# Sooty mould

> A black, soot-like fungal film that grows on the honeydew left by sap-feeding insects; it does not infect the plant but blocks light and signals an aphid, whitefly, mealybug or scale problem.

- Canonical: https://genetixbay.com/knowledge/glossary/sooty-mould.html
- Also written: sooty mold, black mould on leaves, black mold on leaves
- Glossary category: Pests and diseases · Evidence: Established
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

Sooty mould is a group of dark fungi that live on [honeydew](https://genetixbay.com/knowledge/glossary/honeydew.md), the sugary excretion of [aphids](https://genetixbay.com/knowledge/glossary/aphid.md), [whiteflies](https://genetixbay.com/knowledge/glossary/whitefly.md), [mealybugs](https://genetixbay.com/knowledge/glossary/mealybug.md) and soft scales. It grows on the leaf surface without infecting the tissue, but it blocks light and makes leaves and buds unusable (UC [IPM](https://genetixbay.com/knowledge/glossary/ipm.md)).

## In practice

Sooty mould is a sign, not the problem: find and control the sap-feeding insect producing the honeydew, usually directly above the blackened leaves. Once the honeydew stops, the mould dries and flakes off over time. Contaminated buds should not be used.

## Don't confuse with

- **Botrytis**: grey fuzzy mould inside buds that rots the tissue.
- **Frass**: [insect droppings](https://genetixbay.com/knowledge/glossary/frass.md) as pellets or specks, not a film.
- **Necrosis**: dead brown tissue within the leaf, not a surface coating.

## Related terms

[Honeydew](https://genetixbay.com/knowledge/glossary/honeydew.md), [Aphid](https://genetixbay.com/knowledge/glossary/aphid.md), [Whitefly](https://genetixbay.com/knowledge/glossary/whitefly.md), [Mealybug](https://genetixbay.com/knowledge/glossary/mealybug.md)

## Read more

- [Aphids on cannabis: species, life tables, population growth and IPM](https://genetixbay.com/knowledge/pests/aphids.md)
- [Whiteflies on cannabis: life table, temperature, parasitoids and IPM](https://genetixbay.com/knowledge/pests/whiteflies.md)
- [White or grey coating on cannabis: the master decision tree](https://genetixbay.com/knowledge/diagnosis/white-grey-coatings-decision-tree.md)

## Sources

1. UC IPM. Sooty Mold. https://ipm.ucanr.edu/home-and-landscape/sooty-mold/