morning.Wet foliage overnight brings other problems even if it does not drive mildew. Direct the can under the leaves at the crown.
- Remove the worst-affected leaves.Take off the oldest, most heavily coated leaves and bin them. It reduces the spore load and opens the plant to air. Do not strip it — the plant still needs leaves.
- Improve airflow.Courgettes planted close together in a sheltered corner get mildew earliest. A metre of space each and an open position makes a real difference next year.
- Keep picking.A plant with mildew still fruits, often for several more weeks. Regular picking keeps it flowering rather than winding down.
- Feed to keep the plant going.A high-potash feed supports continued fruiting on a plant that is under pressure. Do not use a nitrogen feed, which produces soft growth that succumbs faster.
- Clear and bin the plant when it is finished.Do not leave a collapsing mildewed plant on the bed and do not add it to a cool compost heap. Clear it and put in a late salad or green manure instead.
Tips
- Mildew-tolerant varieties exist for courgettes, squash and cucumbers and stay clean weeks longer — the most effective single change for anyone who gets it every year.
- Milk sprays and bicarbonate sprays are widely recommended online. Evidence is thin and inconsistent, and neither substitutes for watering properly. There is no harm in trying them, but do not rely on them.
- A late sowing of courgettes in early summer gives a young plant coming into production just as the first one succumbs — a good way to extend the season.
- Squash and pumpkins tolerate mildew better than courgettes because their fruit is already sizing up by the time it arrives.
Common mistakes
- Assuming it is caused by watering the leaves — Powdery mildew is unusual among fungal diseases in that it does not need free water to germinate. Under-watering at the root is more of a trigger than wet foliage.
- Stripping every affected leaf off — The plant loses its ability to feed the developing fruit and stops entirely. Take the worst, leave the rest.
- Composting the plant on a cool heap — Spores survive and start the next season earlier. Bin it or use council green waste.
- Panicking in August — By then the plant has usually done most of its job. Treat it as the end of the run, not a rescue.
Questions people ask
Can I still eat courgettes from a mildewed plant?
Yes. The fruit is unaffected and perfectly good. Powdery mildew is a leaf disease and does not make the crop unsafe or unpleasant.
Will it spread to my other vegetables?
The powdery mildews are largely host-specific groups, so courgette mildew will move readily to other cucurbits — squash, pumpkin, cucumber — but is not the same organism affecting your peas or your apples.
Is it worth spraying?
Home garden fungicide availability changes, and most treatments are protective rather than curative, so by the time you see white patches they have limited value. Watering, spacing, tolerant varieties and removing the worst leaves are the reliable levers.