White spots on the leaves: mildew, bugs, or residue?

By Curtis Olson — retired software developer, home grower, and the builder of the device this site is about. More about me and why I built it. · Last reviewed 2026-10-14

The short answer

• Wipe one spot with a dry fingertip. If it comes away as a powder and the leaf under it is green, it is powdery mildew — a fungus that lives on the surface. If it smears or flakes and the leaf under it is green, it is residue from a spray or hard water. If nothing comes off and the pale mark is in the leaf — silvery, dry, a little sunken — something fed there, and the thing that fed is on the underside or in the growing tip.
• Mildew is patches, not spots. It starts as small round white patches on the upper side of a leaf and grows outward until patches merge; a week later it is on stems and, in flower, in the buds.
• Pest scars are flecks with company. Silvery or white flecks along the veins, with tiny black specks nearby, is thrips. A fine pale stipple all over the blade with a bronze cast is mites. Both are worst on the newest growth.
• Septoria is not white. The leaf-spot fungus growers keep finding on search pages makes brown-to-grey spots with yellow halos on the oldest, lowest leaves. Bright white on the upper canopy is not that.

IG-015A dry fingertip, five seconds

Wipe one spot with a dry fingertip

You seeComes away as a powder; green leaf under it

It isPowdery mildew

SoA fungus living on the surface. It regrows on the wiped patch within days. In flower, urgent.

You seeSmears or flakes off; green leaf under it

It isResidue

SoSpray, foliar feed or hard water that dried in rings and runs. Stays gone once wiped.

You seeNothing comes off; the pale mark is in the leaf

It isA feeding scar

SoSilvery flecks along the veins with black specks: thrips. A fine stipple with a bronze cast: mites. Turn the leaf over.

The wipe test and the three-day re-check are first-party practice and general plant pathology, as the page says.

What each of them actually is

Powdery mildew on cannabis is a specific fungus, Golovinomyces ambrosiae, which earlier reports named G. cichoracearum. It was documented on cannabis in Canada in 2018, in Oregon in 2021 and in Quebec in 2022, and it is the one the licensed industry worries about because its presence on the harvested flower is a product problem, not just a leaf problem. It is an obligate biotroph: it can only live on a living plant, and it lives on the leaf, sending feeding structures into the surface cells while the visible fungus sits on top. That is the whole reason the wipe test works — you are wiping the organism off — and the reason a wiped patch comes back within days, because the part you cannot see is still in the surface cells. Like powdery mildews generally, it does not need a wet leaf; it wants humid, still, shaded air, which is why a tent with no free water on any leaf can still get it. It attacks leaves, stems and flowers, and left alone it costs photosynthesis, drops leaves early and spoils flower quality.

Thrips feed by punching a surface cell and sucking it empty. The cell dies pale, so the leaf ends up with white flecks or silvery scars, usually along the veins, and the insect leaves dark specks of frass beside its work. Feeding on leaves that are still expanding can twist them. Western flower thrips is the species most often on cannabis; the adults are about a millimetre long and rarely sit still for a photo. Spider mites do the same thing one cell at a time with a much finer mouth, so the result is a stipple of pale pinpricks across the whole blade, then a bronze or dull cast, then webbing at the tips when the colony is large. The which-pest page works through the four common ones by their damage.

Residue is anything you or the tap put there: neem or soap that dried in droplets, a foliar feed, hard water from a mister, sulfur dust. It sits in the shapes the liquid dried in — rings, runs, droplet outlines — often on both leaf surfaces, and it does not spread on its own or grow back once it is wiped. It is the most common “white spots” of all in the week after somebody sprayed.

Septoria leaf spot is in this list only because it keeps turning up in search results for the other three. On hemp it begins as small, irregular, brown-to-grey spots that expand to roughly five to eight millimetres, mostly on the lower leaves and inside the canopy where it stays humid; the centres can pale as the lesion ages and a yellow halo spreads around it. It is a leaf-spot disease of the bottom of the plant, brown before it is anything else. A grower who has compared his bright-white upper-canopy flecks against Septoria photos and found they do not match has already done the right test.

Why the photo cannot settle it

At phone resolution a fresh mildew colony, a cluster of thrips scars and a dried spray droplet are all “pale marks on green,” and the thing that separates them — whether the mark is on the leaf or in it — is a question of depth, not colour. A photo is flat. That is why the same picture gets five different answers from five people who are each pattern-matching honestly, and why the thread never converges. The fingertip resolves depth in a way no camera does.

