First Phalaenopsis: Diagnosing Black Leaf Spots—Sunburn vs. Fungus

A new Phalaenopsis in a north greenhouse window developed dime-sized black spots with central streaking. Two likely causes: concentrated sunbeam burn or fungal infection from overnight leaf wetness.

Differentiating Causes

• Sunburn: Dry, crisp spots; cosmetic only. Remove affected leaf section with sterile blade.
• Fungus: Soft, spreading spots; serious. Caused by water on leaves overnight.

Treatment and Prevention

• Water in morning only.
• Minimize leaf wetness; use small fan for air movement.
• Swab spots with cinnamon, Listerine, or sulfur; or use commercial fungicide (e.g., Banrot).
• Keep plant on dry side temporarily.

Root Check Imperative

New store-bought orchids often have rotting roots in degraded media. Unpot, remove mushy roots, repot in fresh bark mix (soak first). Annual repotting prevents this.

Key Takeaways

• Morning watering + air flow = fungus prevention.
• Sunburn = dry spots; fungus = soft/spreading.
• Always check roots on new plants.

Act fast on black spots: diagnose, treat, and repot for a healthy start.