White crust or residue that wipes off and stays gone

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.

1. PossibleHard-water or spray residue

   Physical damageLow urgencyRead: White powder or white spots on cannabis leaves: a differential

   Fix: Harmless in itself; stop misting with hard water (use RO for foliar sprays) and wipe off heavy residue that blocks light.

   Recovery: Doesn't come back after wiping.

   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.

2. PossibleSpray or foliar burn (pesticide, oil, soap, foliar feed)

   Physical damageLow urgencyRead: What can I safely spray on plants in flower?

   Fix: Stop spraying; rinse residue off with plain water at lights-off if damage is fresh. In future spray only at lights-off or with lights raised, test one leaf 24-48 h first, never mix sulfur with oils, and never spray buds.

   Recovery: No new damage on leaves that grow after the spray.

   How to confirm it

   • Magnifier check of leaf undersides and new growth. Use a 10-20x loupe for spider mites, thrips, aphids and whiteflies, and a 30-60x lens or clip-on phone macro for broad and russet mites. Check the undersides of 10 affected leaves across several plants and the newest shoot tips, in white light. Look for moving dots, eggs, shed skins, webbing, slender insects with black specks, and glassy specks on new growth.
   • 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.

3. Less likelyHard, alkaline water pushing pH up over weeks

   Root zoneLow urgencyRead: Water quality for cannabis: RO vs tap, alkalinity, bicarbonates and chloramine

   Fix: Alkalinity, not water pH, drives the climb. Above about 100-150 mg/L as CaCO3 (2-3 meq/L), neutralise most of it with acid to an input pH of 5.8-6.2, or blend with RO water. Track runoff pH weekly.

   Recovery: Runoff pH stops climbing over 2-3 weeks.

   How to confirm it

   • Get a water report (or test your water). Ask your water utility for the annual report or test well water: alkalinity (mg/L CaCO3), pH, EC, Ca, Mg, Na, Cl, B, and whether it uses chlorine or chloramine. Alkalinity above ~100-150 mg/L as CaCO3 pushes root-zone pH up; sodium above ~50 mg/L and chloride above ~70 mg/L are a problem; softened water is high in sodium.
   • Measure input pH and root-zone (runoff or reservoir) pH. Calibrate the meter with fresh pH 7 and pH 4 buffers, then measure the mixed feed and the runoff (containers) or reservoir (hydro). Soil and living soil should sit at 6.0-7.0, coco, rockwool and peat mixes at 5.5-6.5, hydro at 5.5-6.2. Track 3+ readings; one reading is not a trend.
   • Pour-through extraction for container pH and EC. Water normally, wait 30 minutes to 2 hours, pour enough pH-adjusted water on top to collect about 50 mL of leachate (roughly 75 mL for a 15 cm pot, 150 mL for a 4 L pot, 350 mL for 11 L), and measure pH and EC of that 50 mL. Less disruptive than full runoff and more repeatable.

Look for

Crusty droplet-shaped white marks where water dried, often on top leaves..