Fungal Infection on calathea
What's Happening
Calathea leaves are thin and highly sensitive to prolonged wetness. When water droplets from misting or overhead watering remain on leaf surfaces for 4+ hours in temperatures above 70°F, fungal pathogens (particularly Colletotrichum and Myrothecium species) germinate and penetrate the leaf epidermis. The plant's tropical understory adaptation—large leaf surface area for light capture in shade—becomes a liability indoors where air circulation is limited. Misting to increase humidity actually creates localized areas of 100% humidity on leaf surfaces, triggering opportunistic fungal infection that manifests as brown/black spots with yellow halos, often starting at leaf margins or where water pools. Unlike root rot, this foliar infection spreads rapidly across leaf surface tissue.
How to Fix It
- 1
Immediate halt: Stop all misting and overhead watering immediately—use bottom-watering or water at soil level only
- 2
Prune affected tissue: Cut individual infected leaves at base with sterile scissors; remove above the node if partial leaf removal needed
- 3
Treat with fungicide: Apply copper-based fungicide (Bonide) or 1 tsp baking soda per liter water as foliar spray to remaining leaves; ensure good coverage but not runoff
- 4
Improve air circulation: Position oscillating fan 3-4 feet from plant on low setting for 6-8 hours daily to speed leaf drying
- 5
Switch to indirect humidity: Replace misting with pebble trays, humidifiers, or plant grouping to achieve 60-80% ambient humidity without wetting leaves
- 6
Monitor for 2 weeks: Watch for new spots; if spreading continues, repot in fresh sterile soil as root system may be compromised
How to Prevent It
Never mist Calatheas directly; water at soil level; maintain 60-80% ambient humidity with humidifiers, not leaf wetting; ensure bright indirect light and good airflow; water in morning so any accidental splashes dry quickly; avoid crowded plant arrangements that trap moisture.