# Septoria leaf spot (yellow leaf spot) in cannabis

> Septoria leaf spot (Septoria cannabis) starts as small brown spots with bright yellow halos on lower, older leaves and climbs the plant in wet, humid weather by rain and water splash. It is mainly an outdoor and greenhouse problem; manage it with clean lower canopies, dry foliage, mulch, airflow and debris removal, and protectants only where registered.

- Canonical: https://genetixbay.com/knowledge/diseases/septoria-leaf-spot.html
- Category: Diseases · Level: Beginner
- Evidence: Supported · Status: Draft: awaiting expert review (some figures are being checked)
- Stage: Veg, Flower · Systems: Soil, Living soil, Any system
- Updated: 2026-10-31 · Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay Knowledge Base (genetixbay.com/knowledge)"

## Questions and answers

**What causes brown spots with yellow halos on lower cannabis leaves?** Small brown spots with bright yellow halos on the lowest, oldest cannabis leaves, enlarging and merging into blotches, are the typical sign of septoria leaf spot, according to extension sources. Look with a 10 to 20x lens for tiny black fruiting bodies (pycnidia) in older lesions. The disease climbs the plant during rain, overhead watering and high humidity.

**What is septoria leaf spot on cannabis?** Septoria leaf spot, also called yellow leaf spot, is a fungal disease caused by Septoria cannabis, which infects only cannabis and hemp. It overwinters on infected plant debris and spreads by spores splashed from black fruiting bodies onto wet leaves. Kentucky extension reports it as the most common leaf spot of field hemp; it is mainly an outdoor and greenhouse problem.

**Is it leaf spot disease or a nutrient deficiency on my cannabis?** Leaf spot disease and nutrient deficiency on cannabis differ in pattern. Septoria spots have yellow halos and black dots, start on the oldest leaves and spread leaf to leaf after wet weather. Nutrient spots usually lack halos and black dots and follow a symmetrical pattern by leaf age, such as new leaves for calcium. Check [pH](https://genetixbay.com/knowledge/glossary/ph.md) and the [root zone](https://genetixbay.com/knowledge/glossary/root-zone.md) first.

**How do I stop leaf spot spreading up my cannabis plant?** Stop septoria leaf spot spreading up a cannabis plant by removing and bagging spotted lower leaves, lifting the lowest leaves to about 20 to 30 cm above the soil, mulching to stop splash, and watering at the base in the morning so foliage dries. Space and prune for airflow. New spots should stop on middle and upper leaves within 7 to 14 days.

**Does septoria affect cannabis buds or yield?** Septoria usually has a modest effect on cannabis yield when it arrives late in the season; Utah State extension notes no [cannabinoid](https://genetixbay.com/knowledge/glossary/cannabinoid.md) yield losses observed from leaf spots in other states. Severe early infection is different, causing stunting and loss of vigour according to Kentucky extension. A few spotted lower leaves in late flower are a cosmetic problem, not an emergency.

**Short answer.** Septoria leaf spot is caused by the fungus *Septoria cannabis*, which only infects cannabis/hemp [1]. It starts on the lowest, oldest leaves as small brown spots with bright yellow halos that enlarge and merge into blotches [2]. Spores move upward in rain splash and overhead watering, so wet, humid weather spreads it fast [1]. It is mostly an outdoor and greenhouse disease. Keep foliage dry, clean up the lower canopy, mulch the soil, and remove debris; use protectants only where registered.

## What's happening

The fungus overwinters on infected plant debris. In spring and summer, spores are released from tiny black fruiting bodies (pycnidia) in old lesions and splash onto nearby leaves. Infection needs leaf wetness, and spread is rapid during extended rain or high humidity [1]. Lower leaves get hit first because they are closest to splash and stay wet longest. Kentucky reports it as the most common leaf spot of field hemp there [1]; Florida lists it among the most common hemp leaf spots [3].

Yield impact is usually modest when it arrives late. Utah State notes no cannabinoid yield losses observed from leaf spots in other states [2], while Kentucky notes stunting and loss of vigour with severe early infection [1].

## How to recognise it

- **Where:** lower, older [fan leaves](https://genetixbay.com/knowledge/glossary/fan-leaf.md) first; moves upward.
- **What:** small (a few mm) round-to-angular brown or tan spots with a bright yellow halo [2]. Spots enlarge and coalesce into irregular brown blotches; [leaflets](https://genetixbay.com/knowledge/glossary/leaflet.md) yellow and drop.
- **Hand lens (10–20x):** tiny black dots (pycnidia) in the centre of older lesions are a strong sign of Septoria.
- **Pattern:** worst after rain, on the side facing irrigation splash, on plants in the wettest part of the garden.

