# Spots, brown patches and dead edges on cannabis leaves: the master decision tree

> Cannabis leaf spots and dead tissue sort into four groups: pest feeding (pale stippling or silver scars, seen with a 10–20× lens under the leaf), disease (spots with yellow halos and tiny black dots, spreading from the bottom in humidity), nutrient or root-zone disorders (Ca, P, K, Mn, B, salts, in a pattern tied to leaf age) and physical damage (spray droplets, fans, heat, light). Flip the leaf first, then check runoff pH and EC, and only then pick a nutrient.

- Canonical: https://genetixbay.com/knowledge/diagnosis/spots-necrosis-decision-tree.html
- Category: Diagnosis · Level: Intermediate
- Evidence: Supported · Status: Draft: awaiting expert review (some figures are being checked)
- Stage: Veg, Flower, Ripening · Systems: 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 on cannabis leaves?** Brown spots on cannabis leaves come from four groups of causes: pests such as spider mites and [thrips](https://genetixbay.com/knowledge/glossary/thrips.md), diseases such as [septoria leaf spot](https://genetixbay.com/knowledge/glossary/septoria-leaf-spot.md), nutrient or [pH](https://genetixbay.com/knowledge/glossary/ph.md) problems with calcium, phosphorus, potassium or manganese, and physical damage from spray droplets, fans or heat. Turn the leaf over with a 10–20× lens first, then check [runoff](https://genetixbay.com/knowledge/glossary/runoff.md) pH and [EC](https://genetixbay.com/knowledge/glossary/ec.md) before adding any nutrient.

**Is it calcium deficiency or spider mites on my cannabis?** Spider-mite damage on cannabis is many tiny pale dots, evenly peppered across the upper leaf, with moving mites, eggs or fine webbing on the underside under a 10–20× lens. Calcium spots are fewer, larger, irregular brown blotches with no insects present, usually on newer or middle leaves, and often follow low pH, high humidity or root damage. Always look under the leaf first.

**What do septoria leaf spots look like on cannabis?** Septoria leaf spot on cannabis appears as round to angular tan or grey-brown spots, often with a yellow halo, that start on the lowest, oldest leaves and climb in humid, wet conditions. Under a lens, mature spots show tiny black dots, the fungal fruiting bodies. Nutrient spots do not show fruiting bodies and do not spread from leaf to leaf in wet weather.

**Why are my cannabis leaf edges brown and crispy?** Brown, crispy edges on cannabis leaves usually mean salt stress from high root-zone EC, or potassium shortage on older leaves, which scorches the margins and serrations. Edges burnt only on the top leaves under a strong light or near a fan point to heat, light or wind. Compare runoff EC with input EC first; more than about 1.0 mS/cm above input points to [salt build-up](https://genetixbay.com/knowledge/glossary/salt-build-up.md).

**Do brown spots on cannabis leaves go away?** Brown spots and dead tissue on cannabis leaves never heal or turn green again, because the cells are dead. Judge any fix by whether new spots stop appearing on fresh growth over the next 7 to 14 days. Remove badly diseased leaves to cut spore load, but keep mildly marked healthy leaves, which still photosynthesise and feed the buds.

**Short answer.** Spots and dead (necrotic) tissue on cannabis come from four places: pests feeding, a leaf disease, a nutrient or root-zone disorder, or physical damage. The fastest split is a 10–20× lens under the leaf: moving dots, eggs, webbing or black specks mean pests. Spots with yellow halos and tiny black dots that climb from the bottom in humidity mean disease. Spots tied to leaf age, with readings out of range, mean the root zone or a nutrient. Droplet-shaped or one-sided burns mean sprays, fans, heat or light.

The GenetixBay symptom checker follows the same order, so answers you gather here carry straight over.

## What is leaf necrosis on cannabis?

Necrosis is dead tissue: brown, tan, black or bleached patches that crack and crumble. It is the end stage of many problems, so its *shape* and *location* matter more than its colour. Nutrient necrosis usually follows a yellowing phase and keeps to a pattern (margins, between veins, tips). Pest necrosis starts as many tiny pale feeding points that merge. Disease spots expand as rings with a defined edge and often a halo. Physical burns are sharp-edged and match the shape of whatever caused them [3][6].

