# Cannabis lookalike problems: how to tell them apart

> Many cannabis problems look alike in a photo. The ones growers confuse most are Ca vs Mg vs K, nitrogen toxicity vs overwatering claw, light bleaching vs Mg deficiency, spider-mite stippling vs calcium spots, heat vs light stress, Pythium vs low dissolved oxygen, Fe vs Mn, and nutrient burn vs K deficiency. Each pair separates on three things: where on the plant it started, the exact pattern, and one cheap test (a lens, a runoff EC reading, a root check, a light meter or a lifted pot).

- Canonical: https://genetixbay.com/knowledge/diagnosis/lookalikes.html
- Category: Diagnosis · Level: Intermediate
- Evidence: Supported · Status: Draft: awaiting expert review (some figures are being checked)
- Stage: Seedling, 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

**How do I tell calcium, magnesium and [potassium deficiency](https://genetixbay.com/knowledge/glossary/potassium.md) apart in cannabis?** Look at where it starts and the pattern. Magnesium yellows between the veins of old, lower leaves while veins stay green. Potassium yellows then browns the margins and serrated tips of old leaves. Calcium hits new or recently matured growth with distorted leaves and brown spots, and is usually a delivery problem ([pH](https://genetixbay.com/knowledge/glossary/ph.md), low transpiration, root damage) rather than an empty feed. Check root-zone pH before dosing any of them.

**Is my cannabis clawing from too much nitrogen or [overwatering](https://genetixbay.com/knowledge/glossary/overwatering.md)?** [Nitrogen toxicity](https://genetixbay.com/knowledge/glossary/nitrogen.md) gives very dark, glossy green leaves whose tips hook down like a claw, on a plant that is otherwise vigorous, with high runoff [EC](https://genetixbay.com/knowledge/glossary/ec.md) and a recent feed increase. Overwatering gives drooping, swollen-looking leaves that curl down along their whole length, a heavy pot, a medium still wet two to three days after watering and perkier leaves once it dries. Lift the pot and read runoff EC to separate them.

**Is it light bleaching or [magnesium deficiency](https://genetixbay.com/knowledge/glossary/magnesium.md) on my cannabis?** Light bleaching is pale-to-white leaves and bracts only at the tops nearest the lamp, often with green veins at first, while lower leaves are fine. Magnesium deficiency starts on old, lower leaves with green veins and yellow blades and climbs upward. If the damage sits where [PPFD](https://genetixbay.com/knowledge/glossary/ppfd.md) is highest, measure it and raise or dim the light; if it starts at the bottom, check pH and the Mg in the feed.

**Are these tiny spots from [spider mites](https://genetixbay.com/knowledge/glossary/two-spotted-spider-mite.md) or calcium deficiency?** Spider mite stippling is hundreds of pin-prick pale dots, usually starting near the leaf midrib, with mites, eggs or fine silk visible underneath through a 10 to 30x lens. Calcium spots are fewer, larger, irregular tan-to-brown patches, often with a yellow halo, on new or recently matured leaves and with no insects underneath. A lens check of the leaf underside settles it in under a minute.

**How do I tell Pythium root rot from low dissolved oxygen in hydro?** Low dissolved oxygen gives limp, tan roots and afternoon wilting that recover within a day or two once you cool the solution and add air. Pythium gives brown, slimy roots whose outer sheath slips off leaving a thin white thread, a sour or rotten smell, and decline that continues after oxygen is fixed. Warm solution above about 23 °C (73 °F) drives both, and low oxygen often comes first.

**Is it iron or manganese deficiency on my cannabis?** Both cause [interveinal yellowing](https://genetixbay.com/knowledge/glossary/interveinal-chlorosis.md) on new growth with green veins. Iron [chlorosis](https://genetixbay.com/knowledge/glossary/chlorosis.md) is smooth, pale and concentrated on the very newest leaves, which can turn almost white. Manganese chlorosis is more netted, sits a little lower on the upper and middle leaves, and develops small tan spots between the veins. Both are usually caused by high root-zone pH, so check pH before adding either.

**Is brown leaf edge nutrient burn or potassium deficiency?** Nutrient burn starts as dry brown tips on the newest mature leaves and creeps back along the margin, with high input or runoff EC. Potassium deficiency starts on older, lower leaves as yellow margins and serration tips that then go tan, with a normal or low EC. A runoff EC reading compared with input EC usually separates them: runoff well above input means salt, not hunger.

