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Brown or Rust Spots on Cannabis Leaves: Calcium, pH or Pests?
By the Cultivar grow team · published 2026-08-04 · updated 2026-08-04 · how we source these numbers
Brown, rust, or bronze speckling is one of those symptoms where four different problems produce something that looks broadly the same in a photo. The distinguishing details are small but reliable: where on the plant the spots are, what shape they take, and what your runoff pH reads.
Start here: three questions
1. Where are the spots?
- Lower and middle leaves first, working upward — a mobile-nutrient problem. The plant is pulling resources from old growth to feed new growth. Points toward magnesium or potassium.
- New and upper growth first — an immobile-nutrient problem. Points toward calcium.
- Scattered anywhere, worst on undersides and lower canopy — points toward pests.
2. What shape are they?
- Fine, even stippling — many tiny dots of similar size, almost like the leaf was airbrushed. This is spider mite feeding damage.
- Irregular blotches with yellow haloes, growing over days. A nutrient problem.
- Round tan spots with dark borders, often with a yellow ring, starting low. Leaf septoria, a fungal issue.
3. What is your runoff pH?
This is the question that most often changes the answer. Measure it before you buy anything.
Why pH usually decides this
Nutrient availability is chemical. Each nutrient has a pH band in which roots can actually take it up, and calcium in particular becomes progressively harder to absorb as the root zone drifts acidic. A pot can hold plenty of calcium and still starve the plant.
This is why "I added cal-mag and it came back" is such a common story. The supplement was never the missing piece.
Target ranges, consistent with our pH chart:
| Medium | Target pH |
|---|---|
| Soil | 6.3–6.8 |
| Coco | 5.8–6.2 |
| Hydro | 5.5–6.0 |
If your runoff sits outside these, correct pH first and change nothing else for a week. Very often that is the whole fix. If runoff pH is in range and spots are still spreading, then a genuine deficiency or a pest is on the table.
The four usual causes
Calcium deficiency. New growth affected first; spots often start at leaf edges and work in; new leaves may look distorted or hooked. Common in coco, which holds calcium poorly, and under LEDs, where higher light intensity raises demand. Fix: confirm pH, then supplement cal-mag.
Magnesium deficiency. Lower leaves first, classically yellowing between the veins while the veins stay green, with rust-coloured spots developing in the yellow areas. Also common in coco and under strong light. Fix: confirm pH, then cal-mag.
Potassium deficiency. Older leaves first, with browning and crisping at the margins and tips alongside the spotting. Sometimes confused with nutrient burn — the difference is that burn hits the biggest, best-fed leaves and potassium deficiency hits the oldest.
Spider mites. Fine even stippling, undersides first, often with visible specks that move under magnification and fine webbing where infestation is advanced. This is not a feeding problem and will not respond to any nutrient change — go to spider mites.
Leaf septoria, briefly
If the spots are distinctly round, tan-to-grey in the centre with a darker border, appear on lower leaves first, and spread upward in humid conditions, you are likely looking at a fungal leaf spot rather than a nutrient issue. It thrives in warm humid air with poor circulation.
Response: remove the worst-affected leaves, improve airflow through the lower canopy, avoid wetting foliage, and bring humidity toward your stage-appropriate VPD targets. Sanitation matters more than any spray.
Confirming the fix
Spotted tissue never heals. What you are watching for:
- New leaves emerging clean.
- Existing spots not spreading to leaves that were previously unmarked.
Give it 5 to 10 days before judging. If new growth is still coming in spotted after that, your first diagnosis was wrong — go back to the three questions rather than adding a second supplement on top of the first.
The mistake worth avoiding
Reaching for cal-mag as the reflex answer to any brown speckle. It is the right fix often enough that the habit forms, and wrong often enough that it wastes weeks. The pH pen costs less than a season of guessing, and it answers the question that decides everything else.
- Digital pH pen + calibration solution — the single most useful purchase here, because pH decides whether calcium is available at all
- Cal-mag supplement — the standard fix once pH is confirmed in range, especially in coco and under LEDs
- Jeweller's loupe or pocket microscope — settles the deficiency-versus-mites question in ten seconds
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FAQ
What causes brown rust spots on cannabis leaves?
Most commonly a calcium or magnesium shortfall — often caused by pH being out of range rather than by an actual lack in the medium. Other causes are potassium deficiency, spider mite damage, and leaf septoria. Where the spots appear on the plant and their shape separate them.
Are brown spots calcium deficiency?
Frequently, especially in coco and under LED lighting where calcium and magnesium demand runs higher. But measure runoff pH before adding cal-mag — calcium becomes hard to absorb as pH drifts low, and in that case more calcium in the pot does not help.
How do I tell mite damage from a deficiency?
Mite damage is fine, even stippling — many tiny pale dots of similar size, usually starting on the undersides and lower leaves, sometimes with fine webbing. A deficiency makes larger, irregular, blotchy patches. Check the leaf underside with magnification.
Will spotted leaves recover?
No. Damaged leaf tissue stays damaged. Judge the fix on whether new leaves come in clean and whether the spotting stops spreading to previously healthy leaves.
Why do brown spots keep coming back after I add cal-mag?
Because the cause is probably not a lack of calcium. Recurring spots despite supplementing almost always means pH is out of range, so the calcium you are adding cannot be taken up. Fix the pH and the spots usually stop without more supplement.
Keep reading
- Yellow Leaves on Cannabis: a 6-Question Decision Tree
- Nutrient Lockout: When Feeding More Makes It Worse
- Spider Mites on Cannabis: Stippling, Webbing and How to Break the Cycle
- Bud Rot (Botrytis): Catching It Before It Takes the Harvest
- Drooping Cannabis Leaves: Overwatered or Underwatered?
- Fungus Gnats: Small Flies in Your Grow and the Larvae That Matter
- Heat Stress in Cannabis: Taco Leaves, Foxtails and What to Fix First
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