## The spots decision tree, step by step

### Fork 1: Flip the leaf. What is underneath?

Turn over five affected leaves from different parts of the plant and look with a 10–20× lens.

| Under the leaf | Suspect | Go to |
|---|---|---|
| Tiny moving dots, round eggs, fine silk | Spider mites [5] | [Spider mites](https://genetixbay.com/knowledge/pests/spider-mites.md) |
| Slender fast insects, silver scars, black frass specks | Thrips [10] | [Thrips](https://genetixbay.com/knowledge/pests/thrips.md) |
| Sticky honeydew, soft-bodied insects | Aphids or whiteflies | [Aphids](https://genetixbay.com/knowledge/pests/aphids.md), [whiteflies](https://genetixbay.com/knowledge/pests/whiteflies.md) |
| Nothing visible, but new growth twisted and glossy | Broad or russet mites (need 30–60×) | [Broad and russet mites](https://genetixbay.com/knowledge/pests/broad-russet-mites.md) |
| Nothing | Fork 2 |  |

### Fork 2: What does a single spot look like?

| Spot appearance | Suspect |
|---|---|
| Many pinpoint pale dots merging into a speckled, bleached area | Mite [stippling](https://genetixbay.com/knowledge/glossary/stippling.md) (even if no mites seen, check again) |
| Silvery, papery streaks with black dots | Thrips scarring |
| Round to angular tan or grey-brown spots with a yellow halo; tiny black dots in older spots | Septoria leaf spot [4] |
| Irregular rusty-brown blotches, no halo, on newer or middle leaves | Calcium uptake problem [1] |
| Dark, dull, olive to brown spots on older leaves, purple petioles | Phosphorus [1][2] |
| Yellow then tan scorch on margins and serrations of older leaves | Potassium [1] |
| Small tan specks within netted [interveinal yellowing](https://genetixbay.com/knowledge/glossary/interveinal-chlorosis.md), upper-middle leaves | Manganese deficiency |
| Brown-black specks along veins of older leaves, low pH | Manganese toxicity [3][7] |
| Marginal yellowing then necrosis of older leaves, boron-rich water or feed | Boron toxicity [1] |
| Crispy tips on otherwise healthy leaves | Salt stress / high EC (see [brown tips](https://genetixbay.com/knowledge/diagnosis/brown-burnt-tips.md)) |
| Round or teardrop spots where spray droplets sat, on the light-facing side | [Foliar](https://genetixbay.com/knowledge/glossary/foliar-feeding.md) or pesticide burn |
| Dry, papery, torn patches on leaves facing a fan | [Wind burn](https://genetixbay.com/knowledge/glossary/wind-burn.md) |
| Bleached or crisp edges on the top leaves only | Heat or light stress |

### Fork 3: Where on the plant, and how is it spreading?

| Location and spread | Points to |
|---|---|
| Oldest leaves first, climbing slowly, worse in humid weeks | Septoria, or a mobile nutrient (P, K, Mg) |
| Newest or middle leaves, appearing in flushes after humid or wet-root days | Calcium (transpiration-limited) |
| Whole plant, all ages, many tiny dots | Mites |
| Top of canopy only | Heat, light, foliar spray under lights |
| One side only, facing a fan or vent | Wind burn |
| One plant first, then neighbours | Pests or disease; isolate |

### Fork 4: What do the root-zone readings say?

Readings decide between nutrient look-alikes. Calcium spots at runoff pH 5.2 are a pH problem. Potassium scorch with runoff EC 3.5 mS/cm over a 1.8 input is salt build-up.

| Reading | Interpretation |
|---|---|
| Runoff pH below ~5.5 ([soilless](https://genetixbay.com/knowledge/glossary/soilless.md)) | Ca and Mg less available; Mn can rise to toxic |
| Runoff pH above ~6.5 (soilless) or ~7.0 (soil) | P, Mn, Zn, Fe less available |
| [Runoff EC](https://genetixbay.com/knowledge/glossary/runoff-ec.md) more than ~1.0 mS/cm above input | Salt build-up; tips and margins burn |
| [RH](https://genetixbay.com/knowledge/glossary/relative-humidity.md) above ~70% for days, little airflow | Ca transport falls; septoria spreads |
| Root-zone temperature below ~16–18 °C (61–64 °F) | P uptake slows |

