White spots on cannabis leaves usually point to a limited set of issues—most of them are diagnosable by looking closely at the pattern, location, and spread. Here are the primary causes and how to distinguish them:

1. Powdery Mildew (Fungal Disease)

Appearance: White, powder-like coating on leaf surfaces (often starts as small circular spots).

Spread: Rapid, especially in high humidity with poor airflow.

Common in: Indoor grows, dense canopies.

Action:

Use a grow room dehumidifier to reduce humidity (<50% in veg, <40% in flower)

Improve air circulation

Apply fungicides (e.g., potassium bicarbonate, sulfur in veg stage)

2. Spider Mites

Appearance: Tiny white or yellow speckles (stippling), often with fine webbing under leaves.

Cause: Mites sucking plant sap.

Where to check: Underside of leaves (use magnification).

Action:

Use miticides or insecticidal soap

Introduce beneficial insects (e.g., predatory mites)

Lower temperature and increase humidity slightly (mites thrive in hot, dry conditions)

3. Thrips

Appearance: Silvery-white streaks or spots, sometimes with tiny black dots (feces).

Damage pattern: Scraping marks rather than round spots.

Action:

Sticky traps to monitor

Neem oil or spinosad treatments

Maintain clean grow environment

4. Calcium Deficiency

Appearance: Irregular white or pale spots, often with brown edges; new growth is affected first.

Cause: Poor nutrient uptake, often due to pH imbalance.

Action:

Check pH (soil: ~6.2–6.5, hydro: ~5.8–6.2)

Add Cal-Mag supplement

Ensure proper watering practices

5. Light Burn / Water Droplet Damage

Appearance: White or bleached spots, often on top leaves closest to light.

Cause: Intense light or water droplets acting like magnifying lenses.

Action:

Increase distance from grow lights

Avoid spraying leaves under strong light

6. Leaf Septoria (Less common but possible)

Appearance: White to gray spots with darker borders.

Progression: Leaves yellow and drop over time.

Action:

Remove affected leaves

Improve airflow and sanitation

Apply appropriate fungicide

Quick Diagnostic Tips

Powder wipes off? → Likely powdery mildew

Tiny moving dots/webbing? → Spider mites

Streaky/scratch-like marks? → Thrips

New growth affected + nutrient issues? → Calcium deficiency

Only top leaves near light? → Light stress