**Short answer.** Most misdiagnoses happen between a handful of pairs that look the same in a photo. They almost always separate on three things: **where the symptom started** (old leaves, new leaves, top of the canopy, whole plant), **the exact pattern** (between the veins, margins, tips, spots, stipples), and **one cheap test**: a 10–30× lens under the leaf, a runoff EC and pH reading, a root check, a light meter, or simply lifting the pot. This page puts each pair side by side.

## What's happening

Cannabis has a limited vocabulary of distress. Chlorophyll fades (yellowing), cells die (brown necrosis), leaves change angle (clawing, tacoing, drooping), and pigments show through (purple, red). Very different causes speak that same vocabulary. The way to separate them is the clinician's method in [how to diagnose any problem](https://genetixbay.com/knowledge/diagnosis/how-to-diagnose-any-problem.md): location first, because nutrient mobility decides whether a shortage shows on old or new growth [3]; then pattern; then the test that rules one side out.

Two controlled cannabis studies give us the reference pictures for single-element shortages: Cockson et al. in soilless sand culture [1] and Llewellyn et al. in deep-water culture through flower [2]. They do not always agree on timing, and several deficiencies (Fe, Mn, Mo) produced weak or no symptoms in the shorter hydroponic flower trial [2], so treat any "textbook" picture as a starting point, not proof.

## How do I tell calcium, magnesium and potassium deficiency apart?

**Magnesium yellows between the veins of old leaves; potassium burns the margins of old leaves; calcium distorts and spots new or recently matured leaves.** Mg and K are mobile and are stripped from old leaves first; Ca moves only in the transpiration stream and cannot be remobilised, so its damage lands on growth that is expanding while supply is short [1][3].

| Feature | Calcium (Ca) | Magnesium (Mg) | Potassium (K) |
|---|---|---|---|
| Where it starts | New and recently matured leaves, growing tips; in DWC flower, also lower fan leaves from week 3 [2] | Old, lower fan leaves [1][2] | Old, lower fan leaves; in DWC flower, sugar leaves early [2] |
| Pattern | Irregular brown spots with yellow halos, distorted or cupped new leaves, dead growing tips in severe cases [1] | Interveinal yellowing, veins and midrib stay green [1][2] | Yellow margins and serration tips, moving inward, then tan crispy scorch [1] |
| Colour | Rust to tan spots | Lemon yellow blade, green "skeleton" | Yellow edge, then tan to brown |
| Speed | Fast in high humidity or after root damage | Slow, climbs upward over 1–3 weeks | Slow; shows from flower week 3 in DWC [2] |
| Typical readings | Low root-zone pH, [RH](https://genetixbay.com/knowledge/glossary/relative-humidity.md) above ~70%, unbuffered coco, RO water | Low root-zone pH, excess K or Ca, RO water | Low input EC, or salt stress mimicking it |
| Healthy leaf tissue | ~1.0–2.5% Ca | ~0.40–0.81% Mg [1] | ~1.8–2.9% K [1] |
| The separating test | Look at *which* leaves: new growth means Ca, old growth means Mg or K. Then read runoff pH | Interveinal and lower: Mg | Margin and lower: K |
| Fix | Correct pH, raise transpiration ([VPD](https://genetixbay.com/knowledge/glossary/vpd.md) 0.8–1.2 kPa), Ca to ~150–200 mg/L | Correct pH, then magnesium sulfate to ~40–70 mg/L Mg | Confirm EC is not high, then raise K toward ~175–240 mg/L |

**Why the confusion.** All three can end in brown, dead leaf tissue, and all three are commonly induced rather than real shortages. Excess K suppresses Mg and Ca uptake, and excess Ca suppresses Mg and K: cation antagonism is one of the few interactions that reliably matters in practice [3][15]. Cal-Mag thrown at every spot often makes K worse. The house position applies: LEDs do not create a Cal-Mag need; demand is driven by cultivar, water source, coco cation exchange and pH (see [Cal-Mag](https://genetixbay.com/knowledge/nutrients/cal-mag.md)).

