# White powdery spots that wipe off and come back: likely causes and fixes

> White powdery spots that wipe off and come back on cannabis is most often Powdery mildew or Humidity too high / VPD too low. How to tell them apart, the fix for each and what recovery looks like.

Also called: powdery mildew, white powder on leaves, PM, flour on leaves, white fuzzy spots.

Run the checker with this sign: https://genetixbay.com/knowledge/diagnose/?at=whole&s=white-powder

## Likely causes, most likely first

### Powdery mildew (Likely)

- Kind: Disease; urgency: Act within a day or two
- 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.
- Confirm: 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.
- Confirm: 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.
- Article: [Powdery mildew in cannabis](https://genetixbay.com/knowledge/diseases/powdery-mildew.md)

### Humidity too high / VPD too low (Less likely)

- Kind: Environment; urgency: Act within a day or two
- 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.
- Confirm: 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.
- Article: [Humidity by stage: RH targets from clone to cure](https://genetixbay.com/knowledge/environment/humidity-by-stage.md)

## Note

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.