Cucumber: plant wilting although the soil is moist

What it is likely to be on cucumber

1. Powdery mildew

   Candidate 1occasional on this plant

   ·source

   Sourced guidancemoderate confidence

   • Royal Horticultural Society: 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 white powdery film on the upper leaf surface is the identification
   • · the oldest leaves are colonised first on a cucurbit

   Argues against it

   • · growth on the UNDERSIDE of the leaf is downy mildew, which needs wet conditions rather than dry ones

   Check this next

   Rub the white film with a finger. Powdery mildew wipes off and sits on the upper surface; downy mildew is fuzz on the underside beneath angular yellow patches, and the two want opposite conditions.

   What to do now

   Improve airflow, keep the roots evenly moist — dry roots make powdery mildew worse — and remove the worst leaves. On a courgette late in the season this is close to inevitable and rarely worth treating; the plant usually crops on regardless.

   ·source

   Sourced guidancehigh confidence

   • Royal Horticultural Society: Powdery mildew — control

   Checked 2026-08-13

2. Root rot

   Candidate 2common on this plant

   ·source

   Sourced guidancemoderate confidence

   • Royal Horticultural Society: Root rot

   Checked 2026-08-13

   The consequence of prolonged waterlogging rather than an independent event: water moulds that are present in most soil and compost become active when roots are deprived of air, and colonise tissue that is already dying. The treatment is therefore the watering, not the fungus.

   Points towards it

   • · cucurbits collapse fast in waterlogged ground
   • · a bed or growbag that does not drain is the usual setting

   Argues against it

   • · the soil is dry, so the plant is short of water rather than drowning in it — this rules it out entirely.
   • · a white powdery film points to mildew, which wilts a plant slowly rather than collapsing it

   Check this next

   Cut into the base of the main stem just above soil level. Healthy tissue is white or pale green throughout; brown, water-soaked or hollow tissue at the collar means the stem base has rotted and the plant will not recover, however much the leaves are misted or shaded.

   What to do now

   Stop watering and improve the drainage before replanting anything in the same spot. A cucurbit that has collapsed at the collar is finished — the useful action is protecting the plants either side of it, which usually means watering the soil rather than the crown.

   ·source

   Sourced guidancemoderate confidence

   • Royal Horticultural Society: Root rot

   Checked 2026-08-13

3. Heat stress

   Candidate 3very common on this plant

   ·source

   Sourced guidancehigh confidence

   • Royal Horticultural Society: Courgettes and marrows — cultivation
   • Home and Garden Information Center — Clemson Cooperative Extension: Cucurbit wilting and heat

   Checked 2026-08-13

   The response of a plant whose water loss has outrun what its roots can supply: wilting through the hottest part of the day, flower and fruit drop, and stalled growth. A plant that wilts at midday and recovers by evening is showing heat stress rather than drought, and watering it at midday does not address the cause.

   Points towards it

   • · the soil is moist and the plant is still wilting, which for a plant carrying this much leaf is a transport limit rather than a shortage
   • · it appears in hot bright weather and disappears when the weather breaks
   • · every leaf droops together, since they are all losing water at the same rate

   Argues against it

   • · dry soil makes this ordinary thirst, which needs watering rather than leaving alone — this rules it out entirely.
   • · wilting confined to the oldest leaves while the growing tips stand up is a root or stem problem, not a midday water deficit

   Check this next

   Look at the same plant in the evening. A cucurbit wilting from heat is upright again by nightfall and flat again the next lunchtime, on soil that is moist both times — nothing else in this differential recovers on that daily cycle. If it is still flat at dusk, or if it never recovers after a cool night, stop treating it as heat and look at the base of the stem and the roots.

   What to do now

   Do nothing at midday, which is the whole of the advice and the hardest part of it. Water in the early morning at the base of the plant, mulch to keep the root zone cool, and judge whether the plant needs water by pushing a finger into the soil rather than by how the leaves look in the sun. Watering a moist-rooted cucurbit every time it droops is the commonest route to the root rot the reader was afraid of.

   ·source

   GrowingHQ editorial positionhigh confidence

   The action is a decision not to act, which no source states as advice, so it is recorded as GrowingHQ's judgement rather than attributed.

   • GrowingHQ editorial synthesis — GrowingHQ

   Checked 2026-08-13

4. Cold damage

   Candidate 4common on this plant

   ·source

   Sourced guidancemoderate confidence

   • Royal Horticultural Society: Cucumbers — growing under cover
   • Agriculture and Horticulture Development Board — AHDB: Protected crops — root zone temperature

   Checked 2026-08-13

   Blackened, translucent or limp tissue appearing suddenly after a cold night or a spell beside a draught. Tender plants show it well above freezing — basil blackens around 10 °C — so 'there has been no frost' does not rule it out.

   Points towards it

   • · a clear night that drops the greenhouse into single figures chills the root zone, and a cucumber stops taking up water within hours of that
   • · an unheated greenhouse loses more heat overnight than open ground does, so the plant most at risk is the one that looks best protected
   • · the compost is wet and the plant is flagging, because chilled roots cannot move water however much of it is there
   • · temperature acts on the whole plant at once, so it goes over from top to bottom together

   Argues against it

   • · a run of hot nights rules the mechanism out completely — this rules it out entirely.
   • · damage confined to one side of a plant points at scorch or at a stem problem rather than at the root zone temperature

   Check this next

   Ask what the night did and what the water was, not what the leaves look like. A cold-checked cucumber goes over suddenly, whole plant at once, in compost that is plainly wet, within a day of a clear cold night or of a can filled straight from an outside tap. Put a minimum thermometer in the greenhouse and a thermometer in the watering can: below about 12 °C in either and this is the likeliest answer. The plant often stands up again within two days of being kept warm, which is the confirmation — a rotted root system does not.

   What to do now

   Keep the water and the root zone warm, and stop watering until the compost has dried back a little, because cold plus wet is what does the damage rather than either alone. Fill the can and leave it standing in the greenhouse overnight so the water is at air temperature, close the vents in the late afternoon to trap the day's heat, and stand pots on a bench or on polystyrene rather than on a cold concrete floor. Fleece the plant over a clear night in early summer. Do not feed until it is growing again.

   ·source

   Sourced guidancemoderate confidence

   • Royal Horticultural Society: Cucumbers — cultivation under glass
   • Agriculture and Horticulture Development Board — AHDB: Protected crops — establishment

   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 plant wilting although the soil is moist page lists causes across all plants, and the Cucumber profile lists the problems it is prone to.