Powdery mildew on cannabis looks like small, round patches of white flour on the tops of the leaves that spread and join up over a few days. It’s a fungal disease that thrives in humid, stagnant rooms. Catch it in veg, remove the worst leaves, fix airflow and humidity, and treat the rest before flowers form.
Powdery mildew (often shortened to PM) is one of the most common diseases in indoor and greenhouse cannabis. On cannabis it’s usually caused by fungi in the Golovinomyces group. Unlike most molds, it doesn’t need wet leaves to get started. It grows on the surface of living leaves, sends tiny feeding structures into the plant’s cells, and releases huge numbers of spores that drift through the air to the next plant. If you’re not sure that’s what you’re looking at, use the Cannabis Doctor to compare symptoms, or check our guide to cultural, mechanical and biological IPM for the bigger prevention picture.
How to identify white powdery mildew
The early spots are easy to miss, especially under bright grow lights. Look at leaves at an angle and check the lower and inner canopy, where air moves least.
- Small, circular white or gray dusty spots on the upper side of fan leaves
- Spots that wipe off with a finger but come back within days
- Patches that grow and merge until whole leaves look coated
- Powder moving onto stems, sugar leaves and eventually buds
- Leaves under the patches yellowing, curling and drying out
- Slower growth and lighter yields in heavy cases
A white powder that stays put and doesn’t wipe off may be leftover spray residue or mineral deposits. Trichomes on sugar leaves can also fool new growers. PM sits on the leaf surface as a loose, dusty film, not as tiny glassy stalks.
What causes powdery mildew on weed
Spores are almost everywhere. What matters is why they took hold in your room. These conditions make an outbreak far more likely:
- High humidity. Moist air helps spores germinate, especially when humidity climbs at lights off.
- Poor airflow. Still air pockets in a dense canopy let spores settle and grow undisturbed.
- Big day and night temperature swings. A sharp drop after lights out pushes humidity up fast.
- A crowded canopy. Leaves that touch pass the infection along and trap moist air.
- Moderate temperatures. PM likes the same comfortable range your plants do, which is why indoor rooms are such good hosts.
- Infected plants or unfiltered air. Clones from another garden and dirty intake air can bring spores in.
Our article on how much dehumidification you need helps you size equipment so humidity doesn’t spike at night, and air movement and ventilation covers fan placement and air exchange.
How to treat powdery mildew on cannabis
1. Remove the worst leaves
Carefully cut off leaves with heavy patches and drop them straight into a bag. Shaking or brushing them releases spores, so move slowly and bag them inside the room. Clean your scissors and wash your hands before touching healthy plants.
2. Open up the canopy
Defoliate inner and lower growth so air can move through the plant. Space plants so leaves don’t touch, and aim oscillating fans so the whole canopy gets gentle, constant movement, including the bottom. Our guide on how to space plants is a good reference.
3. Bring humidity down
Lower relative humidity and keep it steady through lights off. Avoid big temperature drops at night, since that’s when humidity usually spikes. Use the targets in the table below as a starting point.
| Stage | Target relative humidity |
|---|---|
| Veg | About 50 to 60% |
| Flower | About 40 to 50% |
| Late flower | Lower end of the flower range |
4. Use a foliar treatment in veg only
Once the room is fixed, treat the remaining plants while they’re still in veg. Potassium bicarbonate sprays change the leaf surface so it’s harder for the fungus to grow. Biological fungicides based on beneficial Bacillus bacteria are another option and work best as a preventive or at the first sign of spots. Sulfur works well in veg too, but never use sulfur and oil sprays close together, because the combination can burn leaves badly. Follow every label and spray the tops and bottoms of the leaves.
Diluted milk sprays and baking soda are popular home remedies. Growers report mixed results with milk, and baking soda tends to be less effective than potassium bicarbonate and can leave sodium on the leaves. They’re better than nothing on a small grow, but don’t rely on them for a serious outbreak.
5. Stop spraying once buds form
Don’t spray flowers, especially late in bloom. Residue can stay in the buds, and wet flowers invite bud rot. If PM shows up in flower, focus on removing leaves, airflow and humidity control, and harvest on time.
6. Keep checking
One clean week doesn’t mean it’s gone. Spores can sit in the room and on equipment, so check the canopy every few days for the rest of the run.
