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 |
| Slender fast insects, silver scars, black frass specks | Thrips [10] | Thrips |
| Sticky honeydew, soft-bodied insects | Aphids or whiteflies | Aphids, whiteflies |
| Nothing visible, but new growth twisted and glossy | Broad or russet mites (need 30–60×) | Broad and russet mites |
| 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 (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, 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) |
| Round or teardrop spots where spray droplets sat, on the light-facing side | Foliar or pesticide burn |
| Dry, papery, torn patches on leaves facing a fan | Wind burn |
| 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) | 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 more than ~1.0 mS/cm above input | Salt build-up; tips and margins burn |
| RH 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 | 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, 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 |
| 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 |
| Feed Mn | ~0.3–0.8 mg/L | See manganese |
| 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.
- Lens under the leaf, five leaves, top, middle and bottom.
- Spot shape. Halo plus black dots = disease. Pinpoint speckle = pests. Droplet shapes = spray.
- Spray and equipment log. Anything sprayed in the last 7 days? Fan moved? Light lowered?
- Calibrated runoff pH and EC.
- RH and leaf temperature log for the past week.
- Root inspection if spots come with wilting or slow growth.
- 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.
- 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).
- Potassium: confirm EC is not excessive; if the feed is short, move towards ~175–240 mg/L K (see potassium).
- 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 otherbeing reviewed.
- 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 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.
Slender insects about 1-2 mm long, chiefly western flower thrips (Frankliniella occidentalis), that rasp leaf surfaces, leaving silvery streaks dotted with black frass.A pattern of many tiny pale dots on leaves where a sucking pest such as a spider mite has emptied individual cells; the first visible sign of a mite infestation.A fungal leaf disease caused by Septoria cannabis that produces tan to grey spots with dark margins and yellow halos, starting on lower leaves.Yellowing of the leaf tissue between the veins while the veins themselves stay green; on old leaves it points to magnesium, on new leaves to iron or manganese.pH measures how acidic or alkaline a solution is on a 0–14 scale; in growing it controls nutrient uptake, with soil best near 6.0–7.0 and hydro 5.5–6.5.EC (electrical conductivity) measures how well a solution conducts electricity, a proxy for total dissolved salts; growers read it in mS/cm to gauge nutrient strength.Foliar feeding is spraying a dilute nutrient solution onto leaves for quick uptake through the leaf surface; it suits micronutrient fixes but cannot replace root feeding.Leaf damage from a fan blowing directly and constantly on the same leaves: curled, clawed or crispy leaves on the side facing the fan, sometimes with brown, dried patches.Runoff is the water or nutrient solution that drains out of the bottom of a container after irrigation; growers test it to see what is happening in the root zone.Salt build-up is the accumulation of unused fertiliser and water salts in the growing medium, raising root-zone EC and causing osmotic stress and lockout-type symptoms.A soilless medium contains no mineral soil, such as peat, coco, rockwool or perlite blends, and relies mostly on the grower to supply nutrients.Runoff EC is the conductivity of solution draining from the pot; compared with input EC, it shows whether salts are building up or being used up in the root zone.Relative humidity is the water vapour in air as a percentage of what it could hold at that temperature, so the same RH dries plants differently at different temperatures.Nutrient toxicity is damage from an excess of an element in the plant, either directly or by blocking uptake of others; boron, manganese, copper and chloride are common culprits.Leaf temperature is the actual surface temperature of a leaf, which often differs from air temperature by 1–3 °C depending on light source and transpiration.A nutrient deficiency is a shortage of an essential element in the plant tissue, causing characteristic symptoms; the cause is often pH or uptake rather than an empty feed.
