Powdery mildew (PM) is a surface fungus that thrives in cool, humid conditions. It's airborne, it travels on clones, and once it's in your room it's hard to evict.

Prevention

  • Quarantine every clone for 10+ days, including ones from trusted friends
  • Keep RH under 55% in late flower
  • Air circulation under the canopy
  • Sulfur burner in veg only (not within 3 weeks of harvest)

If you find PM in flower

  • Cut affected leaves and bag them — don't shake into the room
  • Drop RH and increase airflow immediately
  • Spot treat with potassium bicarbonate (food-safe) up to 2 weeks before chop
  • Don't bother with milk sprays late in flower — they leave residue