## Is it nitrogen toxicity or overwatering claw?

**Nitrogen toxicity makes leaves very dark and glossy with downward-hooked tips on an otherwise vigorous plant; overwatering makes leaves droop and curl down along their whole length on a heavy, wet pot.** The first is a feed problem confirmed by EC and the feed record; the second is a root-oxygen problem confirmed by weight and by recovery as the pot dries.

| Feature | Nitrogen toxicity | Overwatering |
|---|---|---|
| Where it starts | Upper and middle leaves first, then whole plant | Whole plant at once, usually hours after watering |
| Pattern | Leaf *tips* hook downward ("claw"); leaf blade stays firm; long petioles; soft lush growth | Whole leaf droops from the petiole and curls down; leaves look swollen and firm but hang |
| Colour | Very dark, almost blue-green, glossy | Normal to slightly pale green; yellowing follows if it persists |
| Speed | Builds over days after a feed increase | Appears within hours of watering; eases as the pot dries |
| Readings | High input or runoff EC; ammonium above ~30% of total N is a known cause of toxicity in cannabis (see [nitrogen](https://genetixbay.com/knowledge/nutrients/nitrogen.md)) | Normal EC; pot still heavy 2–3 days after watering; medium smells sour if chronic |
| Separating test | Read runoff EC and check the feed record | Lift the pot: heavy and wet means overwatering |
| Fix | Cut N 20–30% or drop to a lower-N formula; if runoff EC is high, do a root-zone flush with pH'd water | Let the pot dry until noticeably light; improve drainage and air (perlite, fabric pots); water less often, not less per watering |
| Confirm | New leaves emerge mid-green and flat in 5–7 days | Leaves lift within a day of the medium drying |

**Watch for the third option.** A waterlogged root zone and an N-heavy feed often occur together in heavy-fed soil. Twisted, clawed new growth with glassy, shiny leaflets is neither of these: think broad mites (see below and [clawing leaves](https://genetixbay.com/knowledge/diagnosis/clawing-leaves.md)).

## Is it light bleaching or magnesium deficiency?

**Light bleaching is pale-to-white tissue confined to the canopy tops closest to the fixture; magnesium deficiency starts on the lowest, oldest leaves and climbs.** The location settles it before any test does.

| Feature | Light bleaching | Magnesium deficiency |
|---|---|---|
| Where it starts | Top colas, upper fan and sugar leaves, directly under the hottest part of the fixture | Old, lower fan leaves [1][2] |
| Pattern | Leaves and bracts fade from the centre or tips; early leaves may keep green veins, which is why it gets mistaken for Mg | Crisp interveinal yellowing with green veins and midrib |
| Colour | Pale yellow to bone white, bracts and pistils included | Lemon yellow; later necrotic patches |
| Speed | Days after a light raise, a canopy gaining height or a heat spike | Weeks, steady upward progress |
| Readings | Top-canopy PPFD high for the conditions; [leaf temperature](https://genetixbay.com/knowledge/glossary/leaf-temperature.md) well above air | Low root-zone pH, high K or Ca, RO water |
| Separating test | Measure PPFD at the tallest top versus a healthy spot lower down; check whether lower leaves are fine | Check the bottom third of the plant: if it is clean, it is not Mg |
| Fix | Raise or dim the light 10–20%, cool the canopy, even out tall colas | Correct pH; magnesium sulfate to bring Mg to ~40–70 mg/L |

**The numbers.** Cannabis yield kept rising up to about 1,800 µmol/m²/s in a well-run trial [10], so bleaching usually means *too much light for the conditions*: heat, low CO₂, sick roots or a sensitive cultivar. In one gas-exchange study, net photosynthesis fell when high temperature and high light were combined [11]. Bleached tissue does not recover; judge the fix by new growth. See [heat and light stress](https://genetixbay.com/knowledge/environment/heat-light-stress.md).

