# Leaf spot

> Any disease that causes discrete dead spots on leaves; on cannabis, mainly fungi such as Septoria cannabis and Alternaria, plus some bacteria.

- Canonical: https://genetixbay.com/knowledge/glossary/leaf-spot.html
- Also written: leaf spots, leaf spot disease, Alternaria, brown leaf spot, fungal leaf spot
- 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"

Leaf spot is a descriptive term for any disease that produces distinct [necrotic spots](https://genetixbay.com/knowledge/glossary/necrosis.md) on leaves. On cannabis the commonest cause is *[Septoria cannabis](https://genetixbay.com/knowledge/glossary/septoria-leaf-spot.md)* (Septoria leaf spot); *Alternaria* species, *Cercospora* species and some bacteria also cause leaf spots, mostly outdoors. Spots often have a darker margin, a yellow halo, or concentric rings, depending on the organism.

## In practice

Most leaf spot pathogens need wet leaves, so they appear after rain, heavy dew or overhead watering, and on lower leaves first where splash and humidity are highest. Growers check spots with a hand lens for tiny black fruiting bodies, which point to a fungus, and send samples to a plant diagnostic lab when unsure. Management is the same across most types: remove infected leaves, clear debris, water at the base, improve airflow, and rotate outdoor sites. Many "spots" turn out to be nutrient or [pH](https://genetixbay.com/knowledge/glossary/ph.md) problems, so check the [root zone](https://genetixbay.com/knowledge/glossary/root-zone.md) before blaming a pathogen.

## Don't confuse with

- **Calcium or potassium deficiency**: patterns linked to feeding and pH, no fruiting bodies.
- **Light or spray burn**: spots where droplets or hot spots hit, not spreading.
- **Spider mite stippling**: many tiny pale dots.

## Related terms

[Septoria leaf spot](https://genetixbay.com/knowledge/glossary/septoria-leaf-spot.md), [Downy mildew](https://genetixbay.com/knowledge/glossary/downy-mildew.md), [Sanitation](https://genetixbay.com/knowledge/glossary/sanitation.md), [Scouting](https://genetixbay.com/knowledge/glossary/scouting.md)

## Read more

- [Septoria leaf spot (yellow leaf spot) in cannabis](https://genetixbay.com/knowledge/diseases/septoria-leaf-spot.md)
- [Rust-coloured spots on cannabis leaves: a differential](https://genetixbay.com/knowledge/diagnosis/rust-spots.md)

## Sources

1. Utah State University Extension. Hemp leaf spot (IPM Notes). https://extension.usu.edu/planthealth/ipm/notes_ag/hemp-leaf-spot
2. University of Kentucky Plant Pathology. Septoria leaf spot of hemp (PPFS-AG-H-01). https://plantpathology.mgcafe.uky.edu/files/ppfs-ag-h-01.pdf