Short answer. A white or grey coating on a cannabis plant is usually one of six things: powdery mildew, spray or hard-water residue, trichomes, thrips silvering, spider-mite webbing, or grey Botrytis mould on buds. Where it sits and what it does when you rub it decide most cases. Flour-like circles that smear off and return are powdery mildew [1][2]. White on the medium is usually harmless mycelium (fuzz) or salt (crust). A 20–60x lens settles the rest.
What's happening?
"White stuff" is a symptom family, not a diagnosis. Three very different processes produce it:
- A living organism on the surface. Powdery mildew (Golovinomyces ambrosiae and relatives) grows on the outside of leaves and sends feeding structures into epidermal cells [1][2]. Botrytis cinerea grows grey, dusty spore masses on dying tissue, mostly inside dense buds [1][3]. Saprophytic fungi grow white mycelium on wet organic media.
- Insect damage or insect products. Thrips rasp cells empty so they fill with air and look silver [4]. Spider mites spin silk [5]. Whiteflies and aphids drip honeydew that grows black sooty mould [6]. Mealybugs and root aphids coat themselves in white wax [7][8].
- Non-living deposits. Dissolved minerals in hard water, foliar feeds, silica and sulfur products dry into white films [10]. Salts wick to the top of the medium and crystallise.
The danger split is clear: powdery mildew and Botrytis are urgent, insects are urgent within days, and residues are cosmetic unless they block light or come from overfeeding.
Where is the white, and what does it do when you rub it? (the decision tree)
Work through the forks in order. Stop at the first one that fits, then confirm with the key test.
- Is it on the medium, pots or reservoir, not the plant? - Soft, cottony, spreads on a wet surface, smells earthy → saprophytic mycelium. Harmless in itself; dry the top 3–5 cm and move air (white fuzz on soil). - Hard, crunchy, white or off-white crust at the rim or top, worse where water evaporates → salt crust. Measure runoff EC (runoff testing). - Green or brown slime on wet media, net pots or in the reservoir → algae, from light reaching nutrient solution (algae in the reservoir). - White woolly flecks on roots when you lift the plant, small pear-shaped insects → root aphids or root mealybugs [8] (root aphids).
- Is it grey and fuzzy, on or inside a bud, or on a stem wound? → Botrytis (grey mould). Usually a single sugar leaf turns brown or tan and pulls out easily, then grey spores appear inside the cola [1][3]. This is critical urgency in flower: go to bud rot.
- Is it only on buds and sugar leaves, glittery, and fixed in place? → Trichomes. Under a lens they are stalks with clear, cloudy or amber glass-bead heads. Healthy, and the point of the exercise.
- Does it rub off as a flour-like powder, in round patches, mainly on upper leaf surfaces, and come back within days? → Powdery mildew [1][2]. Patches start small and circular, often on lower and inner leaves in still air, then merge until the leaf looks dusted.
- Does it rub off as a film but follow drip lines, leaf edges and the surfaces you sprayed? → Residue (hard water, foliar feed, silica, sulfur, oils or soap). It does not regrow, has no circular pattern and sits on whatever the spray hit.
- Is it shiny silver or pewter, flat, and does not rub off? → Thrips feeding scars, usually with tiny black frass dots on the patch [4] (thrips).
- Is it fine pale speckling with fine silk under leaves or between bud tips? → Spider mites [5] (spider mites). Heavy webbing over the tops is a late, severe infestation.
- Is it a sticky shine, then a black soot film? → Honeydew and sooty mould from whiteflies or aphids [6] (whiteflies, aphids).
