Powdery mildew is a fungal disease that settles as a white, floury coating on leaves and shoots, and it arrives when the summer is at its driest and warmest. The word mildew covers two quite different diseases: powdery mildew, which grows on top of the leaf, and downy mildew, which sits on the underside and goes with rain and damp. The difference decides what you should do, so start by turning the leaf over.
How to tell powdery mildew from downy mildew
The two diseases want opposite weather and respond to different measures. Powdery mildew is caused by fungi in the family Erysiphaceae, which live on the leaf surface and send feeding organs into the outermost cells. Downy mildew is caused by water moulds (oomycetes) and is not a fungus in the strict sense.
| Feature | Powdery mildew | Downy mildew |
|---|---|---|
| Where the coating sits | On top of the leaf, often on shoots and buds too. | On the underside, grey-violet to brown and fuzzy. |
| What the upper side looks like | White, floury patches that merge. | Yellow, angular patches that stop at the leaf veins. |
| Weather | Dry and warm, 15-27°C, with dew at night. | Cool and wet, 10-21°C, leaves that stay wet for 6-12 hours. |
| Common plants | Cucumber, courgette, phlox, roses, gooseberry, apple, peas. | Cucumber, spinach, basil, grapes, roses. |
| What helps | Air, watering at the root, cutting out, bicarbonate of soda, sulphur. | Dry leaves, water at the root, pick off, resistant variety. |
Downy mildew on cucumber (Pseudoperonospora cubensis) produces most spores at 15°C and shows 4-12 days after infection. The rest of this article is about powdery mildew.
Why it comes in drought and heat
The spores of powdery mildew carry their own water. They germinate without droplets on the leaf, thrive at 15-27°C and spread on the wind. Water that stays on the leaves for a long time holds back germination and kills the spores of most species, so a rainy early summer gives little powdery mildew and a dry high summer a lot. Spore production is driven by high humidity in still air, as next to a wall or inside a dense stand. Above 32-35°C the fungus stops.
Two things make the plant more susceptible: drought stress, when it is allowed to dry out between waterings again and again, and too much nitrogen, which gives soft leaves. In late summer small black dots form in the coating. These are the overwintering spores, which survive the winter on fallen leaves or as mycelium in buds and shoots.
The fungus on the cucumber does not infect the rose
Powdery mildew is not one species but around a thousand, and most of them attack a single genus or a few. The cucumber's mildew therefore does not move to the rose next to it, and the garden phlox does not infect the gooseberry bush. You do not need to clear the border when one plant is attacked, but moving it does not help either, the fungus overwinters on it.
| Plant | Type of mildew | What helps most |
|---|---|---|
| Cucumber, courgette, melon | Powdery (Podosphaera xanthii, in older literature Sphaerotheca fuliginea). Cucumber also gets downy mildew. | Resistant variety, even watering at the root, remove the oldest leaves. |
| Garden phlox | Powdery, a species of its own on phlox. Starts from the bottom at the end of July. | Thin to 5-6 stems in May, 50-60 cm between plants, a tough variety. |
| Roses | Powdery (Podosphaera pannosa), often on the underside. Also downy (Peronospora sparsa). | Sunny position, air around the bush, a tough variety. |
| Gooseberry | American gooseberry mildew. The coating turns brown and felt-like on the berries. | Tough variety, open pruning, little nitrogen. |
| Apple tree | Powdery (Podosphaera leucotricha). Overwinters in the buds. | Cut out silver-grey shoots when pruning. |
| Peas | Powdery (Erysiphe pisi). Late in the season in dry weather, brown patches on the pods. | Sow early, rinse off in the morning. |
The powdery mildew year
Most of the work is done before the coating shows. The months follow a temperate season in most of the UK and northern Europe, and in milder areas everything runs a few weeks earlier.
April to May: The apple tree's infected buds open as silver-grey, stunted shoots. Cut them out. In June the year's new shoots are infected the most.
May to June: Thin the garden phlox and plant cucumber and courgette at about 5 plants per square metre. This is when you decide how airy the stand will be in August.
July to August: Dry, warm weeks bring the first patches on cucumber, courgette and phlox. Treat at the first patch and repeat every 7-10 days.
September to October: Black dots in the coating are the overwintering spores. Cut down, rake up the leaves and put them in the household waste.
Winter: A hard winter kills infected apple buds, which cope with cold worse than healthy ones. After a mild winter the attack gets worse.
What works and in which order
Air and spacing: Do not plant next to a wall or a dense hedge, keep 50-60 cm between garden phlox and 1-1.5 metres between berry bushes, and prune bushes so that light and air get in.
Water at the root, evenly: Water generously and seldom at the base so that the moisture reaches the root zone. Rinse the leaves in the morning if you like, never in the evening, when they stay damp all night.
Cut out: Remove the first infected leaves and shoot tips and put them in the bin. On cucumber and courgette we take the oldest leaves at the bottom first.
Bicarbonate of soda: Holds the fungus back but does not remove it. A 1 percent solution, 10 g per litre, can give yellowing leaves, so we mix 5 g per litre with a few drops of soft soap, try it on one leaf and then spray the whole plant every 7-10 days.
Sulphur: Protects new leaves but does not remove a coating that is already there, so it has to go on before the attack shows. Do not spray above 32°C or within two weeks of an oil treatment, as the leaves are damaged. Which products you may use as a home gardener is stated on the label.
Milk, in moderation: Diluted cow's milk had an effect on powdery mildew on courgette in a greenhouse trial in 1999, probably because the whey proteins form oxygen radicals in sunlight. We use 1 part milk to 9 parts water on cucumber and courgette in sun. A stronger mix smells sour after a few warm days.
Five common mistakes
Starting to spray when the plant is already white: Bicarbonate and sulphur act on new spores and new leaves. A plant that is half covered in August is not worth treating. Cut it back and choose a tougher variety next year.
Keeping the leaves dry at all costs: Humid air favours powdery mildew, not water on the leaf. Rinsing in the morning in dry weather reduces the attack. Leaves that are wet all night, on the other hand, favour downy mildew.
Feeding plenty of nitrogen in July: Nitrogen gives soft, dense leaves that the fungus prefers. Give the feed in spring and hold back in midsummer.
Putting infected leaves on the compost: The fungus overwinters on the leaves. A well-run hot compost kills the spores, but an ordinary garden heap does not. The bin is safest.
Treating once only: New leaves grow out untreated and new spores arrive on the wind. Repeat every 7-10 days as long as it is dry and warm.
Choose varieties for resistance
With cucumber and courgette the choice of variety matters most. The mini cucumber Iznik F1, which gives cucumbers about 10 cm long, is listed as resistant to cucumber powdery mildew. More varieties are among the cucumber seeds and squash seeds. Annual phlox (Phlox drummondii) copes better than garden phlox, most annual phloxes count as resistant, and you sow it from summer phlox seeds. Among garden phlox, David and Robert Poore stay healthiest in August, see the article on growing garden phlox. For gooseberry, Invicta is the first choice, with more in the article on growing gooseberries.
Resistance is not immunity. A resistant cucumber also gets patches in a dry August, but later and to a lesser extent, so the crop has time to finish. More pests and diseases are covered in the article on pests and diseases.
Author: Emma Vogiatzi
Fact-checked by Erik Hoekstra
Last updated 2026-10-20