Powdery Mildew and Grey Mould: How to Prevent Them Indoors

Powdery mildew and grey mould are the two diseases that turn up most often in grow tents and greenhouses, and both are far easier to prevent than to cure. The spores are almost always present already; what decides whether they take hold is the air around your plants. Humid air, air that does not move, and a damp, cool night after a warm day give them everything they need. This guide explains how the two differ, why the hour after lights-off is the riskiest time of the day, and the five steps that keep both out: measure, move the air, dry the night, keep it clean and protect early.

Two different diseases, one set of causes

They are often lumped together as “mould”, but they are different fungi that behave differently, and it helps to know which you are looking at.

Powdery mildew looks like someone has dusted the leaves with flour: small white spots that spread into powdery patches, usually on the upper surface first, then onto stems and the undersides. It lives on the surface of living leaves. Unusually for a fungus it does not need liquid water on the leaf to infect; humid air is enough, and it spreads fastest where the air is still and the plants are crowded.

Grey mould, caused by the fungus Botrytis cinerea, is a soft brown rot that grows a grey, fuzzy coat of spores. It prefers tissue that is already weak, dying or damaged: a dead leaf lying on the medium, a wound, a stem that has been pinched or snapped. It needs free moisture on the surface or air close to saturation, which is why it so often starts after a cool, damp night.

Why they take hold indoors

Spores of both fungi float in ordinary air, travel in on new plants, clothes and pets, and survive on old plant debris. You cannot keep every spore out. What you can control is whether the conditions let them germinate and spread, and three things do most of the damage.

The humidity jump at lights-off

Relative humidity is a measure of how full the air is compared with what it could hold at that temperature, and cooler air holds less. So if the lights go off and the tent cools with the same water still in the air, the reading climbs without a drop of water being added. Take air at 24°C and 60% RH: cool it to 20°C and it reads about 77%; at 18°C, about 87%; at around 16°C it reaches 100% and water starts to condense on the coolest surfaces, including leaves. Plants keep releasing water for a while after the lights go off, so the real rise is often bigger.

That is why a tent that looks fine every time you check it during the day can still be giving grey mould ideal conditions every night. The daytime reading tells you almost nothing about the risk.

Still air

Every leaf is wrapped in a thin layer of air that it keeps humid by releasing water. In a still tent that layer sits there, so the leaf surface lives in much damper air than your hygrometer is reading. A gentle, constant breeze strips it away, keeps the surface dry and makes it much harder for spores to settle and germinate.

Crowding, debris and cold spots

Plants packed edge to edge make dense, humid pockets where the air never moves. Dead leaves left on the medium or caught in the plant give grey mould the weak tissue it likes to start on. And any surface colder than the air, a tent wall against a cold outside wall, the area in front of a winter air intake, can drop below the dew point and run with condensation.

Step 1: measure, day and night

You cannot manage humidity you are not measuring, and you especially cannot manage the night-time peak if you only look during the day. The single most useful thing is a hygrometer that records the highest and lowest reading since you last reset it. Check it each morning, write down the overnight high, and reset it.

• Hang it at plant height, in the middle of the tent, out of direct light and not right in front of a fan or the intake. A sensor on the floor or near the roof reads a different climate from the one your leaves are in.
• Watch the overnight maximum, not the daytime figure. If it regularly heads into the 80s or above, that is your problem, whatever the daytime reading says.
• Note the temperature drop too. A night that runs several degrees cooler than the day is what drives the humidity jump.

For the right humidity range at each stage of growth, see our guide to grow tent humidity. As a general rule for established plants, aim to keep the overnight peak well below 80% and avoid long periods at the top of your range.

Step 2: keep the air moving

Air movement is the cheapest protection there is, and in most tents it is the thing missing. You are after two separate jobs: exchange, which swaps the damp air inside the tent for drier air from outside, and circulation, which keeps the air inside moving across every leaf.

• Exchange: the extractor fan does this. If it is undersized, or throttled back so far that it barely runs, humid air builds up however good your circulation is. Our fan and filter sizing guide shows how to size it.
• Circulation: clip fans on the tent poles. In anything bigger than a small tent use two: one blowing across the top of the plants and one lower down, through the stems and the lower leaves, where the air is stillest and the humidity highest.
• The right strength: leaves should flutter gently all over the plant. You do not want one plant blasted in a fixed stream, which can dry and damage the leaves nearest the fan; an oscillating fan or one aimed at the tent wall to set up a loop works better.
• Leave them on at night. Circulation matters most after lights-off, when humidity is at its peak. Do not put clip fans on the lighting timer.

Step 3: dry the night

Once you know your overnight peak, take the steps below in order. Most tents only need the first two or three.

1. Keep extracting after lights-off. Many growers turn the extractor right down at night to keep the tent warm. A lower speed is fine, but the air still needs changing; a fan controller with a humidity setting can raise the speed when RH climbs.
2. Narrow the temperature drop. The smaller the gap between lights-on and lights-off temperature, the smaller the humidity jump. In a cold room in winter that may mean a small heater on a thermostat, or simply moving the tent away from an outside wall.
3. Water early in the light period. Water given in the morning has hours to drain and evaporate while the lights and extraction are running. Water given just before lights-off adds moisture right when the air can least cope with it. Empty standing water from saucers and trays.
4. Add a dehumidifier on its own night setting. If extraction alone cannot hold the overnight peak down, a dehumidifier removes the water instead of relying on drier air coming in. A controller with separate day and night targets lets you hold the night lower without over-drying the day.