## Is it spider mite stippling or calcium spotting?

**Spider mite stippling is many pin-prick pale dots with mites, eggs or silk under the leaf; calcium spots are fewer, larger, irregular brown patches with no insects underneath.** Flip the leaf and look through a lens before you touch the feed.

| Feature | Two-spotted spider mite | Calcium spotting |
|---|---|---|
| Where it starts | Often lower and inner leaves, near the midrib and leaf base; hot, dry, still spots in the room [7] | New and recently matured leaves; in DWC flower, lower fan leaves [1][2] |
| Pattern | Hundreds of tiny pale-yellow or white specks (stipples), merging into a speckled, dusty look; fine webbing at heavy infestations [7] | A few to dozens of irregular spots 1–5 mm across, often with yellow halos, plus margin necrosis or distorted new leaves |
| Colour | Pale yellow to white stipples, later bronze or grey leaves | Tan to rust brown |
| Speed | Explosive in heat: populations can double in days above ~27 °C (80 °F) | Steady; tracks humidity, pH and root health |
| Readings | Low RH, high temperature | Low pH, high RH (low transpiration), RO or unbuffered coco |
| Separating test | 10–30× lens on the leaf underside: moving 0.5 mm mites, round eggs, cast skins, silk [7] | Lens shows nothing alive; spots are dead tissue |
| Fix | Isolate, wash or treat in veg, release predatory mites; see [spider mites](https://genetixbay.com/knowledge/pests/spider-mites.md) | Correct pH and VPD; Ca ~150–200 mg/L; buffer coco |

Thrips damage is the other stippling lookalike: silvery, papery streaks with tiny black faecal dots rather than round stipples ([thrips](https://genetixbay.com/knowledge/pests/thrips.md)). Septoria spots have dark margins and, through a lens, tiny black fruiting bodies ([septoria](https://genetixbay.com/knowledge/diseases/septoria-leaf-spot.md)).

## Is it heat stress or light stress?

**Heat stress shows wherever the air or leaf is hottest, including leaves far from the lamp; light stress shows only where the light is most intense, usually the top of the canopy directly under the fixture.** The two often arrive together, because high light raises leaf temperature.

| Feature | Heat stress | Light stress (excess PPFD) |
|---|---|---|
| Where it starts | Whole upper canopy and any hot pocket: near walls, drivers, poor airflow; also leaves far from the fixture | The tops nearest the fixture's peak PPFD, often the centre of the footprint |
| Pattern | Leaves taco (fold upward along the midrib) or canoe, edges curl up, droop by late afternoon; foxtailing on colas | Leaves point up toward the light ("praying" if mild), then pale and bleach; bracts bleach |
| Colour | Green with crispy, curled edges; brown edges if severe | Pale yellow to white, often with green veins early |
| Speed | Hours; worst at the end of the photoperiod | Days |
| Readings | Air above ~30 °C (86 °F), leaf VPD above ~1.5–1.7 kPa, leaf temperature above air | High PPFD at the top; leaf temperature may be normal under LED |
| Separating test | IR thermometer on the leaves and a thermometer at canopy height: hot leaves = heat | [PAR](https://genetixbay.com/knowledge/glossary/par.md) meter at the tops: high PPFD with normal leaf temperature = light |
| Fix | Lower air temperature, raise RH to bring VPD to 0.8–1.5 kPa, add airflow across the canopy | Dim or raise the light; level the canopy |

Under LEDs, leaf temperature at a given air temperature is lower than under HPS because there is less radiant heat [9], so growers often run warmer air under LED. A leaf that is hot to the IR gun under LED usually means poor transpiration (dry air, still air, sick roots), not radiant heat.

## Is it Pythium root rot or low dissolved oxygen?

**Low dissolved oxygen gives limp, tan roots and afternoon wilting that recover within a day or two of cooling and aerating the solution; Pythium gives brown, slimy roots whose outer sheath slips off, a rotten smell, and a decline that continues after oxygen is fixed.** Warm solution drives both: oxygen solubility falls as temperature rises, and Pythium thrives in warm, oxygen-poor water [5][6].

| Feature | Low dissolved oxygen | Pythium root rot |
|---|---|---|
| Where it starts | Whole root mass, worst at the bottom of the reservoir | Patches of root, spreading; often after a hot spell |
| Root look | Tan or cream, limp, fewer fuzzy white tips; no slime | Brown, soft, slimy; outer cortex slides off leaving a thin white stele [4] |
| Smell | Flat or slightly stale | Sour, rotten, "swampy" |
| Plant above | Mild wilting in the afternoon, slow growth | Wilting by day, yellowing lower leaves, stunting [4] |
| Speed | Hours after a pump or air failure; recovers in 24–48 h once fixed | Days to a week; progresses despite aeration |
| Readings | Solution above ~23–24 °C (73–75 °F), DO below ~6 mg/L (grower threshold) | Same warm solution; pH may drift oddly |
| Separating test | Fix air and temperature first, then re-check roots in 48 h: new white tips mean it was oxygen | Pull a root and rub it: the sheath slipping off is the hallmark [4] |
| Fix | Air stones and pump checked, solution to 18–22 °C (65–72 °F), clean reservoir | Same, plus remove rotted roots, sanitise or establish beneficial microbes; see [root rot](https://genetixbay.com/knowledge/diseases/root-rot-pythium.md) |