- Is it white cottony tufts in leaf axils and along stems, with slow-moving oval insects? → Mealybugs [7]. Uncommon indoors on cannabis, more common where houseplants share the room.being reviewed
- Is it a pale grey-violet fuzz on the underside of yellow, angular leaf patches? → possibly downy mildew.being reviewed
How do I tell them apart? (differential table)
| Pattern | Most likely | Rule out first | Key test |
|---|---|---|---|
| Round flour-like patches, upper leaf surface, return after wiping | Powdery mildew [2] | Residue, trichomes | Rub test, then check 48 h later; lens shows cottony hyphae |
| White film in drip lines, only where sprayed | Hard-water or spray residue | Powdery mildew | Wet a cloth and wipe: residue dissolves or flakes, never regrows |
| Glittering heads on buds and sugar leaves only | Trichomes | PM on sugar leaves | 30–60x lens: glass-bead heads on stalks |
| Grey dusty fuzz inside a cola; brown sugar leaf pulls out | Botrytis bud rot [3] | Normal senescence of a sugar leaf | Pull the bud open: brown, mushy core |
| Silver, flat, shiny patches with black dots | Thrips [4] | Mite stippling, sun bleaching | 10–20x lens under the leaf; tap leaves over white paper |
| Fine pale dots, silk under leaves | Spider mites [5] | Ca spotting, thrips | Lens on the underside: moving dots, eggs |
| Sticky shine, black film | Whiteflies or aphids (honeydew) [6] | Sap from damage, spray oil | Shake the plant; yellow card for 48 h |
| White cottony tufts in axils | Mealybugs [7] | PM, wax from aphids | Lens: segmented oval insects under the wax |
| White wool on roots | Root aphids or root mealybugs [8] | Root hairs, mycelium | Lens on roots; check the pot base and drainage holes |
| Soft white fuzz on top of the medium | Saprophytic mycelium | PM spores on media (rare) | It sits only on wet organic surface; dry back 3–5 cm |
| Hard white crust on medium or pot rim | Salt build-up | Lime from hard water | Runoff EC vs input EC |
| Green slime on media or in reservoir | Algae | Biofilm | Is light reaching the solution? |
Which numbers matter?
| Parameter | Target or threshold | Notes |
|---|---|---|
| RH, late flower | Below ~50% | Botrytis prevention [3]; see humidity by stage |
| Botrytis growth range | ~10–27 °C (50–81 °F), optimum near 20 °C (68 °F) | With high humidity and free water [1][3] |
| PM-favouring conditions | Moderate temperatures, still air, humid canopy; free water is not needed | Established for PM in general; cannabis-specific optima are thin [1][2] |
| Lens magnification | 10–20x for insects; 30–60x for mildew hyphae and trichome heads | Grower practice |
| Water alkalinity | Above ~100–150 mg/L as CaCO₃ leaves white mineral residue and raises root-zone pH | [10]; see water quality |
| Runoff EC above input | More than ~0.5 mS/cm suggests salt build-up | Grower practice; see runoff testing |
| Top of medium dry-back | ~3–5 cm (1–2 in) between waterings | Discourages mycelium and fungus gnats [11] |
| Spider mite outbreak conditions | Hot, dry air (roughly above 27 °C and below 50% RH) | [5]being reviewed |
What should I check first?
Do these in order, cheapest first. They take about five minutes.
- Location. Medium vs plant; buds vs fan leaves; upper vs lower surface; top of canopy vs interior.
- Rub test. Wipe with a damp finger or cloth. Powder that smears = mildew or residue. Fixed silver = thrips. Fixed glitter = trichomes.
- Spray history. Did anything get sprayed or misted on these leaves in the last 2 weeks? Hard tap water in a mister alone can leave white films.
- Lens check under the leaf. 10–20x for insects, eggs, silk and black frass; 30–60x for mildew hyphae.
- Shake test. Shake a branch over white paper. Whiteflies rise in a cloud; mites and thrips fall and move.
- Watch for 48 hours. Wipe a marked patch. Mildew returns; residue does not.
- Climate log. RH and temperature at lights-off, when the canopy is coolest and dew is most likely.
How do I fix each one?
- Powdery mildew. In veg: remove the worst leaves into a bag, raise airflow through (not just over) the canopy, keep RH stable, and use a least-toxic contact option that is legal where you live, such as potassium bicarbonate or a horticultural oil, at label rate, testing on one leaf first. In flower there is no clean fix: anything sprayed lands on what you consume. Remove affected material and decide whether to harvest early. See powdery mildew and home remedies for PM.
- Botrytis. Cut the infected bud out at least 5 cm (2 in) below visible rot, bag it inside the room, sanitise the blade between cuts, drop RH below ~50% and increase airflow [3]. Never eat or extract rotted buds. Inspect every cola every 2 days until harvest. See bud rot.
- Residue. Switch foliar sprays to RO or low-alkalinity water; stop foliar feeding in flower. A gentle wipe with a damp cloth removes heavy films on fan leaves. Residue blocks a little light but does no other harm.
- Thrips. Predatory mites on foliage plus a soil-level predator or nematodes for the pupae, blue or yellow cards, and remove heavily scarred leaves [4]. See thrips.
- Spider mites. Act the day you see silk: isolate, knock numbers down in veg with a contact option, release predatory mites, and lower canopy temperature and raise RH within safe limits [5]. See spider mites.
