Is it a deficiency or a disease?

The short answer

Where is it, and is it still moving?

1. Where. Is the damage on the oldest leaves down low, the newest growth up top, or the leaves most exposed to the lamp? Those three point in three different directions.
2. When. Has it appeared in leaves that opened after you first noticed it? Something living keeps making new damage. An event that already finished does not.

Almost every “what is wrong with my plant” question I have seen is answerable further than people expect from those two, before anyone buys anything.

Question one: where is it?

Plants move some nutrients around inside themselves and cannot move others. That single fact sorts a lot of damage by position.

When a plant runs short of something it can move — nitrogen is the clearest example — it strips that element out of its old leaves and sends it to the new growth. So the damage shows up at the bottom first, and the top looks fine. When it runs short of something it cannot move — calcium and iron, for instance — the old leaves keep what they already built and the new growth is what suffers.

• Oldest leaves first, working up: the mobile group. Nitrogen, potassium, magnesium.
• Newest growth, working down: the immobile group. Calcium, magnesium supplements, iron. Iron in particular tends to show up when the root-zone pH has drifted above roughly 6.5, where the plant can no longer take it up even though it is present.
• Uniform, everywhere including new growth: more likely sulphur, or a broad feed or pH problem, and worth checking your numbers before you chase an element.
• Only the leaves nearest the lamp, shaded ones fine: not a feeding pattern at all. That sorts by light, and it belongs with the heat question.

Now the honest limit, because this axis is genuinely imperfect. Magnesium can present on younger leaves, which makes it easy to confuse with iron, and the reference text says so plainly. Position narrows the field. It does not close the case, and anyone telling you it does is overselling it.

Question two: is it still moving?

This is the test I reach for most, and it costs nothing.

Mark the affected leaves today — a twist tie, a photo, whatever you like — and look again in four or five days. In the meantime the plant will have opened new leaves.

• New damage on leaves that were not there before: something is still acting. A pathogen or a pest is the first thing to think about, and a feeding problem that is still uncorrected is the second.
• The old marks are still there but nothing new has appeared: whatever did it has stopped. Very often it was a single event — a hot week, a hard feed, a spray, time spent somewhere else before the plant reached you — and the damaged tissue simply cannot heal. Leaves do not repair. They just carry the scar until they drop.

That second case is the one people waste the most effort on, treating damage that finished happening a fortnight ago.

Where it is	Still spreading?	Look at
Oldest leaves, low down	Yes	A mobile nutrient running short — and your feed strength
Newest growth, up top	Yes	An immobile nutrient, and your pH before your bottles
Scattered, no age pattern	Yes	Something living — pathogen or pest
Anywhere	No	A finished event. Stop treating it and watch the new growth
Only the most exposed tops	Either	Light and heat, not feeding

Why picture-matching goes wrong

The usual approach is to search images until one looks like your leaf. It fails for a reason worth understanding: a leaf has a limited vocabulary. Yellow, brown, spotted, curled, crisp edges. Very different causes converge on the same few appearances, especially once damage is advanced.

Which is the other reason to look early. Once lesions run together and a leaf is mostly dead, almost every cause looks the same. The boring early photo — a few discrete marks on tissue that is otherwise still green — carries far more information than the dramatic one.

If you want to compare against something better than search results, Cockson and Whipker published an open-access atlas of cannabis nutrient disorders (2019) with controlled single-element photographs. Comparing to a controlled series beats comparing to a stranger’s tent.

What would show that I have this wrong

If the damage sorts by neither age nor light exposure and is not spreading, these two questions have not helped you and you should go looking at what happened to that plant a couple of weeks ago instead. And if the roots are involved — wilting in a medium that is already wet — the pattern on the leaves is downstream of the real problem, and that is a different page.

Where my device fits, stated narrowly

Croplock does not settle deficiency versus disease for you, and I would not trust anything that claimed to from a photograph alone. What it does is keep the record that makes the second question answerable: it photographs the same plants on a schedule, so “is this new since last week” is something you can look up rather than something you have to remember. The two questions above are yours to answer either way, and they work with no equipment at all.

Sources

• Cannabis in Controlled Environment Production (CRC Press). DOI 10.1201/9781003150442 — nutrient mobility and symptom position, and the disease chapter’s account of how living causes progress.
• Cockson, P. & Whipker, B. (2019). Nutrient disorder atlas for Cannabis sativa. Applied Sciences 9(20): 4432. Open access.
• The caveat that magnesium can present on younger leaves, and is therefore confusable with iron, is stated in the reference text above, not softened by me.

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