Could calcium deficiency be causing brown spots?

Small brown/rust spots scattered across young-to-middle leaves. New growth may be twisted or distorted. Common in coco coir and RO/filtered water setups. Fix: Add Cal-Mag supplement at 3-5 ml/gal every feeding Check pH — calcium absorbs best at 6.2-6.5 in soil If using RO water, ALWAYS add Cal-Mag

Could magnesium deficiency be causing brown spots?

Yellowing BETWEEN the veins (interveinal chlorosis) turning brown. Starts on lower leaves and moves up. Leaf tips may curl upward. Fix: Foliar spray with 1 tsp Epsom salt per quart of water Add Cal-Mag to feed Check pH — Mg locks out below 6.0 in soil

Could leaf septoria (fungal) be causing brown spots?

Yellow spots that turn brown with distinct circular edges. Starts on lower leaves, spreads upward in humid conditions. Worst after rain or high humidity. Fix: Remove and destroy affected leaves (don't compost) Improve airflow with oscillating fans Drop humidity below 60% in veg, 50% in flower Apply a copper-based fungicide if outdoors

Could light burn from water droplets be causing brown spots?

Spots appear only on upper leaves under the light. Happened after a foliar spray. Fix: Only foliar spray with lights off Spray in early evening so leaves dry overnight

Leaf IssuesSerious

Brown Spots on Cannabis Leaves

Brown spots on cannabis leaves can mean a nutrient deficiency, a fungal infection, or damage from water droplets and light. The pattern and location of the spots tells you which.

What it looks like

●Small rust-colored, dark brown, or yellow-brown spots
●Spots often on older (lower) leaves first
●Spots may be surrounded by yellow halos
●Progressive — they usually spread if untreated

Quick diagnostic checks

1If you use RO or distilled water, add Cal-Mag immediately

2Remove any spotted leaves below the canopy to limit fungal spread

3Check runoff pH — Ca and Mg both lock out at wrong pH

Causes ranked by likelihood

1. Calcium deficiency

Most common

Signs

●Small brown/rust spots scattered across young-to-middle leaves
●New growth may be twisted or distorted
●Common in coco coir and RO/filtered water setups

Fix

✓Add Cal-Mag supplement at 3-5 ml/gal every feeding
✓Check pH — calcium absorbs best at 6.2-6.5 in soil
✓If using RO water, ALWAYS add Cal-Mag

2. Magnesium deficiency

Common

Signs

●Yellowing BETWEEN the veins (interveinal chlorosis) turning brown
●Starts on lower leaves and moves up
●Leaf tips may curl upward

Fix

✓Foliar spray with 1 tsp Epsom salt per quart of water
✓Add Cal-Mag to feed
✓Check pH — Mg locks out below 6.0 in soil

3. Leaf septoria (fungal)

Common

Signs

●Yellow spots that turn brown with distinct circular edges
●Starts on lower leaves, spreads upward in humid conditions
●Worst after rain or high humidity

Fix

✓Remove and destroy affected leaves (don't compost)
✓Improve airflow with oscillating fans
✓Drop humidity below 60% in veg, 50% in flower
✓Apply a copper-based fungicide if outdoors

4. Light burn from water droplets

Possible

Signs

●Spots appear only on upper leaves under the light
●Happened after a foliar spray

Fix

✓Only foliar spray with lights off
✓Spray in early evening so leaves dry overnight

⚠️When to worry

Spots that spread across more than 20% of the canopy in a week, or that appear on buds directly, need immediate action. Bud-level spots during flower can lead to bud rot.

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Brown Spots on Cannabis Leaves

What it looks like

Quick diagnostic checks

Causes ranked by likelihood

1. Calcium deficiency

2. Magnesium deficiency

3. Leaf septoria (fungal)

4. Light burn from water droplets

⚠️When to worry

Not sure what you're looking at?

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