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Step 4: keep it clean

Grey mould almost always starts on something already dead or damaged, and powdery mildew carries over on old leaves and in a dirty tent. Hygiene takes a few minutes a week.

• Remove dead and yellowing leaves as you see them, and pick up anything that has fallen onto the medium or the tent floor. Put them straight into a bag rather than carrying them across the tent.
• Leave space between plants. If leaves from neighbouring plants are pressed together, the air between them cannot move. Fewer plants with room around them do better than a crowded tent.
• Clean your tools. Wipe scissors and blades with a disinfectant between plants, especially after cutting out anything infected.
• Check new plants before they come in. Look under the leaves for white spots and keep new arrivals apart for a week or two if you can.
• Start every crop clean. Spores survive on tent walls, poles and old debris. Our cleaning guide sets out the full routine for an empty tent.

Step 5: protect early

Mildew treatments work best before infection takes hold, so the time to use them is when conditions are risky, or if you have had mildew before, not after the leaves have turned white. They fall into two groups.

Foliar sprays

Most of these are gentle, non-toxic products used on a routine schedule. Some form a thin protective film on the leaf, some use plant extracts, and some, like CX Horticulture PMT, work by prompting the plant’s own defences. Whichever you use:

• Cover both sides of every leaf. A spray only protects what it touches.
• Spray when the lights are off or about to go off, so wet leaves are not sitting under strong light, and make sure the circulation fans are running so the leaves dry.
• Follow the label for rate, interval, the growth stages it is suitable for and any restrictions on edible crops.
• Rotate between products that work in different ways if you are spraying for long periods.

Sulphur vaporisers

A sulphur vaporiser gently heats sulphur powder and releases a fine vapour that settles over every surface in the room, including leaf undersides and corners a sprayer never reaches. Nothing wet goes on the plants. It is a preventative rather than a rescue treatment, and growers usually run it on a timer for a short spell each night.

• Follow the maker’s instructions for running time and room size, and never let the sulphur burn: the unit is designed to evaporate it, not to scorch it.
• Keep the vapour out of your living space. Run it while the room is closed and ventilate before you go in.
• Check before combining it with oil-based sprays. Sulphur and oils used close together can damage leaves, so read both labels.

Insects are a separate job: our pest prevention guide covers the layered approach that keeps them out.

If you find it

Powdery mildew

1. Take off the worst leaves into a bag held right next to them, so you are not shaking spores across the tent.
2. Treat the whole plant and its neighbours with a mildew product, covering both sides of every leaf, and repeat at the interval on the label.
3. Fix the cause the same day. More circulation, a lower overnight peak and more space between plants. A treatment buys time; if the air stays humid and still it will come back.
4. Check every day for a week. New spots on fresh growth mean the conditions are still wrong.

Grey mould

1. Cut it out generously with clean scissors, going back well into healthy tissue, and bag it inside the tent. Grey fuzz is a mass of spores; handle it gently.
2. Do not mist or spray water onto the area. Wet tissue is exactly what it needs.
3. Disinfect the blade before you touch another plant.
4. Dry the room: check the overnight humidity peak, add circulation and look for condensation on walls and leaves. Grey mould that appears once usually means the nights are too damp.

Seven mistakes that let mildew and mould in

1. Only checking humidity in the daytime. The risk is at night, after lights-off.
2. Turning the fans off at night to keep the tent warm, just when the air is dampest.
3. One fan, aimed at the top. The lower leaves, where humidity is highest, get no air at all.
4. Watering just before lights-off. Water it in the morning instead.
5. Leaving dead leaves where they fall. They are where grey mould starts.
6. Spraying only once the leaves are white. Mildew products work best as prevention.
7. Treating without fixing the air. The spray buys time; humidity and airflow decide whether it comes back.

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Quick answers

What causes powdery mildew in a grow tent?

Powdery mildew spores are almost always present; they take hold when the air around the leaves is humid and still, plants are crowded, and the humidity swings between day and night. More airflow, more space and a lower overnight humidity peak are the main fixes.

What humidity causes grey mould?

Grey mould needs water on the surface or air close to saturation, so the risk climbs sharply once RH goes above about 85% and is highest when anything condenses. That usually happens at night, when the tent cools after lights-off.

Why does humidity go up when the lights go off?

Cooler air can hold less water, so the same air reads higher once the temperature falls. Air at 24°C and 60% RH reads about 77% at 20°C and about 87% at 18°C, before you count the water the plants keep releasing.

Should I leave my fans on at night?

Yes. Keep the circulation fans running and keep the extractor changing the air, even at a lower speed. Night-time is when humidity peaks and moving air matters most.

Can I wipe powdery mildew off the leaves?

Wiping it off removes what you can see but spreads spores and leaves the infection underneath. Remove the worst leaves into a bag, treat the plant with a mildew product and fix the humidity and airflow.

When should I spray for mildew?

Before it appears, if conditions are risky or you have had it before. Spray when the lights are off or about to go off, cover both sides of the leaves, keep the fans running so they dry, and follow the label.

Is a sulphur vaporiser safe to use?

Used exactly as the maker directs, it is a widely used way to protect a whole room. Run it on a timer while the room is closed, ventilate before you go in, never let the sulphur burn, and check before combining it with oil-based sprays.

Is this the same for tomatoes, cucumbers and other crops?

Yes. Powdery mildew is a familiar problem on cucumbers, courgettes, tomatoes and roses, and grey mould on tomatoes, strawberries and lettuce. Glasshouse growers control both with humidity, airflow and hygiene.