Is it a virus?

The short answer

• “Mosaic” is a look, not a diagnosis. Light-and-dark mottling between the veins is produced by magnesium and iron shortages, by pH lockout of either, by some cultivars’ own variegation, by a seedling recovering from a rough transplant — and by a virus. The photo does not sort them.
• The pathogens confirmed in cannabis and hemp do not look the way forum posts say they do. The one that causes a true mosaic is an outdoor, leafhopper-borne virus. The one growers fear indoors, hop latent viroid, mostly shows as stunting, brittle stems and lost yield — often with no leaf pattern at all.
• “Throw it all out” is not an answer to an unconfirmed guess. A virus is confirmed by a lab assay. A nutrient or pH cause is confirmed by the plant itself, in about ten days, for free. Do the free test first, isolate the plant while you wait, and send a sample if the free test fails.

What is actually confirmed in cannabis

When researchers sequenced everything viral in Colorado hemp fields, they found seven viruses and one viroid: a cannabis mitovirus, beet curly top virus in two strains, citrus yellow vein-associated virus, an opuntia-like virus, tobacco streak virus, cannabis cryptic virus, grapevine line pattern virus, and hop latent viroid. Tobacco mosaic virus was not among them. The “TMV” that gets named in nearly every mosaic thread has, as far as the published surveys go, not been the thing on the leaf.

• Beet curly top virus is the one that produces a genuine mosaic in cannabis: the survey lists “yellowing, mottling, curled leaves and stunting”, and the first report described leaves fading from the base outward into a yellow-green pattern, with new leaves narrow and curled sideways. It was in 81 percent of the field samples tested. It is carried by the beet leafhopper, which is why it is an outdoor and greenhouse problem in leafhopper country and a very unlikely one in a sealed tent.
• Hop latent viroid is the one indoor growers should actually think about, and it is the one a photo is worst at. The confirmed symptom set is “stunting, malformation or chlorosis of leaves, brittle stems, and reduction in yields”, plus cuttings that root poorly. It got its name in hops because the plant carries it with no visible symptoms. It spreads on tools, hands and sap — not on the wind — and it was first confirmed in cannabis in 2019 by two California groups within weeks of each other.

So the two real candidates pull in opposite directions from the forum picture: the mosaic-maker is an outdoor insect-borne virus, and the indoor threat usually does not make a mosaic.

Why the photo cannot settle it

Interveinal mottling on an otherwise green, turgid leaf is one of the least specific things a cannabis leaf can do. The field’s reference handbook describes magnesium and iron shortages both as yellowing between the veins with the veins staying green; below a root-zone pH of roughly 6.2 the magnesium group locks out even when it is present, and above roughly 6.5 iron does. A young plant that was moved too early can sit stalled and blotchy for two weeks while its roots catch up. Some cultivars carry a mottled or variegated leaf as a genetic trait and are perfectly healthy. Every one of those produces the same photograph.

There is also a reason to expect less from the photo than usual under indoor lights. In a controlled study of pathogen symptoms under different lamps, the plants under a blue-and-red-peaked LED spectrum expressed viral symptoms poorly compared with broad-spectrum light; the plant made less symptom, so there was less to see. The hosts in that study were tobacco relatives, cucurbits and citrus, not cannabis, and the pathogens were viruses and bacteria, so take it as a caution rather than a finding about your plant — but it is the direction you would expect: a viral pattern under a purple lamp is the one your camera is least likely to catch early.

The two tests that do settle it

The free one: fix the likely cause and watch new growth. Read the runoff pH with a calibrated pen and correct it into range for your medium (soil roughly 6.0–7.0, coco and hydro 5.5–6.5). Feed normally. Mark the newest leaf with a twist of tape and wait seven to ten days. A nutrient or pH cause shows in the leaves that open after the correction: they come in clean, and the old mottled leaves stay mottled, because damage does not repair. A virus or viroid does the opposite: the new growth is where it shows next, and it keeps showing regardless of what you feed. Position tells you more than colour here — a systemic pathogen moves into the growing tip; a shortage that has been corrected does not.