### Look-alikes (differential)
| Looks like | How to tell |
|---|---|
| Calcium or magnesium issue, "rust spots" | Nutrient spots usually lack yellow halos and black dots; often appear on new (Ca) or mid (Mg) leaves symmetrically. Check pH first. See [rust spots](https://genetixbay.com/knowledge/diagnosis/rust-spots.md) |
| [Nitrogen deficiency](https://genetixbay.com/knowledge/glossary/nitrogen.md) | Uniform yellowing of whole lower leaves, no discrete spots. See [yellowing lower leaves](https://genetixbay.com/knowledge/diagnosis/yellowing-lower-leaves.md) |
| Spray or [light burn](https://genetixbay.com/knowledge/glossary/light-stress.md) | Pattern matches droplets or the hottest canopy spot; doesn't spread from leaf to leaf |
| Other leaf spots (*Cercospora*, *Alternaria*, bacterial) | Lab confirmation needed; management is similar [3] |
| Pest stippling | Tiny pale dots, insects or mites on undersides |

## Numbers that matter

| Parameter | Target | Notes |
|---|---|---|
| Leaf wetness duration | Minimise; foliage dry within 1–2 h after watering or rain | Infection requires wetness (*Established* for Septoria genus) |
| Irrigation | Drip or soil-level watering | Splash spreads spores [1] |
| Lowest leaves | Keep 20–30 cm (8–12 in) above soil outdoors | Reduces splash infection (*Grower practice*) |
| [Plant spacing](https://genetixbay.com/knowledge/glossary/plant-density.md) | Leaves of neighbouring plants not touching | Airflow and faster drying [2] |
| Indoor RH | 55–70 % veg, 40–50 % late flower | Indoors Septoria is uncommon with dry foliage (*Grower practice*) |

## What to check first

1. **Old or new leaves?** Septoria starts low and old. Spots appearing first on new growth point elsewhere (pH, calcium, pests).
2. **Look for halos and black dots** with a hand lens.
3. **Check pH and the root zone** before concluding it's a disease, because nutrient spotting and fungal spotting are confused often.
4. **Check the weather and watering:** recent rain, overhead watering, [foliar sprays](https://genetixbay.com/knowledge/glossary/foliar-feeding.md).
5. **Is it spreading leaf to leaf?** Disease spreads; nutrient problems follow a pattern by leaf age.

## How to fix it

### Cultural (first, and usually enough)
- **Remove spotted lower leaves** and bag them; don't leave them on the ground.
- **Lift the skirt:** remove the lowest leaves that touch or sit near soil.
- **Mulch** the soil surface (straw, wood chips) to stop splash.
- **Water at the base**, in the morning, so foliage dries during the day.
- **Space and prune** for airflow [2].
- **End of season:** remove and destroy all debris; don't till infected residue into next year's bed without a gap. Rotate outdoor beds where practical [1].

### Biological and least-toxic options
Registrations vary by jurisdiction; Utah State notes not all listed materials are registered for edible crops [2]. Check local regulations and the label before using anything.
- **Protectant classes** listed for hemp leaf spots include sulfur, copper-based products and biofungicides (e.g. *Bacillus*-based) [2]. They protect healthy leaves; they don't cure spots.
- **Biofungicides (*Bacillus* spp., *Streptomyces* spp.):** *Supported in other crops*; limited cannabis data.

### What not to do in flower
- **No copper or sulfur sprays on buds.** Residue on flowers is unacceptable and can fail testing where required.
- **No foliar feeding or overhead watering** once spotting starts, and none on flowers ever.
- **Don't strip a flowering plant bare** to remove every spotted leaf. Remove the worst; leaves still feed the buds. A few spotted lower leaves in late flower are a cosmetic problem, not an emergency.

## How to confirm it worked

- New spots stop appearing on middle and upper leaves within 7–14 days of drier conditions and lower-leaf cleanup.
- New growth is clean.
- Next season, fewer early spots after debris removal and mulching.

## Common mistakes

- Treating halo spots as [calcium deficiency](https://genetixbay.com/knowledge/glossary/calcium.md) and adding more [Cal-Mag](https://genetixbay.com/knowledge/glossary/cal-mag.md).
- Leaving fallen leaves under plants.
- Watering with a hose wand over the foliage at dusk.
- Spraying protectants after the disease is everywhere instead of fixing wetness and splash.

## Evidence

- *Established:* *Septoria cannabis* causes leaf spot on hemp, starting on lower leaves and spreading in wet weather [1][2][3].
- *Supported:* [cultural controls](https://genetixbay.com/knowledge/glossary/cultural-control.md) (debris removal, spacing, avoiding splash) reduce disease [1][2].
- *Supported:* yield impact is often limited when infection is late [2]; early severe infection reduces vigour [1].
- *Grower practice:* lifting lower canopy, mulching, numeric splash heights.

## Sources

1. 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
2. Utah State University Extension. Hemp leaf spot (IPM Notes). https://extension.usu.edu/planthealth/ipm/notes_ag/hemp-leaf-spot
3. University of Florida IFAS Plant Pathology. Hemp diseases. https://plantpath.ifas.ufl.edu/extension/hemp-diseases/
4. Punja ZK et al., 2019. Pathogens and molds affecting production and quality of Cannabis sativa L. Frontiers in Plant Science 10:1120. https://www.frontiersin.org/articles/10.3389/fpls.2019.01120/full