## The spots and necrosis differential table

| Symptom pattern | Most likely | Rule out first | Key test |
|---|---|---|---|
| Fine pale stippling, upper surface | Spider mites | Ca spotting | 10–20× lens underside |
| Silver streaks with black specks | Thrips | Mites, PM residue | Lens; blue or yellow sticky card |
| Halo spots climbing from the bottom | Septoria | P or K deficiency | Lens for black fruiting dots; spread with humidity |
| Rust blotches, new/middle leaves | Calcium | Low pH, high RH, root damage, mites | Runoff pH, RH log, lens |
| Olive-brown spots, old leaves, dull dark leaves | Phosphorus | Cold roots, high pH | Root-zone temperature, runoff pH |
| Margin scorch, old leaves | Potassium | Salt build-up, B or Mn [toxicity](https://genetixbay.com/knowledge/glossary/nutrient-toxicity.md) | Runoff EC vs input |
| Black-brown vein specks, old leaves | Mn toxicity | Septoria | Runoff pH (low), tissue test |
| Tip burn only | High EC / salts | K deficiency | Runoff EC vs input |
| Droplet-shaped burns | Foliar or pesticide burn | Septoria | Spray log, pattern on light side |
| Torn, papery patches near fan | Wind burn | Heat | Fan position |
| Crisp top-canopy edges | Heat or light stress | K | [Leaf temperature](https://genetixbay.com/knowledge/glossary/leaf-temperature.md), PPFD |

## What numbers matter for spots and necrosis?

| Parameter | Target | Notes |
|---|---|---|
| Root-zone pH, hydro | 5.5–6.0 | [8] |
| Root-zone pH, coco/rockwool | 5.5–6.5 | Low pH reduces Ca and Mg availability |
| Root-zone pH, soil | 6.0–7.0 | Grower practice |
| Runoff EC vs input | within ~0.5 mS/cm | > 1.0 above input = build-up (grower practice) |
| Feed Ca, soilless | ~150–200 mg/L | See [calcium](https://genetixbay.com/knowledge/nutrients/calcium.md) |
| Feed P | ~30–60 mg/L | Higher P adds nothing and can tie up Zn and Fe |
| Feed B | ~0.2–0.5 mg/L | Narrow margin to toxicity; see [boron](https://genetixbay.com/knowledge/nutrients/boron.md) |
| Feed Mn | ~0.3–0.8 mg/L | See [manganese](https://genetixbay.com/knowledge/nutrients/manganese.md) |
| RH, veg / flower | ~55–70% / ~40–55% | Grower practice; high RH favours septoria and slows Ca |
| Leaf temperature, lights on | ~24–28 °C (75–82 °F) | Photosynthesis fell above ~30 °C in one cultivar [9] |
| Leaf Ca, healthy | ~1.5–4.4% dry weight | [1] |
| Leaf P, healthy | ~0.24–0.49% | [1] |

## What should I check first when I see spots?

Check for pests and disease before nutrients, because they spread and nutrients don't.

1. **Lens under the leaf**, five leaves, top, middle and bottom.
2. **Spot shape.** Halo plus black dots = disease. Pinpoint speckle = pests. Droplet shapes = spray.
3. **Spray and equipment log.** Anything sprayed in the last 7 days? Fan moved? Light lowered?
4. **Calibrated runoff pH and EC.**
5. **RH and leaf temperature log** for the past week.
6. **Root inspection** if spots come with wilting or slow growth.
7. **Only now** choose between Ca, P, K, Mn and B by location and pattern.

## How do I fix spots and necrosis on cannabis?

- **Mites and thrips:** follow the pest articles; act early, because populations double in days in warm rooms. Predatory mites are the backbone of indoor control.
- **Septoria:** remove the worst lower leaves (bag them, don't drop them), lower RH below ~60%, increase airflow under the canopy, stop wetting foliage; see [septoria leaf spot](https://genetixbay.com/knowledge/diseases/septoria-leaf-spot.md).
- **Calcium:** correct pH first; lower RH towards range so the plant transpires; feed about 150–200 mg/L Ca in soilless; buffer new coco; add Ca only if the recipe and water report are short.
- **Phosphorus:** warm the root zone to 18–22 °C; correct high pH; P at about 30–60 mg/L is enough. Bloom boosters are not the fix (see [bloom boosters](https://genetixbay.com/knowledge/nutrients/bloom-boosters.md)).
- **Potassium:** confirm EC is not excessive; if the feed is short, move towards ~175–240 mg/L K (see [potassium](https://genetixbay.com/knowledge/nutrients/potassium.md)).
- **Mn toxicity:** raise root-zone pH into range; Mn availability falls sharply as pH rises.
- **Boron toxicity:** check the water report; switch water or reduce B-containing additives; a root-zone flush of the medium.
- **Salt build-up:** root-zone flush with pH-adjusted, low-EC solution until runoff EC is near input, then resume normal feed.
- **Foliar or pesticide burn:** stop spraying under lights; spray at lights-off, at label rate, test on one branch first; never combine sulfur and oil products within about two weeks of each other.
- **Wind burn:** angle fans across, not at, the canopy; leaves should flutter, not thrash.
- **Heat or light:** raise or dim the light; bring leaf temperature under ~28–30 °C (82–86 °F).