The definitive one: a lab assay. Hop latent viroid and the confirmed hemp viruses are detected by RT-PCR — a published multiplex test covers three viruses and the viroid from one sample. Commercial plant-pathology labs run it on a leaf or stem sample for a modest fee. That is the only thing that confirms a viroid. If a cutting or mother is valuable enough to keep, it is worth testing; if it is a seedling three weeks in, the free test usually answers the question first.

While you wait, isolate and stop sharing tools. The viroid moves mechanically, so a plant under suspicion gets its own scissors, or scissors that are wiped down between plants, and it does not get pruned with the same blade that then touches the healthy ones. That costs nothing and protects everything, whichever way the test comes out.

How to check this yourself

• Indoors or out? A true mosaic in a sealed tent with no leafhoppers is far less likely than a pH or magnesium problem. Outdoors in leafhopper country, the mosaic virus is a real candidate.
• Runoff pH, calibrated. Under 6.2 in soil, or over 6.5, explains most interveinal mottling on its own.
• Mark the newest leaf and wait ten days after correcting. Clean new growth: it was never a pathogen. Mottle that moves into the new growth after the fix: send a sample.
• Look at the stems and the internodes. Brittle stems, short internodes and a plant that has simply stopped, with or without a leaf pattern, is the viroid presentation — and it is a lab question, not a photo question.
• Own scissors for the suspect plant until you know.

What would show that we have this wrong

If the runoff pH is squarely in range on a calibrated pen, the plant is not recovering from a transplant, the mottle is spreading into leaves that opened after you corrected everything, and the internodes are shortening at the same time, that is the pattern this page cannot argue with — isolate it and send the sample. And if the “mottle” has been there since the seedling leaves and every leaf carries it evenly while the plant grows at full speed, that is a cultivar trait, and neither test is needed.

Where our device fits, stated narrowly

Croplock does not detect viruses or viroids, and it will not tell you a leaf is infected. A viroid is confirmed by a lab, and the honest thing a camera can do with a mottled leaf is what this page does: report the pattern and its position, and say what the pattern is consistent with. What the device can do is the free test’s bookkeeping — keep the dated pictures so that “is the new growth clean?” ten days from now is a comparison rather than a memory.

Sources

• Chiginsky et al. (2021), First Insights Into the Virus and Viroid Communities in Hemp (Cannabis sativa), Frontiers in Agronomy. DOI 10.3389/fagro.2021.778433 — the eight pathogens found, the absence of tobacco mosaic virus, the 81 percent beet curly top incidence, and the two quoted symptom lines.
• Giladi et al. (2020), First Report of Beet Curly Top Virus Infecting Cannabis sativa in Western Colorado, Plant Disease. DOI 10.1094/PDIS-08-19-1656-PDN.
• Bektas et al. (2019), Plant Disease, DOI 10.1094/PDIS-03-19-0459-PDN, and Warren et al. (2019), Plant Disease, DOI 10.1094/PDIS-03-19-0530-PDN — the two first reports of hop latent viroid in cannabis, California.
• A Multiplex RT-PCR for the Detection of Three Viruses and One Viroid Infecting Hemp, PhytoFrontiers (2023). DOI 10.1094/PHYTOFR-02-23-0021-R — that confirmation is an assay.
• Comparison and selection of LEDs for disease assays, PLOS One (2025), journal.pone.0328277 — viral symptom expression under blue-and-red-peaked spectra. Hosts were not cannabis; given as a caution, not a result about cannabis.
• Cannabis in Controlled Environment Production (CRC Press), Ch. 5. DOI 10.1201/9781003150442 — the magnesium and iron presentations and the pH ranges at which each locks out, restated in our own words.
• The ten-day new-growth test and the tool discipline are first-party practice, not citations.

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

Related: Is it cal-mag? · Is it a deficiency or a disease? · pH or EC — which number is actually telling me something?