Cucumber: white powdery coating on leaves

What it is likely to be on cucumber

1. Downy mildew

   Candidate 1common on this plant

   ·source

   Sourced guidancemoderate confidence

   • Royal Horticultural Society: Downy mildews
   • Agriculture and Horticulture Development Board — AHDB: Cucurbits — downy mildew

   Checked 2026-08-13

   Pale yellow patches on the upper leaf surface with a greyish or purplish furry growth directly beneath them, the patches often bounded by the leaf veins into angular blocks. It is routinely confused with powdery mildew and behaves in the opposite way: downy mildew needs wet conditions, powdery mildew thrives in dry ones.

   Points towards it

   • · the fruiting growth is felted rather than dusty and appears under angular patches bounded sharply by the leaf veins
   • · unlike powdery mildew it needs free water on the leaf and long humid nights, so it follows wet weather rather than dry
   • · the lower shaded leaves that stay wet longest go first
   • · a wet outdoor patch and a badly ventilated tunnel both hold the leaf wetness it needs

   Argues against it

   • · a dusty coating that rubs off is the other mildew, and the advice differs
   • · it cannot establish without leaf wetness, which a hot dry spell removes

   Check this next

   Turn the leaf over and look at the shape of the patch from above. Downy mildew makes yellow patches on the upper surface with straight edges, because the growth stops at the veins and the patch ends up angular rather than round — and directly beneath each patch, on the underside, is a grey-purple felt visible best first thing in the morning. It does not rub off. Powdery mildew is the opposite on both counts: dusty, on top, and it wipes away with a thumb.

   What to do now

   Get the leaves dry rather than the soil, which is the reverse of the powdery mildew answer. Water the ground in the morning only, never overhead and never in the evening, open every vent and door on a tunnel day and night in warm weather, and take out the lowest leaves so air moves under the canopy. Remove and bin badly affected leaves rather than composting them. Where it has taken hold late in the season the practical answer is to keep picking; where it arrives in July, a later sowing in a different part of the plot often outruns it.

   ·source

   Sourced guidancemoderate confidence

   • Royal Horticultural Society: Downy mildews — control
   • Agriculture and Horticulture Development Board — AHDB: Cucurbits — downy mildew management

   Checked 2026-08-13

2. Powdery mildew

   Candidate 2very common on this plant

   ·source

   Sourced guidancehigh confidence

   • Royal Horticultural Society: Cucumbers — growing under cover
   • Agriculture and Horticulture Development Board — AHDB: Cucurbits — powdery mildew

   Checked 2026-08-13

   A white, floury coating on the upper leaf surface, most familiar on courgettes and cucumbers late in the season. Unlike most fungal diseases it does not need the leaves to be wet — it needs the roots to be dry and the air to be humid, which is why it arrives in a warm dry August.

   Points towards it

   • · a closed greenhouse in July holds humid air around dry leaves for weeks, which is the exact condition powdery mildew wants and the one a garden never provides for as long
   • · surface growth that wipes off with a thumb, sitting on top of the leaf rather than in it
   • · a cucumber in a pot that dries between waterings is under root stress in humid air, and stressed plants go first
   • · it begins on the oldest shaded leaves at the base of the plant and climbs

   Argues against it

   • · a felt on the underside beneath an angular yellow patch is downy mildew, and the ventilation advice is the same but the watering advice is not
   • · an outdoor ridge cucumber in moving air gets it later and less, and is far more likely to be showing something else

   Check this next

   Judge how far up the plant it has climbed and how early in the season it arrived, because on a cucumber that decides whether it matters. Powdery mildew starts on the lowest, oldest, most shaded leaves as discrete white circles that merge, and it wipes off with a thumb leaving green tissue. A greenhouse cucumber that is white to head height in July has lost its working leaf area and will stop cropping; the same plant showing a few patches at the base in September will crop until it is cleared.

   What to do now

   Ventilate rather than damp down, which is the opposite of what many greenhouse routines do: open the vents and the door day and night in warm weather so the humid air is replaced, and stop misting. Water the growing bag or pot generously and consistently at the root and never over the leaves. Strip the lowest three or four leaves to open the base of the plant up, and keep stripping as it climbs. For next year, grow a mildew-resistant F1 — that is the single change that makes the biggest difference to a greenhouse cucumber.

   ·source

   Sourced guidancehigh confidence

   • Royal Horticultural Society: Cucumbers — cultivation under glass
   • Agriculture and Horticulture Development Board — AHDB: Cucurbits — mildew management

   Checked 2026-08-13

Narrow it down

This page lists everything at once, which is the right shape for reading and the wrong shape for deciding. The interactive tool asks one question at a time, starting with whichever would separate these causes fastest, and stops as soon as further answers would not help.

Opens with the plant and the symptom already set. Nothing you answer is stored or sent anywhere — the answers live in the address bar and nowhere else.

What this does not cover

These are the causes GrowingHQ holds rules for on cucumber — not every cause there is. Where several problems look alike, the honest answer is often that they cannot be told apart by eye, and a page that named one would be guessing. If none of the above fits, the general white powdery coating on leaves page lists causes across all plants, and the Cucumber profile lists the problems it is prone to.