Black sooty coating on leaves
Grows on honeydew; follows sticky leaves by days to weeks..
- Also called
- black mould on leaves
- sooty mould
- black film
- soot on leaves
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.
PossibleWhiteflies
Fix: Yellow cards; identify the species; release Encarsia formosa (greenhouse whitefly) or Eretmocerus (warmer rooms, silverleaf whitefly) plus predatory mites; keep sprays out of flower.
Recovery: Card counts fall within 3-4 weeks; parasitised black nymphs appear.
How to confirm it
- Sticky cards for 1-2 weeks. Hang yellow cards at canopy height and lay one flat on the medium surface; add blue cards for thrips. Count catches every 3-4 days. Rising counts of small dark flies at the medium mean fungus gnats; slender insects mean thrips; white wedge-shaped fliers mean whiteflies; winged aphids mean a colony nearby.
- 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.
PossibleAphids
Fix: Squash or rinse small colonies; release parasitoid wasps (Aphidius), predatory midges or lacewings; insecticidal soap or oil only before flower. Control ants that farm them.
Recovery: Colonies shrink and mummified (parasitised) aphids appear within 1-2 weeks.
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.
- Sticky cards for 1-2 weeks. Hang yellow cards at canopy height and lay one flat on the medium surface; add blue cards for thrips. Count catches every 3-4 days. Rising counts of small dark flies at the medium mean fungus gnats; slender insects mean thrips; white wedge-shaped fliers mean whiteflies; winged aphids mean a colony nearby.
Less likelyMealybugs
Fix: Remove colonies with a cotton bud dipped in 70% isopropyl alcohol (veg only, test a leaf first); release mealybug predators (e.g. Cryptolaemus beetles) or parasitoids; quarantine new stock; discard heavily infested plants.
Recovery: No new cottony clusters after 2 weeks.
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.
Look for
Black film that rubs off, on leaves already sticky..
The full guide: White or grey coating on cannabis: the master decision tree
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.
