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 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 yellow and drop.
- Hand lens (10–20x): tiny black dots (pycnidia) in the centre of older lesions are a strong sign of Septoriabeing reviewed.
- 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 |
| Nitrogen deficiency | Uniform yellowing of whole lower leaves, no discrete spots. See yellowing lower leaves |
| Spray or light burn | 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)being reviewed |
| 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 | 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
- Old or new leaves? Septoria starts low and old. Spots appearing first on new growth point elsewhere (pH, calcium, pests).
- Look for halos and black dots with a hand lens.
- Check pH and the root zone before concluding it's a disease, because nutrient spotting and fungal spotting are confused often.
- Check the weather and watering: recent rain, overhead watering, foliar sprays.
- 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 and adding more Cal-Mag.
- 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 (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.
