White powdery spots that wipe off and come back
Small round white colonies on upper leaf surfaces, often older or shaded leaves first; merge and spread to buds..
- Also called
- powdery mildew
- white powder on leaves
- PM
- flour on leaves
- white fuzzy spots
Likely causes, most likely first
Ranked for this sign on its own. Your stage, medium and readings change the order, so run the checker for your plant.
LikelyPowdery mildew
Fix: Treat early in veg: remove the worst leaves into a bag, improve airflow and stable humidity (avoid big day-night RH swings), and use least-toxic contact options (e.g. potassium bicarbonate, sulfur, or biological products, in veg only; never sulfur near oils or in flower). Once it reaches flowers there is no clean fix. Choose resistant genetics and clean clones.
Recovery: No new colonies on treated or new leaves within 7-10 days.
How to confirm it
- Wipe test for white coatings. Wipe a white spot gently with a damp finger or cotton bud and mark the leaf. Powdery mildew comes off as powder and reappears in 2-5 days on the same and nearby leaves; spray or hard-water residue wipes off and stays gone; trichomes and thrips scars do not wipe off. Wash hands afterwards: spores travel on fingers.
- Check temperature and RH at canopy height with a calibrated sensor. Put a calibrated sensor (salt test: a sealed bag with damp table salt reads 75% RH after 8-12 hours) at canopy height, out of direct light, and log for 24 hours including lights-off. Look for lights-off RH spikes and dew point.
Less likelyHumidity too high / VPD too low
Fix: Target about 55-70% RH in veg, 50-60% early flower, 40-50% late flower (leaf VPD 1.0-1.5 kPa in flower). Dehumidify, raise exhaust, keep air moving under the canopy and watch the lights-off spike (dew point). High RH in flower raised bud-rot risk and in one study delayed flowering and lowered cannabinoids.
Recovery: RH holds in band through lights-off for 3+ days; no new mould signs.
How to confirm it
- Check temperature and RH at canopy height with a calibrated sensor. Put a calibrated sensor (salt test: a sealed bag with damp table salt reads 75% RH after 8-12 hours) at canopy height, out of direct light, and log for 24 hours including lights-off. Look for lights-off RH spikes and dew point.
Look for
Talc-like circular patches on the upper side of fan leaves; wipe test photo before and after..
The full guide: Powdery mildew in cannabis
Guidance, not a guarantee. The checker weighs common causes against what you tell it; confirm with the tests before you treat, change one thing at a time and watch the new growth.
