Powdery Mildew on Courgettes: Causes and Fixes

That white, dusty coating on your courgette leaves is powdery mildew — a common fungal disease that shows up on almost every plant by late summer. It looks alarming but is rarely fatal, and you can slow it right down with consistent watering and better airflow. In most UK gardens it is simply part of the courgette's natural end-of-season decline, not a sign you have done anything wrong.

It is one of the most reliable questions we get from beginners growing courgettes, usually around August: the leaves were fine in July, and now they look like someone has dusted them with talcum powder. The good news is that the fix is gentle, the plant usually keeps cropping, and a couple of small changes next year will buy you weeks of extra harvest.

How to recognise it

Powdery mildew starts as small, pale, dusty patches on the upper surface of the older leaves — the lower, shadier ones nearest the centre of the plant tend to go first. Over a week or two those patches spread and join up until whole leaves look coated in a thin layer of white or greyish powder.

You can usually rub a little of it off with your finger, which helps tell it apart from harmless natural leaf markings. Many courgette varieties have silvery-white blotches and veining as a normal feature — these follow the leaf veins in a fixed pattern and won't smudge. Mildew is dusty, patchy and gets steadily worse; the silver markings stay put all season.

As it advances, affected leaves yellow, then go brown and crispy, and may curl at the edges. The fruit itself is almost never affected directly — you can keep eating courgettes from a mildewed plant quite safely. What suffers is the plant's energy: badly covered leaves can't photosynthesise well, so growth slows and the plant produces fewer, smaller courgettes.

Why it happens

The biggest myth about powdery mildew is that it's caused by wet leaves. It isn't. Unlike most fungal diseases, the spores actually prefer dry conditions on the leaf, and the disease takes hold when the plant is under stress — most often water stress at the roots.

The classic UK trigger is the weather pattern we get from late July onwards: warm days, cool nights, and soil that dries out faster than you can keep up with. A big courgette plant in full summer can drink several litres a day, and if the roots go dry the plant gets stressed and far more susceptible. That stress, not damp air, is what opens the door.

A few things make it worse:

• Dry roots. The single biggest factor. A plant that's allowed to wilt, even briefly, is much more likely to succumb — and once mildew arrives, dry plants get covered fastest.
• Crowding and still air. Courgette plants are huge. Pack them too close, or grow them where no breeze reaches, and humidity builds around the leaves while airflow that would disperse spores is missing.
• Late season. Spore numbers build up over summer, so by August there's simply more mildew in the air. Older leaves, past their best, are the easiest target.
• Container growing. Pots dry out faster than open ground, so courgettes grown in containers often show mildew earlier — keeping them well watered matters even more.

So it's less "my garden is too damp" and more "my plant got thirsty and tired at the time of year when mildew is everywhere."

What to do

Once you can see powdery mildew, you won't get rid of it entirely — but you can slow it down and keep the plant cropping. Work through these in order.

1. Remove the worst leaves. Snip off any leaves that are more than about half-covered, plus any that are yellowing or brown. This takes pressure off the plant and removes a chunk of the spore source. Don't strip it bare, though — the plant still needs healthy leaves to feed the developing fruit. Bin or burn the removed leaves rather than composting them in a cool heap, where spores can survive.

2. Water deeply at the base. This is the most useful thing you can do. Give the plant a thorough soak at the roots — a couple of full watering cans — two or three times a week in dry spells, more in a heatwave. Water in the morning so any splashes dry quickly, and aim the can at the soil, not the leaves. Easing the water stress is what actually helps the plant resist.

3. Improve airflow. If neighbouring plants are crowding it, cut back or remove a few lower leaves to open the plant up and let air move through the centre. Better circulation dries the leaf surface and carries spores away.

4. Keep harvesting. A plant that's still setting and ripening fruit stays more vigorous. Pick courgettes young and often — every couple of days at the peak — and the plant keeps producing rather than tiring itself out on a few marrows. (If you're being buried in them, our guide to dealing with a courgette glut has plenty of ideas.)

5. Try the milk spray — with honest expectations. The best-known home remedy is a milk-and-water spray: roughly 1 part ordinary milk to 9 parts water, sprayed onto the leaves on a dry, sunny morning, repeated weekly. There's reasonable evidence it slows mildew (something in the milk appears to act against the fungus in sunlight), and it's cheap and harmless. But be realistic: it works best as a preventative on lightly affected plants, won't cure a plant that's already plastered in white, and needs reapplying after rain. Treat it as a way to buy a few extra weeks, not a magic cure. Skip strong vinegar or bicarbonate mixes unless well diluted — they can scorch leaves and do more harm than good.

You don't need a fungicide for this. By the time mildew is obvious, the plant is usually late in its life anyway, and the gentle measures above are enough to see out the season.

How to prevent it

You can't stop powdery mildew forever, but next season you can push it back by weeks. Prevention is far more effective than any cure.

• Grow a resistant variety. This is the biggest win. Several modern varieties carry good mildew resistance — 'Defender' F1 is the standard recommendation for UK gardens, with 'Parador' and 'Romanesco' types also performing well. Resistant doesn't mean immune, but these stay productive much longer into autumn.
• Never let the roots dry out. Plant into rich, moisture-retentive ground, water deeply and consistently from the start, and don't wait until the plant wilts. Working plenty of organic matter in first helps — see improving your soil for the easy way to do it.
• Mulch. A thick mulch of compost or well-rotted manure around the base locks moisture in and keeps the roots cooler and steadier, which directly reduces the stress that invites mildew.
• Space plants properly. Give each courgette plant about 90cm of room. It feels wildly generous when they're seedlings, but they fill it — and the airflow between mature plants pays off in August.
• Water in the morning, at the base. Morning watering lets the surface dry through the day, and base watering keeps the leaves drier and stronger.
• Start at the right time. Sowing too early gives you tired, stressed plants by mid-summer. Late April to May is plenty early enough in most of the UK — check our planting calendar for your region. A plant that's still young and vigorous in August shrugs off mildew far better than an exhausted one.

Don't panic — it's mostly the end of the season anyway

Here's the reassuring truth: a courgette plant covered in powdery mildew in late August or September is usually just winding down naturally. These are annual plants that crop hard for a few months and then run out of steam — the mildew often arrives at the same time as that natural decline, which is why it gets the blame for an ending that was coming anyway.

So if your plant has given you a summer of courgettes and is now looking dusty and tired, that's a job well done, not a failure. Pick the last fruit, clear the plant when it stops producing, and bin the foliage rather than composting it cold.

And if it's the sheer quantity of courgettes that's the real problem, you're in good company — work through the recipes and storage tips in our what to do with a courgette glut guide, and start planning a resistant variety and a thicker mulch for next year. With a little watering discipline and the right seed, you'll be eating home-grown courgettes well into the autumn.