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
Measure before you treat
Plant diagnostic essentials
These tools help confirm the cause of brown spots before you change nutrients, watering, or the environment.
Carson MicroBrite Pocket Microscope
Inspect leaf undersides for mites, eggs, mildew, and harvest-ready trichomes.
Check price on AmazonGovee Bluetooth Hygrometer
Track temperature and humidity trends instead of guessing from one reading.
Check price on AmazonVIVOSUN pH and TDS Meter Combo
Check root-zone pH and nutrient strength before changing a feeding plan.
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Causes ranked by likelihood
1. Calcium deficiency
Most commonSigns
- ●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
Recommended products
Botanicare Cal-Mag Plus
The standard Cal-Mag — fixes most spotting
2. Magnesium deficiency
CommonSigns
- ●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)
CommonSigns
- ●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
PossibleSigns
- ●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.
Gear that prevents this problem
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