## How do I know the fix worked?

Dead spots stay dead; success is no new spots on growth that emerged after the fix, usually within 7–14 days. Pest fixes are confirmed by repeated lens checks showing no live mites or thrips for three consecutive weekly scouts. Septoria is confirmed when spots stop climbing. Nutrient fixes are confirmed when runoff readings stay in range and new leaves come in clean.

## What are the common mistakes with leaf spots?

- Adding Cal-Mag for mite stippling. The lens costs less than the bottle.
- Spraying anything under hot lights, then blaming a [deficiency](https://genetixbay.com/knowledge/glossary/nutrient-deficiency.md) for the burn.
- Stripping healthy leaves with a few old spots; they still feed the buds.
- Trusting one leaf; diagnose from the pattern across the plant.
- Treating septoria with nutrients, or calcium spots with fungicide.
- Assuming Cal-Mag is needed because you run LEDs; that is a *Myth*. Ca and Mg needs depend on cultivar, water source and medium.

## How strong is the evidence?

- Nutrient spot and necrosis patterns (Ca, P, K, Mg, Mn, B, Zn) in cannabis are *Established* by controlled trials in substrate and DWC [1][2]; Mn toxicity specks are *Established* in horticulture [3][7] and *Supported* for cannabis.
- Septoria symptoms and spread on cannabis/hemp are *Established* by extension [4][6].
- Mite and thrips damage patterns are *Established* [5][10].
- Foliar burn and wind burn patterns are *Grower practice*: obvious, but rarely studied in cannabis.
- Heat thresholds come from one photosynthesis study [9] and grower practice: *Supported*.

## Sources

1. Cockson P, et al., 2019. Characterization of Nutrient Disorders of Cannabis sativa. Applied Sciences 9(20):4432. https://doi.org/10.3390/app9204432
2. Llewellyn D, Golem S, Jones AMP, Zheng Y., 2023. Foliar Symptomology, Nutrient Content, Yield, and Secondary Metabolite Variability of Cannabis Grown Hydroponically with Different Single-Element Nutrient Deficiencies. Plants 12(3):422. https://doi.org/10.3390/plants12030422
3. Marschner P (ed.). 2012. Marschner's Mineral Nutrition of Higher Plants, 3rd ed. Academic Press. https://doi.org/10.1016/C2009-0-63043-9
4. 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
5. Utah State University Extension. Two-Spotted Spider Mite (hemp). https://extension.usu.edu/planthealth/ipm/notes_ag/hemp-spider-mites.php
6. Punja ZK, et al., 2019. Pathogens and Molds Affecting Production and Quality of Cannabis sativa L. Frontiers in Plant Science 10:1120. https://doi.org/10.3389/fpls.2019.01120
7. Sonneveld C, Voogt W. 2009. Plant Nutrition of Greenhouse Crops. Springer. https://doi.org/10.1007/978-90-481-2532-6
8. Velechovský J, et al., 2024. Frontiers in Plant Science 15:1322824. https://doi.org/10.3389/fpls.2024.1322824
9. Chandra S, Lata H, Khan IA, ElSohly MA., 2008. Photosynthetic response of Cannabis sativa L. to variations in photosynthetic photon flux densities, temperature and CO2 conditions. Physiology and Molecular Biology of Plants 14(4):299-306. https://doi.org/10.1007/s12298-008-0027-x
10. Cranshaw W. Hemp pests (Colorado State University, 2020). https://webdoc.agsci.colostate.edu/bspm/InsectInformation/Talks2020/HempPests.pdf