The short answer

Powdery mildew needs still air, a humid canopy, and soft leaf tissue. Remove any one of those and the fungus stalls. In practice that means holding relative humidity between 40 and 55 percent, keeping canopy temps near 68 to 78 F, and moving air across every leaf from week one of veg through harvest. A preventive spray schedule in veg closes the remaining gaps.

milk method for powdery mildew on marijuana explained

Prerequisites

  • A hygrometer and thermometer placed at canopy height, not on the tent floor
  • Two oscillating fans plus an inline exhaust fan with a carbon filter
  • A dehumidifier if the lung room sits above 60 percent humidity
  • Pruners, 70 percent isopropyl alcohol, and a spray bottle
  • A preventive product: potassium bicarbonate, Bacillus subtilis, or sulfur for veg use only

Prevention steps

  1. Set the target range. Hold 40 to 55 percent humidity in veg and 40 to 50 percent in flower, with lights-on temps of 68 to 78 F.
  2. Measure at the canopy. Read humidity and temperature at the top of the tallest cola, since that is where infection starts.
  3. Turn up air exchange. Run the exhaust fan so the tent's full air volume cycles at least once per minute.
  4. Add internal airflow. Point one oscillating fan across the top of the canopy and a second low across the pots.
  5. Open the canopy. Defoliate lower and interior growth every 7 to 10 days during veg so leaves do not touch.
  6. Space the plants. Leave 6 to 12 inches between pots and keep containers off the tent walls.
  7. Water at the base. Feed the medium directly and never spray overhead during lights-on hours.
  8. Empty drip trays. Remove standing water within an hour of feeding.
  9. Filter incoming air. Fit a fine mesh or HEPA intake filter and keep the surrounding room free of dust and plant debris.
  10. Quarantine new plants. Isolate clones and outside plants in a separate room for 14 days before they enter the tent.
  11. Sanitize between plants. Wipe blades with 70 percent isopropyl alcohol after every plant you prune.
  12. Spray preventively in veg. Alternate potassium bicarbonate and Bacillus subtilis every 7 to 10 days, applying at lights-off to avoid leaf burn.
  13. Scout twice a week. Inspect the undersides of lower fan leaves with a loupe for the first faint white patches.
  14. Ease off nitrogen. Keep nitrogen moderate so new growth stays firm rather than soft and thin.

If you find the first spots

  1. Remove affected leaves and seal them in a plastic bag. Do not compost them.
  2. Lower humidity by 5 points and add one more fan aimed across the affected zone.
  3. Treat the whole tent with potassium bicarbonate or a Bacillus subtilis product. Skip sulfur if buds have formed.
  4. Recheck every 48 hours and cull heavily infected plants in mid-flower rather than letting spores spread.

Mistakes that cause outbreaks

  • Reading humidity at the tent floor instead of canopy level
  • Running fans that stir air only above the plants and leave the lower canopy stagnant
  • Packing plants so leaves overlap and trap moisture
  • Spraying sulfur during flower, which damages buds and leaves residue
  • Bringing outdoor plants or unverified clones straight into the tent