How to check this yourself

• Wipe one spot, dry finger. Powder off, green underneath: mildew. Smear or flake off, green underneath: residue. Nothing off, pale mark stays: a feeding scar.
• Which side? Mildew shows on the upper surface first. Thrips scars are on the upper surface too, but the insects and their frass are on the underside and in the growing tip — turn the leaf over and look at the tip with a hand lens at ten to twenty times.
• Where on the plant? Mildew favours shaded, still, lower leaves and spreads as patches. Thrips and mites favour the newest growth. Septoria is the oldest, lowest leaves.
• Mark the spot and come back in three days. Mildew regrows on the wiped patch. A scar stays exactly the size it was. Residue stays gone. This is the test that still pays off if nobody ever answers your thread.
• Did anyone spray in the last week? If yes, assume residue until the wipe says otherwise.
• In flower, treat mildew as urgent. A colony on a leaf is a leaf problem; a colony in a bud is the harvest. The bud-rot page covers the humidity side of the same tent.

What would show that we have this wrong

If a spot wipes off as powder and the tissue under it is already scarred, you may have both — thrips feeding scars are a common place for mildew to settle, and both like the same still, humid air. Treat the pest and the environment, not just one. And if the marks are white, do not wipe off, and two checks with a hand lens over three days find nothing on the underside or in the tips, a photo cannot go further; take a leaf in a bag to a grow shop or an extension office rather than a forum.

Where our device fits, stated narrowly

Croplock does not identify mildew, thrips or residue from a photo, and it cannot do the wipe test. What it holds is the other half of the mildew question: the humidity and air-movement history of the tent, which is the condition the fungus needs, sits on the dashboard with dates on it. And it keeps dated pictures, so “is the patch bigger than it was on Tuesday?” is a comparison rather than a memory.

Sources

• Pépin, N., Punja, Z.K. & Joly, D.L. (2018), Occurrence of Powdery Mildew Caused by Golovinomyces cichoracearum sensu lato on Cannabis sativa in Canada, Plant Disease. DOI 10.1094/PDIS-04-18-0586-PDN.
• Wiseman, M.S. et al. (2021), First Report of Powdery Mildew Caused by Golovinomyces ambrosiae on Cannabis sativa in Oregon, Plant Disease. DOI 10.1094/PDIS-11-20-2455-PDN. Brochu, A.-S. et al. (2022), the Quebec first report, Plant Disease. DOI 10.1094/PDIS-02-22-0350-PDN.
• Seifi, S. et al. (2025), Mapping and characterization of a novel powdery mildew resistance locus (PM2) in Cannabis sativa L., Frontiers in Plant Science. DOI 10.3389/fpls.2025.1543229 — the pathogen named as an obligate biotroph, its former name, and its attack on leaves, stems and flowers.
• Hybrid genome assembly of the cannabis powdery mildew agent Golovinomyces ambrosiae, BMC Genomics (2026). DOI 10.1186/s12864-026-12995-2 — the product-quality framing for licensed producers.
• Scott, C. & Punja, Z.K. (2020), Evaluation of disease management approaches for powdery mildew on Cannabis sativa, Canadian Journal of Plant Pathology. DOI 10.1080/07060661.2020.1836026 — that the management question has been studied; no specific treatment is recommended here.
• Rahnama, M. et al. (2021), Reemergence of Septoria Leaf Spot Caused by Septoria cannabis on Hemp in Kentucky, Plant Disease. DOI 10.1094/PDIS-12-20-2620-SC — lesion colour, size and position on the plant.
• Utah State University Extension, hemp IPM note Thrips — the punch-and-suck feeding, the white flecks or silvery scars with dark fecal spots, distortion of developing leaves, and damage along the veins. UC IPM photo record Western flower thrips frass and feeding scars (J.K. Clark) for the frass-beside-scar pairing.
• Lemay, J. & Scott-Dupree, C., Ch. 9 in Cannabis in Controlled Environment Production (CRC Press). DOI 10.1201/9781003150442 — the mite and thrips signatures, restated in our own words.
• The wipe test, the three-day re-check, the residue description and the “humid air on a dry leaf” point about powdery mildews as a group are first-party practice and general plant pathology, not citations from the papers above.

More of these at Answers. The device is at croplock.com.

Related: Mites, thrips, aphids or gnats — which one do I have? · Are those bugs still alive? · Bud rot: how humid, for how long? · Is it a deficiency or a disease? · Can I spray a flowering plant? · Is it a hermie?