House position: a beneficial-microbe reservoir is preferred to a half-hearted sterile one; a sterile reservoir works only if you commit to it fully. Fusarium can join Pythium in hydro cannabis [4]; brown pith in the stem base points to it ([fusarium](https://genetixbay.com/knowledge/diseases/fusarium.md)). Light leaking into the reservoir browns roots too, with algae and no slime ([root browning in hydro](https://genetixbay.com/knowledge/diagnosis/root-browning-hydro.md)).

## Is it iron or manganese deficiency?

**Both cause interveinal yellowing on new growth; iron chlorosis is smooth, pale and concentrated on the very newest leaves, while manganese chlorosis is netted, sits on the upper-middle leaves and develops small tan spots.** Both are usually caused by high root-zone pH, not an empty bottle [1][3].

| Feature | Iron (Fe) deficiency | Manganese (Mn) deficiency |
|---|---|---|
| Where it starts | Newest, still-expanding leaves at the shoot tips [1] | Upper and central foliage, a little below the tip [1] |
| Pattern | Starts as yellowing at the base of new leaflets, then smooth interveinal chlorosis; fine veins stay green [1] | Netted interveinal chlorosis spreading out from the midrib; tan necrotic spots develop between veins [1] |
| Colour | Pale yellow to nearly white in severe cases | Bright yellow mesh, then tan flecks |
| Speed | Slow: about 9 weeks in sand culture [1] | About 6 weeks in sand culture [1] |
| Readings | Root-zone pH above ~6.5 in soilless, alkaline water, cold or waterlogged roots, very high P | Same high pH; excess Fe can compete with Mn |
| Separating test | Read root-zone pH first; a leaf tissue test separates them reliably (healthy ranges in [12]) | As left; spots on the yellowed tissue favour Mn |
| Fix | Bring pH to 5.5–6.2 (hydro) or 6.2–6.8 (soil); use a chelate matched to pH (EDTA ≤ ~6.5, DTPA ≤ ~7.5, EDDHA above) at 1.5–3 mg/L Fe | Same pH correction; Mn ~0.3–0.8 mg/L |

In a DWC flowering trial, withholding Fe or Mn produced weak or no symptoms and no significant yield loss, probably because plants carried reserves from veg [2]. If you see sharp new-growth chlorosis in a short hydro run, rule out pH, a cold or waterlogged root zone and light bleaching before blaming the feed. Zinc and sulfur are the other new-growth lookalikes: Zn gives small, narrow leaves with margin necrosis; S yellows the whole new leaf evenly rather than between the veins ([yellow new growth](https://genetixbay.com/knowledge/diagnosis/yellow-new-growth.md)).

## Is it nutrient burn or potassium deficiency?

**Nutrient burn starts as dry brown tips on the newest mature leaves with high EC; potassium deficiency starts as yellowing margins and serration tips on old, lower leaves with normal or low EC.** A runoff EC reading against input EC usually separates them in one watering.

| Feature | Nutrient (salt) burn | Potassium deficiency |
|---|---|---|
| Where it starts | Tips of the newest fully expanded leaves, then down the margin; whole plant | Old, lower leaves first [1] |
| Pattern | A thin, dry, brown tip of a few mm, often slightly twisted; a yellow line ahead of the brown | Yellow along the margin and saw-teeth, moving inward, then tan crispy scorch [1] |
| Colour | Brown tips on otherwise dark-green leaves | Yellow then tan; leaf otherwise fades |
| Speed | Days after a feed increase or as runoff EC climbs | Weeks |
| Readings | [Runoff EC](https://genetixbay.com/knowledge/glossary/runoff-ec.md) well above input (more than ~0.5 mS/cm above is a common warning sign [16]); high input EC | Low input EC, low K in the recipe, or high Na/Ca/Mg competing |
| Separating test | Pour-through or runoff EC versus input EC | Normal EC plus the old-leaf margin pattern; a tissue test confirms (healthy ~1.8–2.9% K [1]) |
| Fix | Lower feed EC 10–20%; if runoff is far above input, a root-zone flush with pH'd water to clear salts | Raise K toward ~175–240 mg/L in veg; check Na and coco buffering |

