Algae vs. Powdery Mildew on Bonsai: Sulfur Treats Both

Likely Algae, Not Mildew

Light green fuzz on bonsai bark/leaves that rubs off is typically algae—common in humid, low-light conditions. True powdery mildew is white, powdery, and on leaves, not bark.

Unified Treatment: Sulfur

• Wettable sulfur powder: Mix per label for ornamentals. Spray all surfaces.
• Bordeaux mixture (copper sulfate + lime): Also effective; copper stains blue—protect surfaces.

Application Protocol

1. Move bonsai outdoors or over drop cloth.
2. Spray to runoff, covering trunk, branches, both leaf sides.
3. Repeat 2–3 times at 7–10 day intervals.
4. Improve airflow and light exposure to prevent recurrence.

Cultural Prevention

• Morning sun, afternoon shade.
• Water soil only—keep foliage dry.
• Space plants for air circulation.
• Remove algae-promoting moss from soil surface.

Sulfur is broad-spectrum, low-toxicity, and handles both algae and true powdery mildew. Treat preventively in humid seasons.