Cannabis Yellow Leaves With Brown Spots: What It Means

Yellow leaves with brown spots on cannabis signal a mobile nutrient deficiency, most often magnesium or potassium. The spots appear because chlorophyll breaks down in patches. A pH problem causes the same look by locking out nutrients. Fungal leaf spot and root pests also produce yellow tissue with brown lesions.

Cannabis Yellow Leaves Diagnosis: Causes and Solutions

Check pH before adding nutrients. In soil, the target range is 6.0 to 7.0. In coco or hydro, 5.5 to 6.5. A pH of 5.0 or 7.5 blocks magnesium and potassium uptake even when those minerals are present.

How to Fix Yellow Leaves in Cannabis Seedlings

Magnesium Deficiency

Symptoms start on lower and middle leaves. The tissue between veins turns yellow. Brown spots follow at the leaf edges or between veins. The veins stay green. Magnesium moves from old leaves to new growth, so the top of the plant looks healthy.

cannabis lower leaves yellowing

Fix: Add 1 to 2 teaspoons of Epsom salt per gallon of water. Apply once. Repeat in 10 days if needed. Check pH after mixing.

why are my cannabis leaves turning yellow

Potassium Deficiency

Lower leaves yellow first. Brown spots or dead patches appear at the leaf margins. Tips curl and dry. Potassium is mobile, so the plant pulls it from old leaves.

Fix: Use a bloom nutrient with potassium sulfate or kelp. Raise the dose by 10 percent. Do not exceed 2.0 EC in soil or 2.4 EC in hydro.

Calcium Deficiency

Brown spots appear on new leaves. The spots are small and random. Leaf edges may curl. Calcium is immobile. The plant cannot move it from old tissue. This problem is common in coco and hydro.

Fix: Add cal-mag at 2 to 5 ml per gallon. Keep pH at 5.8 to 6.2 in hydro.

Nutrient Burn

Leaf tips turn brown and crisp. Yellowing follows the burn. The whole leaf may look yellow with brown spots. This happens when the nutrient solution is too strong.

Fix: Flush the medium with plain pH water. Use three times the pot volume. Then restart at half strength.

Leaf Septoria (Fungal Spot)

Leaf septoria causes round brown spots with yellow halos. Spots appear on lower leaves first. The fungus spreads upward in wet conditions. Leaves dry and fall off.

Fix: Remove infected leaves. Lower humidity to 40 to 50 percent. Increase airflow. Apply a copper-based fungicide if needed.

Pests

Spider mites, thrips, and aphids feed on leaf sap. The damage shows as yellow stippling. Brown spots follow where cells die. Look for webbing, black specks, or insects on leaf undersides.

Fix: Spray with neem oil or insecticidal soap. Repeat every 3 days for 2 weeks. Isolate the plant.

Root Problems and Overwatering

Overwatering drowns roots. Roots cannot take up magnesium or potassium. Leaves yellow with brown spots. The medium stays wet for days. Root rot smells sour and turns roots brown.

Fix: Let the top 2 inches of soil dry. Add perlite for drainage. Use a root inoculant with beneficial bacteria.

Light Burn and Heat Stress

Leaves closest to the light turn yellow with brown spots. The spots are dry and papery. Temperatures above 85°F slow photosynthesis.

Fix: Raise the light 6 to 12 inches. Keep canopy temperature at 75 to 82°F.

Step-by-Step Diagnosis

  1. Check pH of runoff or reservoir. Adjust to 6.0-7.0 soil, 5.5-6.5 hydro.
  2. Inspect leaf undersides with a 10x loupe for pests.
  3. Note which leaves show symptoms. Lower leaves: magnesium or potassium. New leaves: calcium or iron.
  4. Look for halos around spots. Halos point to fungus.
  5. Smell the root zone. A sour smell means root rot.
  6. Measure EC or PPM. High values point to nutrient burn.

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