The root-zone flush here treats the medium, not the plant, and is valid at any stage. A pre-harvest "flush" is a different thing and is a *Myth* that costs yield and resin ([flush debate](https://genetixbay.com/knowledge/harvest-cure/flush-debate.md)). See [brown or burnt tips](https://genetixbay.com/knowledge/diagnosis/brown-burnt-tips.md).

## Is my plant overwatered or underwatered?

**Lift the pot: a heavy pot with drooping leaves is overwatered, a light pot with limp, papery leaves is underwatered.** Both droop, but the leaves feel different and the timing is opposite.

| Feature | Overwatering | Underwatering |
|---|---|---|
| Leaf feel | Firm, swollen, curled down along the length | Thin, limp, papery, hanging from the petiole |
| Timing | Within hours of watering; improves as it dries | Before watering; recovers within an hour or two of water |
| Pot | Heavy, wet 2–3+ days after watering | Light; medium pulling away from the pot wall |
| Long-term signs | Yellowing lower leaves, fungus gnats, sour smell, root rot | Crispy edges, slow growth, salt concentration |
| Fix | Water less often, improve drainage and air | Water thoroughly to 10–20% runoff, then on a schedule |

A plant that wilts while the medium is wet is neither: check the roots for rot or the stem base for wilt disease ([drooping](https://genetixbay.com/knowledge/diagnosis/drooping-overwatering-vs-underwatering.md)).

## Is it nitrogen deficiency or normal late-flower fade?

**Uniform yellowing from the bottom up in the last 2–3 weeks of flower is normal senescence; the same pattern in veg or before about week 5–6 of flower is nitrogen shortage or a root problem.** Timing is the test.

| Feature | Nitrogen deficiency | Late-flower fade |
|---|---|---|
| When | Veg or early to mid flower | Final 2–3 weeks of flower |
| Pattern | Pale to lemon yellow, whole leaf, climbing fast; red-brown petioles in severe DWC cases [2] | Slow yellowing and sometimes purpling of fan leaves; sugar leaves and buds keep developing |
| Buds | Small, slow | Swelling, trichomes maturing |
| Fix | Rule out roots and pH, then raise N 10–20% | Keep feeding P, K, Ca, Mg and micros; taper N; no pre-harvest flush |

The GenetixBay approach is to taper nitrogen in late flower while keeping everything else fed, which per-element dosing makes possible ([yellowing lower leaves](https://genetixbay.com/knowledge/diagnosis/yellowing-lower-leaves.md)).

## Is twisted new growth broad mites, heat or calcium?

**Broad and russet mites twist and glaze the newest growth with no visible insect; heat curls the upper canopy where it is hottest; calcium (or boron) distorts new growth alongside spotting and pH or humidity problems.** Broad mites are about 0.2 mm long and need 20× or more to see, or a microscope [8].

| Feature | Broad / russet mites | Heat stress | Calcium or boron shortage |
|---|---|---|---|
| Where | Shoot tips and newest leaves, spreading plant to plant | Top of the canopy, hot spots | New growth across the plant |
| Pattern | Leaves twisted, cupped, glossy or glassy, edges curled down; bronzed, rusty stems on russet mites; buds stall [8] | Taco and canoe, edges up; fine at night | Distorted, cupped new leaves with brown spots; dead tips |
| Spread | Moves between touching plants | Follows the heat map | Follows pH and humidity |
| Test | 40–60× magnification on the newest leaf and petiole | IR thermometer and canopy thermometer | Runoff pH, RH reading |
| Fix | Isolate; predatory mites or approved treatment; see [broad and russet mites](https://genetixbay.com/knowledge/pests/broad-russet-mites.md) | Cool the canopy | Correct pH, VPD, Ca or B supply |