- Whiteflies, aphids and sooty mould. Treat the insect, not the mould. Yellow cards, parasitoid wasps or predators, contact options in veg [6]. Wipe the sooty film off fan leaves with a damp cloth.
- Mealybugs. Remove by hand with a cotton swab in isopropyl alcohol on small infestations in veg; quarantine incoming houseplants.being reviewed
- Root aphids and root mealybugs. See root aphids; soil-applied entomopathogenic fungi and predators are the usual approaches [8].
- Mycelium on the medium. Water less often, let the top 3–5 cm dry, move air over the pots. Scrape off if you prefer; it is not a disease of the plant.
- Salt crust. Scrape it off and do a root-zone flush (the one valid flush, which treats the medium, not the plant) with pH-adjusted water to 20–30% runoff until runoff EC is near input EC, then resume the normal feed.
- Algae. Block all light from the reservoir, lines and net-pot tops; clean and sanitise between runs.
How do I know the fix worked?
- PM: no new patches on wiped or new leaves for 10–14 days. Spores can lurk; keep checking weekly.
- Botrytis: no new brown sugar leaves or soft spots in colas on 2-day inspections until harvest.
- Insects: card counts fall week on week; no new stippling, silvering or honeydew on new leaves.
- Residue: new leaves stay clean after the spray-water change.
- Salt crust: runoff EC falls to within ~0.5 mS/cm of input and stays there for two checks.
- Mycelium: surface fuzz stops returning once the top dries between waterings.
What are the common mistakes?
- Spraying a fungicide on residue or trichomes. Rub, lens, wait 48 hours.
- Treating sooty mould instead of the whiteflies or aphids making the honeydew.
- Spraying anything on buds in flower. It ends up in the smoke.
- Leaving a Botrytis bud on the plant "to finish". Spores spread to the next cola within days.
- Shaking or carrying infected material through the room, which spreads PM and Botrytis spores.
- Calling white soil fuzz "mould" and drenching the medium with peroxide, which kills beneficial microbes too.
- Missing thrips because the damage looks like bleaching. Bleaching is on the top of the canopy facing the light; thrips scars are random and carry black dots.
How strong is the evidence?
- Powdery mildew and Botrytis identification and the conditions that favour them in cannabis are Established by pathology surveys and extension guides [1][2][3][9].
- Thrips silvering, spider-mite stippling and webbing, and honeydew with sooty mould are Established general IPM knowledge, confirmed on hemp and cannabis by extension [4][5][6].
- Root aphids on cannabis are Established [8]; root and foliar mealybugs on indoor cannabis are Supported (scattered reports).being reviewed
- Downy mildew on indoor cannabis is Contested (rare, mostly hemp field reports).being reviewed
- The rub test, 48-hour regrowth check and drip-line pattern for residue are Grower practice.
- Least-toxic PM options (bicarbonates, oils) are Supported in other crops; cannabis-specific efficacy data are limited [1][9].
A fungal disease, on cannabis caused mainly by Golovinomyces ambrosiae, that grows as white powdery patches on leaves and buds and thrives in moderate humidity.A hair-like outgrowth of the plant's skin; in cannabis the glandular types make and store the resin holding cannabinoids and terpenes.Tetranychus urticae, the commonest cannabis mite indoors: a 0.5 mm sap-feeder that stipples leaves pale and spins webbing, breeding fastest in hot, dry rooms.Small white-winged sap-sucking insects of the family Aleyrodidae, mainly Trialeurodes vaporariorum and Bemisia tabaci, that fly up when disturbed and coat leaves in honeydew.Small soft-bodied sap-sucking insects of the family Aphididae that cluster on stems and leaf undersides, excrete sticky honeydew and multiply rapidly by live birth.The sugary liquid excreted by sap-feeding insects such as aphids, whiteflies, mealybugs and soft scales; it makes leaves sticky and feeds sooty mould.A black, soot-like fungal film that grows on the honeydew left by sap-feeding insects; it does not infect the plant but blocks light and signals an aphid, whitefly, mealybug or scale problem.Soft-bodied sap-sucking insects of the family Pseudococcidae, covered in white waxy filaments, that hide in stem joints and roots and excrete honeydew.Aphids that colonise roots and the crown below the medium surface, on cannabis chiefly the rice root aphid (Rhopalosiphum rufiabdominale), causing slow, unexplained decline.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.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.The dense, elongated cluster of flowers that forms at the end of a stem or branch; the main cola tops the leader.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.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.RO water is water stripped of most dissolved salts by reverse osmosis; it has near-zero EC and little buffering, so pH moves easily and Ca and Mg must be added.