Is weed with powdery mildew safe to smoke?
We don’t recommend it. Buds covered in powdery mildew taste and smell off, and inhaling fungal spores isn’t something anyone should do, especially people with breathing problems or weak immune systems. If only a few fan leaves were hit early and the buds are clean, the harvest is usually fine. Buds with visible powder should be thrown out.
Problems that look like powdery mildew
| Problem | What you see | How it differs |
|---|---|---|
| Leaf septoria | Yellow and brown spots on lower leaves | Spots are inside the leaf tissue and don’t wipe off |
| Spider mites | White or yellow speckles on leaf tops | Tiny dots instead of powder, with mites and webbing underneath |
| Spray or mineral residue | White film after foliar feeding or misting | Doesn’t spread from day to day and appears right after spraying |
| Bud rot | Gray, fuzzy mold inside dense buds | Starts inside the cola and turns brown and mushy |
How to prevent powdery mildew
- Keep humidity in range for each stage and watch the swing at lights off.
- Run oscillating fans above and below the canopy and keep fresh air coming in.
- Filter intake air and don’t bring in clones without a quarantine period.
- Defoliate and space plants so the canopy never turns into a wall of leaves.
- Keep the day and night temperature difference small.
- Clean the room, tools and equipment between runs.
- Scout leaves with a flashlight held at an angle every week.
Powdery mildew in outdoor and greenhouse grows
Outdoors, you can’t control the weather, but you can control how much of an opening you give the fungus. Late summer is the danger zone in most regions: warm days, cool nights and heavy morning dew create the humidity swings PM loves. Greenhouses get hit hardest when the vents close at night and moisture builds up under the roof.
- Plant in full sun with room between plants so leaves dry quickly after dew or rain.
- Prune the lower third of each plant to lift the canopy off the ground and let air move under it.
- In a greenhouse, run circulation fans overnight and vent the warm, wet air before lights out or sunset.
- Water at the base early in the day, never on the leaves in the evening.
- Start preventive treatments in veg if PM has shown up in your area in past seasons.
A five-minute weekly scouting routine
Most growers who lose a crop to PM saw it late, not because it moved too fast. A short, regular check catches it while a few leaves are all you lose.
- Turn on a flashlight and hold it low, almost flat against the leaves. The powder shows up far better at an angle than under the grow light.
- Check the lower and inner leaves first, then the plants closest to walls and corners where air moves least.
- Look at the room’s humidity log for the last week, especially the readings right after lights off.
- Write down any spots and the plant they’re on, so you can tell next week whether it’s spreading.
- If you find anything, start with the steps above that same day.
Frequently asked questions about powdery mildew on cannabis
What does powdery mildew on weed look like?
It starts as small, round white spots that look like someone dusted the leaves with flour. The spots usually appear on the upper side of fan leaves first, then spread and merge. Left alone, the powder covers whole leaves, moves to stems and buds, and the leaves underneath turn yellow and dry out.
Can you get rid of powdery mildew for good?
You can clear an outbreak and keep it from coming back, but spores are common in the air, so prevention never really stops. Fixing humidity, airflow and spacing does most of the work. Clean the room thoroughly between runs and quarantine new plants so you aren’t starting each cycle with spores already present.
Does baking soda kill powdery mildew?
Baking soda can slow powdery mildew a little, but it’s a home remedy with limited results. Potassium bicarbonate works in a similar way and usually performs better without leaving sodium on the leaves. Either way, only spray in veg, and fix the room conditions or the mildew will return.
What humidity prevents powdery mildew?
Aim for roughly 50 to 60% relative humidity in veg and 40 to 50% in flower, with the low end in late bloom. Steady numbers matter as much as the targets. A room that sits at 50% all day but jumps much higher after lights out can still give the fungus what it needs.
Can I spray for powdery mildew during flowering?
Avoid spraying buds, especially late in flower. Residue can end up in what you smoke, and wet flowers are prone to rot. In early flower some growers still treat leaves carefully, but the safer plan is to remove infected leaves and tighten up humidity and airflow.
Still not sure what’s wrong with your plant?
Members can post a photo in the Growing Exposed forums and get help from growers who’ve beaten powdery mildew in their own rooms. Members also get the full video library, where we walk through real gardens and the problems growers hit along the way. See membership plans.