## Is it septoria leaf spot or a calcium or phosphorus spot?

**Septoria spots are round to angular with dark brown margins, start on lower leaves in humid conditions, and show tiny black fruiting bodies through a lens; nutrient spots have no fruiting bodies and follow pH and feed.** [14]

| Feature | Septoria | Ca spots | P shortage |
|---|---|---|---|
| Where | Lower leaves, moving up in wet, crowded canopies [14] | New and recently matured leaves | Old, lower leaves |
| Look | Pale yellow spots that turn brown with darker margins, often with a yellow halo; black specks (pycnidia) inside [14] | Irregular rust spots, yellow halo, no specks | Dark, dull, olive to purple-brown blotches; dark leaves overall |
| Test | 10–30× lens for pycnidia; spots on many leaves at once after leaf wetness | Lens shows nothing; pH and RH readings | Root-zone pH and temperature; feed record |
| Fix | Remove affected leaves, cut leaf wetness and RH, improve airflow; see [septoria](https://genetixbay.com/knowledge/diseases/septoria-leaf-spot.md) | Correct pH and VPD | Correct pH, warm cold roots; P is rarely truly short |

## Is it powdery mildew or residue and trichomes?

**[Powdery mildew](https://genetixbay.com/knowledge/glossary/powdery-mildew.md) is a flat, white, flour-like coating that wipes off and comes back in circles on leaf surfaces; residue does not regrow, and trichomes are stalked glands you can see as tiny mushrooms under a lens.** [13]

| Feature | Powdery mildew | Spray or water residue | Trichomes |
|---|---|---|---|
| Where | Upper surfaces of fan leaves first, then stems and buds | Wherever spray landed; drip edges | Sugar leaves, bracts, buds |
| Look | Fuzzy white spots spreading to patches | Dry crusty film or rings | Glistening, stalked glands |
| Wipe test | Wipes off, reappears in days | Wipes off, does not return | Does not wipe off cleanly; sticky |
| Lens | Cobweb-like hyphae, chains of spores | Crystals, no structure | Clear stalks with round heads |
| Fix | RH below ~55% in flower, airflow, remove infected leaves; see [powdery mildew](https://genetixbay.com/knowledge/diseases/powdery-mildew.md) | Nothing; change spray practice | Nothing; that is the crop |

## What to check first

1. **Where did it start?** Old leaves, new leaves, top of the canopy, or the whole plant.
2. **Lens on the underside** of three affected leaves (10–30×; 40–60× for broad mites).
3. **Lift the pot** or check reservoir temperature and air.
4. **pH and EC of input and runoff**, on a calibrated meter ([runoff testing](https://genetixbay.com/knowledge/ph-ec/runoff-testing.md)).
5. **Air and leaf temperature, RH and PPFD** at the affected spot.
6. **Only then** compare nutrient lookalikes.

## How to fix it

Fix the cause the separating test pointed to, one change at a time, and write it in your journal. Changing pH, feed, light and watering on the same day makes it impossible to know which one worked. The individual fixes are in the tables above and the linked articles.

## How to confirm it worked

- New growth emerging after the fix is clean; damaged leaves do not heal and are not the measure.
- The symptom stops spreading to new leaves within 5–7 days for nutrients, 1–2 days for watering and oxygen, and 3–5 days for light and heat.
- Readings stay in range at the next two checks.

## Common mistakes

- Diagnosing from one leaf. Look at the whole plant and the neighbouring plants.
- Skipping the lens. Mites and thrips cause many "deficiencies".
- Treating several lookalikes at once with Cal-Mag plus a bloom booster plus a pH change.
- Judging a photo taken under purple or warm LED light; colours lie. See [how to describe your plant's problem](https://genetixbay.com/knowledge/diagnosis/how-to-describe-your-plants-problem.md).

## Evidence

- The old-versus-new location rule for mobile and immobile nutrients is *Established* plant nutrition [3] and holds in cannabis trials [1][2].
- Cannabis-specific deficiency pictures for Ca, Mg, K, Fe and Mn are *Supported* by two controlled studies; Fe and Mn gave weak or no symptoms in short hydro flower runs [1][2].
- Pythium on hydro cannabis roots is *Established* [4]; the dissolved-oxygen threshold for cannabis is *Grower practice* extrapolated from other crops [5][6].
- Spider mite and broad mite signs are *Established* extension guidance [7][8].
- Leaf temperature differences between LED and HPS are *Established* [9].

## Sources

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