Why Your Basil Has Black Spots (Fusarium or Cold Damage)
Black spots on basil are usually fusarium wilt or cold damage. Fusarium is fatal. Cold damage is cosmetic if caught early.
Basil is sensitive to both cold and fungal disease. The pattern of spots tells you which.
The 2 Causes
Cause 1: Fusarium Wilt (Serious)
Signs: Black streaks on stems. Leaves yellow, then black, then drop. Plant wilts despite water. Cause: Soil-borne fungus. Fix: No cure. Remove and destroy. Don’t compost.
Cause 2: Cold Damage (Recoverable)
Signs: Black spots on leaves after cold exposure. Stem still firm. Cause: Below 50°F. Basil is tropical. Fix: Move to warmth. Remove affected leaves.
How to Tell Them Apart
| Sign | Fusarium | Cold |
|---|---|---|
| Stem | Black streaks | Firm, green |
| Pattern | Yellow then black | Sudden black spots |
| Spread | Fast | Stops when warm |
| Recovery | None | Yes |
What to Do
- Check the stem. Black streaks → fusarium. Firm → cold.
- If fusarium: Remove plant. Bag it. Discard soil.
- If cold: Move to 65-70°F. Remove damaged leaves.
- Monitor for 1 week. New growth means recovery.
Prevention
For fusarium:
- Don’t reuse soil
- Sterilize pots
- Don’t overwater
For cold:
- Keep basil above 50°F
- Don’t plant outside until nights are warm
Common Questions
Can basil recover from black spots? From cold, yes. From fusarium, no.
Is it safe to eat basil with black spots? Cut off the spots. Or discard the leaf.
Will it spread to other basil? Fusarium spreads through soil. Isolate.
Does it affect other herbs? Fusarium affects basil specifically.
Should I throw away the pot? No. Sterilize with 10% bleach.
Can I replant in the same soil? No. Discard soil and use fresh.