# Black sooty coating on leaves: likely causes and fixes

> Black sooty coating on leaves on cannabis is most often Whiteflies, Aphids or Mealybugs. How to tell them apart, the fix for each and what recovery looks like.

Also called: black mould on leaves, sooty mould, black film, soot on leaves.

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

## Likely causes, most likely first

### Whiteflies (Possible)

- Kind: Pest; urgency: Deal with it this week
- 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.
- Confirm: 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.
- Confirm: 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.
- Article: [Whiteflies on cannabis: life table, temperature, parasitoids and IPM](https://genetixbay.com/knowledge/pests/whiteflies.md)

### Aphids (Possible)

- Kind: Pest; urgency: Deal with it this week
- 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.
- Confirm: 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.
- Confirm: 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.
- Article: [Aphids on cannabis: species, life tables, population growth and IPM](https://genetixbay.com/knowledge/pests/aphids.md)

### Mealybugs (Less likely)

- Kind: Pest; urgency: Deal with it this week
- 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.
- Confirm: 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.
- Article: [Mealybugs and root mealybugs on cannabis: ID, life cycle in numbers and IPM](https://genetixbay.com/knowledge/pests/mealybugs.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.