Powdery Mildew
Mostly cosmeticFungal disease · spreads to other plants
Powdery mildew is the most recognisable plant disease there is, and one of the least dangerous. Unlike almost every other fungal disease it does not need wet leaves — it thrives in warm days, cool nights and high humidity, which is why it arrives reliably in late summer. On squash and cucumbers at the end of the season it is often not worth treating at all.
What it looks like
White or pale grey powder on the upper leaf surface, looking exactly as though the plant has been dusted with flour. It starts as small round patches that merge. Affected leaves yellow, then go brown and papery.
How to confirm it
- White powder on the top of the leaf that rubs off with a finger.
- Appears in dry weather, unlike almost every other fungal disease.
- Round discrete patches early on, merging into a full coating.
- Rarely kills the plant; it shortens the productive season.
What to do
- 1On a squash or cucumber in late season, often nothing. The plant is near the end of its life anyway.
- 2Remove the worst leaves to slow it and improve airflow.
- 3A potassium bicarbonate spray, or a milk-and-water solution at one part to nine, both have reasonable evidence behind them and are gentler than sulphur.
- 4Never apply sulphur within two weeks of an oil spray, or in temperatures above about 90°F.
What it gets confused with
One observation settles each of these.
Downy mildew shows yellow patches on top and grey-purple fuzz underneath, bounded by leaf veins. Powdery mildew is white and on top.
Mineral deposits do not spread outward over days and are not confined to leaf surfaces in growing patches.
Preventing it next time
- Space plants properly. Powdery mildew is largely a crowding and airflow problem.
- Choose resistant varieties — for cucumbers and squash this makes more difference than any spray.
- Plant in full sun, and avoid shaded, still corners.
Plants affected
Check these two things first
- Push a finger into the soil, two knuckles deep. Wet soil plus a wilting plant means too much water, not too little — and it is the commonest reason a plant is dying. Dry soil plus crisp leaves means the opposite. The leaves look nearly identical either way, so the soil is what tells you.
- Turn a damaged leaf over. Half of all diagnoses live on the underside: fuzz means downy mildew, fine webbing means spider mites, clusters of insects mean aphids. Nothing on the top of a leaf is as informative.
Most garden problems are cosmetic and resolve without treatment. Reach for a spray last, not first.